mm: change return values of finish_mkwrite_fault()

Currently finish_mkwrite_fault() returns 0 when PTE got changed before
we acquired PTE lock and VM_FAULT_WRITE when we succeeded in modifying
the PTE.  This is somewhat confusing since 0 generally means success, it
is also inconsistent with finish_fault() which returns 0 on success.
Change finish_mkwrite_fault() to return 0 on success and VM_FAULT_NOPAGE
when PTE changed.  Practically, there should be no behavioral difference
since we bail out from the fault the same way regardless whether we
return 0, VM_FAULT_NOPAGE, or VM_FAULT_WRITE.  Also note that
VM_FAULT_WRITE has no effect for shared mappings since the only two
places that check it - KSM and GUP - care about private mappings only.
Generally the meaning of VM_FAULT_WRITE for shared mappings is not well
defined and we should probably clean that up.

Link: http://lkml.kernel.org/r/1479460644-25076-17-git-send-email-jack@suse.cz
Signed-off-by: Jan Kara <jack@suse.cz>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/memory.c b/mm/memory.c
index bbc25da48a184c9186b9eb5bad1a6418165bcab7..8b7f0656a9211f2d6b88b23af4b95eabd061cc75 100644 (file)
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2295,10 +2295,10 @@ int finish_mkwrite_fault(struct vm_fault *vmf)
         */
        if (!pte_same(*vmf->pte, vmf->orig_pte)) {
                pte_unmap_unlock(vmf->pte, vmf->ptl);
-               return 0;
+               return VM_FAULT_NOPAGE;
        }
        wp_page_reuse(vmf);
-       return VM_FAULT_WRITE;
+       return 0;
 }
 
 /*
@@ -2341,8 +2341,7 @@ static int wp_page_shared(struct vm_fault *vmf)
                        return tmp;
                }
                tmp = finish_mkwrite_fault(vmf);
-               if (unlikely(!tmp || (tmp &
-                                     (VM_FAULT_ERROR | VM_FAULT_NOPAGE)))) {
+               if (unlikely(tmp & (VM_FAULT_ERROR | VM_FAULT_NOPAGE))) {
                        unlock_page(vmf->page);
                        put_page(vmf->page);
                        return tmp;