mm, gup: remove broken VM_BUG_ON_PAGE compound check for hugepages

When operating on hugepages with DEBUG_VM enabled, the GUP code checks
the compound head for each tail page prior to calling
page_cache_add_speculative.  This is broken, because on the fast-GUP
path (where we don't hold any page table locks) we can be racing with a
concurrent invocation of split_huge_page_to_list.

split_huge_page_to_list deals with this race by using page_ref_freeze to
freeze the page and force concurrent GUPs to fail whilst the component
pages are modified.  This modification includes clearing the
compound_head field for the tail pages, so checking this prior to a
successful call to page_cache_add_speculative can lead to false
positives: In fact, page_cache_add_speculative *already* has this check
once the page refcount has been successfully updated, so we can simply
remove the broken calls to VM_BUG_ON_PAGE.

Link: http://lkml.kernel.org/r/20170522133604.11392-2-punit.agrawal@arm.com
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Punit Agrawal <punit.agrawal@arm.com>
Acked-by: Steve Capper <steve.capper@arm.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Hillf Danton <hillf.zj@alibaba-inc.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/gup.c b/mm/gup.c
index 9e472cb835b5195af0cfe208a040ec864c8202e1..1e68bf21330299cd1e04aae8e28f10d082d87115 100644 (file)
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1426,7 +1426,6 @@ static int gup_huge_pmd(pmd_t orig, pmd_t *pmdp, unsigned long addr,
        head = pmd_page(orig);
        page = head + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
        do {
-               VM_BUG_ON_PAGE(compound_head(page) != head, page);
                pages[*nr] = page;
                (*nr)++;
                page++;
@@ -1465,7 +1464,6 @@ static int gup_huge_pud(pud_t orig, pud_t *pudp, unsigned long addr,
        head = pud_page(orig);
        page = head + ((addr & ~PUD_MASK) >> PAGE_SHIFT);
        do {
-               VM_BUG_ON_PAGE(compound_head(page) != head, page);
                pages[*nr] = page;
                (*nr)++;
                page++;
@@ -1503,7 +1501,6 @@ static int gup_huge_pgd(pgd_t orig, pgd_t *pgdp, unsigned long addr,
        head = pgd_page(orig);
        page = head + ((addr & ~PGDIR_MASK) >> PAGE_SHIFT);
        do {
-               VM_BUG_ON_PAGE(compound_head(page) != head, page);
                pages[*nr] = page;
                (*nr)++;
                page++;