mm/mmap.c: extract __vma_unlink_list() as counterpart for __vma_link_list()

[mirror_ubuntu-hirsute-kernel.git] / mm / util.c

// SPDX-License-Identifier: GPL-2.0-only
#include <linux/mm.h>
#include <linux/slab.h>
#include <linux/string.h>
#include <linux/compiler.h>
#include <linux/export.h>
#include <linux/err.h>
#include <linux/sched.h>
#include <linux/sched/mm.h>
#include <linux/sched/signal.h>
#include <linux/sched/task_stack.h>
#include <linux/security.h>
#include <linux/swap.h>
#include <linux/swapops.h>
#include <linux/mman.h>
#include <linux/hugetlb.h>
#include <linux/vmalloc.h>
#include <linux/userfaultfd_k.h>
#include <linux/elf.h>
#include <linux/elf-randomize.h>
#include <linux/personality.h>
#include <linux/random.h>
#include <linux/processor.h>
#include <linux/sizes.h>
#include <linux/compat.h>

#include <linux/uaccess.h>

#include "internal.h"

/**
 * kfree_const - conditionally free memory
 * @x: pointer to the memory
 *
 * Function calls kfree only if @x is not in .rodata section.
 */
void kfree_const(const void *x)
{
        if (!is_kernel_rodata((unsigned long)x))
                kfree(x);
}
EXPORT_SYMBOL(kfree_const);

/**
 * kstrdup - allocate space for and copy an existing string
 * @s: the string to duplicate
 * @gfp: the GFP mask used in the kmalloc() call when allocating memory
 *
 * Return: newly allocated copy of @s or %NULL in case of error
 */
char *kstrdup(const char *s, gfp_t gfp)
{
        size_t len;
        char *buf;

        if (!s)
                return NULL;

        len = strlen(s) + 1;
        buf = kmalloc_track_caller(len, gfp);
        if (buf)
                memcpy(buf, s, len);
        return buf;
}
EXPORT_SYMBOL(kstrdup);

/**
 * kstrdup_const - conditionally duplicate an existing const string
 * @s: the string to duplicate
 * @gfp: the GFP mask used in the kmalloc() call when allocating memory
 *
 * Note: Strings allocated by kstrdup_const should be freed by kfree_const.
 *
 * Return: source string if it is in .rodata section otherwise
 * fallback to kstrdup.
 */
const char *kstrdup_const(const char *s, gfp_t gfp)
{
        if (is_kernel_rodata((unsigned long)s))
                return s;

        return kstrdup(s, gfp);
}
EXPORT_SYMBOL(kstrdup_const);

/**
 * kstrndup - allocate space for and copy an existing string
 * @s: the string to duplicate
 * @max: read at most @max chars from @s
 * @gfp: the GFP mask used in the kmalloc() call when allocating memory
 *
 * Note: Use kmemdup_nul() instead if the size is known exactly.
 *
 * Return: newly allocated copy of @s or %NULL in case of error
 */
char *kstrndup(const char *s, size_t max, gfp_t gfp)
{
        size_t len;
        char *buf;

        if (!s)
                return NULL;

        len = strnlen(s, max);
        buf = kmalloc_track_caller(len+1, gfp);
        if (buf) {
                memcpy(buf, s, len);
                buf[len] = '\0';
        }
        return buf;
}
EXPORT_SYMBOL(kstrndup);

/**
 * kmemdup - duplicate region of memory
 *
 * @src: memory region to duplicate
 * @len: memory region length
 * @gfp: GFP mask to use
 *
 * Return: newly allocated copy of @src or %NULL in case of error
 */
void *kmemdup(const void *src, size_t len, gfp_t gfp)
{
        void *p;

        p = kmalloc_track_caller(len, gfp);
        if (p)
                memcpy(p, src, len);
        return p;
}
EXPORT_SYMBOL(kmemdup);

/**
 * kmemdup_nul - Create a NUL-terminated string from unterminated data
 * @s: The data to stringify
 * @len: The size of the data
 * @gfp: the GFP mask used in the kmalloc() call when allocating memory
 *
 * Return: newly allocated copy of @s with NUL-termination or %NULL in
 * case of error
 */
char *kmemdup_nul(const char *s, size_t len, gfp_t gfp)
{
        char *buf;

        if (!s)
                return NULL;

        buf = kmalloc_track_caller(len + 1, gfp);
        if (buf) {
                memcpy(buf, s, len);
                buf[len] = '\0';
        }
        return buf;
}
EXPORT_SYMBOL(kmemdup_nul);

/**
 * memdup_user - duplicate memory region from user space
 *
 * @src: source address in user space
 * @len: number of bytes to copy
 *
 * Return: an ERR_PTR() on failure.  Result is physically
 * contiguous, to be freed by kfree().
 */
void *memdup_user(const void __user *src, size_t len)
{
        void *p;

        p = kmalloc_track_caller(len, GFP_USER | __GFP_NOWARN);
        if (!p)
                return ERR_PTR(-ENOMEM);

        if (copy_from_user(p, src, len)) {
                kfree(p);
                return ERR_PTR(-EFAULT);
        }

        return p;
}
EXPORT_SYMBOL(memdup_user);

/**
 * vmemdup_user - duplicate memory region from user space
 *
 * @src: source address in user space
 * @len: number of bytes to copy
 *
 * Return: an ERR_PTR() on failure.  Result may be not
 * physically contiguous.  Use kvfree() to free.
 */
void *vmemdup_user(const void __user *src, size_t len)
{
        void *p;

        p = kvmalloc(len, GFP_USER);
        if (!p)
                return ERR_PTR(-ENOMEM);

        if (copy_from_user(p, src, len)) {
                kvfree(p);
                return ERR_PTR(-EFAULT);
        }

        return p;
}
EXPORT_SYMBOL(vmemdup_user);

/**
 * strndup_user - duplicate an existing string from user space
 * @s: The string to duplicate
 * @n: Maximum number of bytes to copy, including the trailing NUL.
 *
 * Return: newly allocated copy of @s or an ERR_PTR() in case of error
 */
char *strndup_user(const char __user *s, long n)
{
        char *p;
        long length;

        length = strnlen_user(s, n);

        if (!length)
                return ERR_PTR(-EFAULT);

        if (length > n)
                return ERR_PTR(-EINVAL);

        p = memdup_user(s, length);

        if (IS_ERR(p))
                return p;

        p[length - 1] = '\0';

        return p;
}
EXPORT_SYMBOL(strndup_user);

/**
 * memdup_user_nul - duplicate memory region from user space and NUL-terminate
 *
 * @src: source address in user space
 * @len: number of bytes to copy
 *
 * Return: an ERR_PTR() on failure.
 */
void *memdup_user_nul(const void __user *src, size_t len)
{
        char *p;

        /*
         * Always use GFP_KERNEL, since copy_from_user() can sleep and
         * cause pagefault, which makes it pointless to use GFP_NOFS
         * or GFP_ATOMIC.
         */
        p = kmalloc_track_caller(len + 1, GFP_KERNEL);
        if (!p)
                return ERR_PTR(-ENOMEM);

        if (copy_from_user(p, src, len)) {
                kfree(p);
                return ERR_PTR(-EFAULT);
        }
        p[len] = '\0';

        return p;
}
EXPORT_SYMBOL(memdup_user_nul);

void __vma_link_list(struct mm_struct *mm, struct vm_area_struct *vma,
                struct vm_area_struct *prev, struct rb_node *rb_parent)
{
        struct vm_area_struct *next;

        vma->vm_prev = prev;
        if (prev) {
                next = prev->vm_next;
                prev->vm_next = vma;
        } else {
                mm->mmap = vma;
                if (rb_parent)
                        next = rb_entry(rb_parent,
                                        struct vm_area_struct, vm_rb);
                else
                        next = NULL;
        }
        vma->vm_next = next;
        if (next)
                next->vm_prev = vma;
}

void __vma_unlink_list(struct mm_struct *mm, struct vm_area_struct *vma)
{
        struct vm_area_struct *prev, *next;

        next = vma->vm_next;
        prev = vma->vm_prev;
        if (prev)
                prev->vm_next = next;
        else
                mm->mmap = next;
        if (next)
                next->vm_prev = prev;
}

/* Check if the vma is being used as a stack by this task */
int vma_is_stack_for_current(struct vm_area_struct *vma)
{
        struct task_struct * __maybe_unused t = current;

        return (vma->vm_start <= KSTK_ESP(t) && vma->vm_end >= KSTK_ESP(t));
}

#ifndef STACK_RND_MASK
#define STACK_RND_MASK (0x7ff >> (PAGE_SHIFT - 12))     /* 8MB of VA */
#endif

unsigned long randomize_stack_top(unsigned long stack_top)
{
        unsigned long random_variable = 0;

        if (current->flags & PF_RANDOMIZE) {
                random_variable = get_random_long();
                random_variable &= STACK_RND_MASK;
                random_variable <<= PAGE_SHIFT;
        }
#ifdef CONFIG_STACK_GROWSUP
        return PAGE_ALIGN(stack_top) + random_variable;
#else
        return PAGE_ALIGN(stack_top) - random_variable;
#endif
}

#ifdef CONFIG_ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT
unsigned long arch_randomize_brk(struct mm_struct *mm)
{
        /* Is the current task 32bit ? */
        if (!IS_ENABLED(CONFIG_64BIT) || is_compat_task())
                return randomize_page(mm->brk, SZ_32M);

        return randomize_page(mm->brk, SZ_1G);
}

unsigned long arch_mmap_rnd(void)
{
        unsigned long rnd;

#ifdef CONFIG_HAVE_ARCH_MMAP_RND_COMPAT_BITS
        if (is_compat_task())
                rnd = get_random_long() & ((1UL << mmap_rnd_compat_bits) - 1);
        else
#endif /* CONFIG_HAVE_ARCH_MMAP_RND_COMPAT_BITS */
                rnd = get_random_long() & ((1UL << mmap_rnd_bits) - 1);

        return rnd << PAGE_SHIFT;
}

static int mmap_is_legacy(struct rlimit *rlim_stack)
{
        if (current->personality & ADDR_COMPAT_LAYOUT)
                return 1;

        if (rlim_stack->rlim_cur == RLIM_INFINITY)
                return 1;

        return sysctl_legacy_va_layout;
}

/*
 * Leave enough space between the mmap area and the stack to honour ulimit in
 * the face of randomisation.
 */
#define MIN_GAP         (SZ_128M)
#define MAX_GAP         (STACK_TOP / 6 * 5)

static unsigned long mmap_base(unsigned long rnd, struct rlimit *rlim_stack)
{
        unsigned long gap = rlim_stack->rlim_cur;
        unsigned long pad = stack_guard_gap;

        /* Account for stack randomization if necessary */
        if (current->flags & PF_RANDOMIZE)
                pad += (STACK_RND_MASK << PAGE_SHIFT);

        /* Values close to RLIM_INFINITY can overflow. */
        if (gap + pad > gap)
                gap += pad;

        if (gap < MIN_GAP)
                gap = MIN_GAP;
        else if (gap > MAX_GAP)
                gap = MAX_GAP;

        return PAGE_ALIGN(STACK_TOP - gap - rnd);
}

void arch_pick_mmap_layout(struct mm_struct *mm, struct rlimit *rlim_stack)
{
        unsigned long random_factor = 0UL;

        if (current->flags & PF_RANDOMIZE)
                random_factor = arch_mmap_rnd();

        if (mmap_is_legacy(rlim_stack)) {
                mm->mmap_base = TASK_UNMAPPED_BASE + random_factor;
                mm->get_unmapped_area = arch_get_unmapped_area;
        } else {
                mm->mmap_base = mmap_base(random_factor, rlim_stack);
                mm->get_unmapped_area = arch_get_unmapped_area_topdown;
        }
}
#elif defined(CONFIG_MMU) && !defined(HAVE_ARCH_PICK_MMAP_LAYOUT)
void arch_pick_mmap_layout(struct mm_struct *mm, struct rlimit *rlim_stack)
{
        mm->mmap_base = TASK_UNMAPPED_BASE;
        mm->get_unmapped_area = arch_get_unmapped_area;
}
#endif

/**
 * __account_locked_vm - account locked pages to an mm's locked_vm
 * @mm:          mm to account against
 * @pages:       number of pages to account
 * @inc:         %true if @pages should be considered positive, %false if not
 * @task:        task used to check RLIMIT_MEMLOCK
 * @bypass_rlim: %true if checking RLIMIT_MEMLOCK should be skipped
 *
 * Assumes @task and @mm are valid (i.e. at least one reference on each), and
 * that mmap_sem is held as writer.
 *
 * Return:
 * * 0       on success
 * * -ENOMEM if RLIMIT_MEMLOCK would be exceeded.
 */
int __account_locked_vm(struct mm_struct *mm, unsigned long pages, bool inc,
                        struct task_struct *task, bool bypass_rlim)
{
        unsigned long locked_vm, limit;
        int ret = 0;

        lockdep_assert_held_write(&mm->mmap_sem);

        locked_vm = mm->locked_vm;
        if (inc) {
                if (!bypass_rlim) {
                        limit = task_rlimit(task, RLIMIT_MEMLOCK) >> PAGE_SHIFT;
                        if (locked_vm + pages > limit)
                                ret = -ENOMEM;
                }
                if (!ret)
                        mm->locked_vm = locked_vm + pages;
        } else {
                WARN_ON_ONCE(pages > locked_vm);
                mm->locked_vm = locked_vm - pages;
        }

        pr_debug("%s: [%d] caller %ps %c%lu %lu/%lu%s\n", __func__, task->pid,
                 (void *)_RET_IP_, (inc) ? '+' : '-', pages << PAGE_SHIFT,
                 locked_vm << PAGE_SHIFT, task_rlimit(task, RLIMIT_MEMLOCK),
                 ret ? " - exceeded" : "");

        return ret;
}
EXPORT_SYMBOL_GPL(__account_locked_vm);

/**
 * account_locked_vm - account locked pages to an mm's locked_vm
 * @mm:          mm to account against, may be NULL
 * @pages:       number of pages to account
 * @inc:         %true if @pages should be considered positive, %false if not
 *
 * Assumes a non-NULL @mm is valid (i.e. at least one reference on it).
 *
 * Return:
 * * 0       on success, or if mm is NULL
 * * -ENOMEM if RLIMIT_MEMLOCK would be exceeded.
 */
int account_locked_vm(struct mm_struct *mm, unsigned long pages, bool inc)
{
        int ret;

        if (pages == 0 || !mm)
                return 0;

        down_write(&mm->mmap_sem);
        ret = __account_locked_vm(mm, pages, inc, current,
                                  capable(CAP_IPC_LOCK));
        up_write(&mm->mmap_sem);

        return ret;
}
EXPORT_SYMBOL_GPL(account_locked_vm);

unsigned long vm_mmap_pgoff(struct file *file, unsigned long addr,
        unsigned long len, unsigned long prot,
        unsigned long flag, unsigned long pgoff)
{
        unsigned long ret;
        struct mm_struct *mm = current->mm;
        unsigned long populate;
        LIST_HEAD(uf);

        ret = security_mmap_file(file, prot, flag);
        if (!ret) {
                if (down_write_killable(&mm->mmap_sem))
                        return -EINTR;
                ret = do_mmap_pgoff(file, addr, len, prot, flag, pgoff,
                                    &populate, &uf);
                up_write(&mm->mmap_sem);
                userfaultfd_unmap_complete(mm, &uf);
                if (populate)
                        mm_populate(ret, populate);
        }
        return ret;
}

unsigned long vm_mmap(struct file *file, unsigned long addr,
        unsigned long len, unsigned long prot,
        unsigned long flag, unsigned long offset)
{
        if (unlikely(offset + PAGE_ALIGN(len) < offset))
                return -EINVAL;
        if (unlikely(offset_in_page(offset)))
                return -EINVAL;

        return vm_mmap_pgoff(file, addr, len, prot, flag, offset >> PAGE_SHIFT);
}
EXPORT_SYMBOL(vm_mmap);

/**
 * kvmalloc_node - attempt to allocate physically contiguous memory, but upon
 * failure, fall back to non-contiguous (vmalloc) allocation.
 * @size: size of the request.
 * @flags: gfp mask for the allocation - must be compatible (superset) with GFP_KERNEL.
 * @node: numa node to allocate from
 *
 * Uses kmalloc to get the memory but if the allocation fails then falls back
 * to the vmalloc allocator. Use kvfree for freeing the memory.
 *
 * Reclaim modifiers - __GFP_NORETRY and __GFP_NOFAIL are not supported.
 * __GFP_RETRY_MAYFAIL is supported, and it should be used only if kmalloc is
 * preferable to the vmalloc fallback, due to visible performance drawbacks.
 *
 * Please note that any use of gfp flags outside of GFP_KERNEL is careful to not
 * fall back to vmalloc.
 *
 * Return: pointer to the allocated memory of %NULL in case of failure
 */
void *kvmalloc_node(size_t size, gfp_t flags, int node)
{
        gfp_t kmalloc_flags = flags;
        void *ret;

        /*
         * vmalloc uses GFP_KERNEL for some internal allocations (e.g page tables)
         * so the given set of flags has to be compatible.
         */
        if ((flags & GFP_KERNEL) != GFP_KERNEL)
                return kmalloc_node(size, flags, node);

        /*
         * We want to attempt a large physically contiguous block first because
         * it is less likely to fragment multiple larger blocks and therefore
         * contribute to a long term fragmentation less than vmalloc fallback.
         * However make sure that larger requests are not too disruptive - no
         * OOM killer and no allocation failure warnings as we have a fallback.
         */
        if (size > PAGE_SIZE) {
                kmalloc_flags |= __GFP_NOWARN;

                if (!(kmalloc_flags & __GFP_RETRY_MAYFAIL))
                        kmalloc_flags |= __GFP_NORETRY;
        }

        ret = kmalloc_node(size, kmalloc_flags, node);

        /*
         * It doesn't really make sense to fallback to vmalloc for sub page
         * requests
         */
        if (ret || size <= PAGE_SIZE)
                return ret;

        return __vmalloc_node_flags_caller(size, node, flags,
                        __builtin_return_address(0));
}
EXPORT_SYMBOL(kvmalloc_node);

/**
 * kvfree() - Free memory.
 * @addr: Pointer to allocated memory.
 *
 * kvfree frees memory allocated by any of vmalloc(), kmalloc() or kvmalloc().
 * It is slightly more efficient to use kfree() or vfree() if you are certain
 * that you know which one to use.
 *
 * Context: Either preemptible task context or not-NMI interrupt.
 */
void kvfree(const void *addr)
{
        if (is_vmalloc_addr(addr))
                vfree(addr);
        else
                kfree(addr);
}
EXPORT_SYMBOL(kvfree);

static inline void *__page_rmapping(struct page *page)
{
        unsigned long mapping;

        mapping = (unsigned long)page->mapping;
        mapping &= ~PAGE_MAPPING_FLAGS;

        return (void *)mapping;
}

/* Neutral page->mapping pointer to address_space or anon_vma or other */
void *page_rmapping(struct page *page)
{
        page = compound_head(page);
        return __page_rmapping(page);
}

/*
 * Return true if this page is mapped into pagetables.
 * For compound page it returns true if any subpage of compound page is mapped.
 */
bool page_mapped(struct page *page)
{
        int i;

        if (likely(!PageCompound(page)))
                return atomic_read(&page->_mapcount) >= 0;
        page = compound_head(page);
        if (atomic_read(compound_mapcount_ptr(page)) >= 0)
                return true;
        if (PageHuge(page))
                return false;
        for (i = 0; i < compound_nr(page); i++) {
                if (atomic_read(&page[i]._mapcount) >= 0)
                        return true;
        }
        return false;
}
EXPORT_SYMBOL(page_mapped);

struct anon_vma *page_anon_vma(struct page *page)
{
        unsigned long mapping;

        page = compound_head(page);
        mapping = (unsigned long)page->mapping;
        if ((mapping & PAGE_MAPPING_FLAGS) != PAGE_MAPPING_ANON)
                return NULL;
        return __page_rmapping(page);
}

struct address_space *page_mapping(struct page *page)
{
        struct address_space *mapping;

        page = compound_head(page);

        /* This happens if someone calls flush_dcache_page on slab page */
        if (unlikely(PageSlab(page)))
                return NULL;

        if (unlikely(PageSwapCache(page))) {
                swp_entry_t entry;

                entry.val = page_private(page);
                return swap_address_space(entry);
        }

        mapping = page->mapping;
        if ((unsigned long)mapping & PAGE_MAPPING_ANON)
                return NULL;

        return (void *)((unsigned long)mapping & ~PAGE_MAPPING_FLAGS);
}
EXPORT_SYMBOL(page_mapping);

/*
 * For file cache pages, return the address_space, otherwise return NULL
 */
struct address_space *page_mapping_file(struct page *page)
{
        if (unlikely(PageSwapCache(page)))
                return NULL;
        return page_mapping(page);
}

/* Slow path of page_mapcount() for compound pages */
int __page_mapcount(struct page *page)
{
        int ret;

        ret = atomic_read(&page->_mapcount) + 1;
        /*
         * For file THP page->_mapcount contains total number of mapping
         * of the page: no need to look into compound_mapcount.
         */
        if (!PageAnon(page) && !PageHuge(page))
                return ret;
        page = compound_head(page);
        ret += atomic_read(compound_mapcount_ptr(page)) + 1;
        if (PageDoubleMap(page))
                ret--;
        return ret;
}
EXPORT_SYMBOL_GPL(__page_mapcount);

int sysctl_overcommit_memory __read_mostly = OVERCOMMIT_GUESS;
int sysctl_overcommit_ratio __read_mostly = 50;
unsigned long sysctl_overcommit_kbytes __read_mostly;
int sysctl_max_map_count __read_mostly = DEFAULT_MAX_MAP_COUNT;
unsigned long sysctl_user_reserve_kbytes __read_mostly = 1UL << 17; /* 128MB */
unsigned long sysctl_admin_reserve_kbytes __read_mostly = 1UL << 13; /* 8MB */

int overcommit_ratio_handler(struct ctl_table *table, int write,
                             void __user *buffer, size_t *lenp,
                             loff_t *ppos)
{
        int ret;

        ret = proc_dointvec(table, write, buffer, lenp, ppos);
        if (ret == 0 && write)
                sysctl_overcommit_kbytes = 0;
        return ret;
}

int overcommit_kbytes_handler(struct ctl_table *table, int write,
                             void __user *buffer, size_t *lenp,
                             loff_t *ppos)
{
        int ret;

        ret = proc_doulongvec_minmax(table, write, buffer, lenp, ppos);
        if (ret == 0 && write)
                sysctl_overcommit_ratio = 0;
        return ret;
}

/*
 * Committed memory limit enforced when OVERCOMMIT_NEVER policy is used
 */
unsigned long vm_commit_limit(void)
{
        unsigned long allowed;

        if (sysctl_overcommit_kbytes)
                allowed = sysctl_overcommit_kbytes >> (PAGE_SHIFT - 10);
        else
                allowed = ((totalram_pages() - hugetlb_total_pages())
                           * sysctl_overcommit_ratio / 100);
        allowed += total_swap_pages;

        return allowed;
}

/*
 * Make sure vm_committed_as in one cacheline and not cacheline shared with
 * other variables. It can be updated by several CPUs frequently.
 */
struct percpu_counter vm_committed_as ____cacheline_aligned_in_smp;

/*
 * The global memory commitment made in the system can be a metric
 * that can be used to drive ballooning decisions when Linux is hosted
 * as a guest. On Hyper-V, the host implements a policy engine for dynamically
 * balancing memory across competing virtual machines that are hosted.
 * Several metrics drive this policy engine including the guest reported
 * memory commitment.
 */
unsigned long vm_memory_committed(void)
{
        return percpu_counter_read_positive(&vm_committed_as);
}
EXPORT_SYMBOL_GPL(vm_memory_committed);

/*
 * Check that a process has enough memory to allocate a new virtual
 * mapping. 0 means there is enough memory for the allocation to
 * succeed and -ENOMEM implies there is not.
 *
 * We currently support three overcommit policies, which are set via the
 * vm.overcommit_memory sysctl.  See Documentation/vm/overcommit-accounting.rst
 *
 * Strict overcommit modes added 2002 Feb 26 by Alan Cox.
 * Additional code 2002 Jul 20 by Robert Love.
 *
 * cap_sys_admin is 1 if the process has admin privileges, 0 otherwise.
 *
 * Note this is a helper function intended to be used by LSMs which
 * wish to use this logic.
 */
int __vm_enough_memory(struct mm_struct *mm, long pages, int cap_sys_admin)
{
        long allowed;

        VM_WARN_ONCE(percpu_counter_read(&vm_committed_as) <
                        -(s64)vm_committed_as_batch * num_online_cpus(),
                        "memory commitment underflow");

        vm_acct_memory(pages);

        /*
         * Sometimes we want to use more memory than we have
         */
        if (sysctl_overcommit_memory == OVERCOMMIT_ALWAYS)
                return 0;

        if (sysctl_overcommit_memory == OVERCOMMIT_GUESS) {
                if (pages > totalram_pages() + total_swap_pages)
                        goto error;
                return 0;
        }

        allowed = vm_commit_limit();
        /*
         * Reserve some for root
         */
        if (!cap_sys_admin)
                allowed -= sysctl_admin_reserve_kbytes >> (PAGE_SHIFT - 10);

        /*
         * Don't let a single process grow so big a user can't recover
         */
        if (mm) {
                long reserve = sysctl_user_reserve_kbytes >> (PAGE_SHIFT - 10);

                allowed -= min_t(long, mm->total_vm / 32, reserve);
        }

        if (percpu_counter_read_positive(&vm_committed_as) < allowed)
                return 0;
error:
        vm_unacct_memory(pages);

        return -ENOMEM;
}

/**
 * get_cmdline() - copy the cmdline value to a buffer.
 * @task:     the task whose cmdline value to copy.
 * @buffer:   the buffer to copy to.
 * @buflen:   the length of the buffer. Larger cmdline values are truncated
 *            to this length.
 *
 * Return: the size of the cmdline field copied. Note that the copy does
 * not guarantee an ending NULL byte.
 */
int get_cmdline(struct task_struct *task, char *buffer, int buflen)
{
        int res = 0;
        unsigned int len;
        struct mm_struct *mm = get_task_mm(task);
        unsigned long arg_start, arg_end, env_start, env_end;
        if (!mm)
                goto out;
        if (!mm->arg_end)
                goto out_mm;    /* Shh! No looking before we're done */

        spin_lock(&mm->arg_lock);
        arg_start = mm->arg_start;
        arg_end = mm->arg_end;
        env_start = mm->env_start;
        env_end = mm->env_end;
        spin_unlock(&mm->arg_lock);

        len = arg_end - arg_start;

        if (len > buflen)
                len = buflen;

        res = access_process_vm(task, arg_start, buffer, len, FOLL_FORCE);

        /*
         * If the nul at the end of args has been overwritten, then
         * assume application is using setproctitle(3).
         */
        if (res > 0 && buffer[res-1] != '\0' && len < buflen) {
                len = strnlen(buffer, res);
                if (len < res) {
                        res = len;
                } else {
                        len = env_end - env_start;
                        if (len > buflen - res)
                                len = buflen - res;
                        res += access_process_vm(task, env_start,
                                                 buffer+res, len,
                                                 FOLL_FORCE);
                        res = strnlen(buffer, res);
                }
        }
out_mm:
        mmput(mm);
out:
        return res;
}

int memcmp_pages(struct page *page1, struct page *page2)
{
        char *addr1, *addr2;
        int ret;

        addr1 = kmap_atomic(page1);
        addr2 = kmap_atomic(page2);
        ret = memcmp(addr1, addr2, PAGE_SIZE);
        kunmap_atomic(addr2);
        kunmap_atomic(addr1);
        return ret;
}