mlock: avoid dirtying pages and triggering writeback

When faulting in pages for mlock(), we want to break COW for anonymous or
file pages within VM_WRITABLE, non-VM_SHARED vmas.  However, there is no
need to write-fault into VM_SHARED vmas since shared file pages can be
mlocked first and dirtied later, when/if they actually get written to.
Skipping the write fault is desirable, as we don't want to unnecessarily
cause these pages to be dirtied and queued for writeback.

Signed-off-by: Michel Lespinasse <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Kosaki Motohiro <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Nick Piggin <[email protected]>
Cc: Theodore Tso <[email protected]>
Cc: Michael Rubin <[email protected]>
Cc: Suleiman Souhlal <[email protected]>
Cc: Dave Chinner <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

diff --git a/mm/memory.c b/mm/memory.c
index 9144fae9a68bcb46eb1f4a500a3d8b605d9e03bd..b8f97b8575b7fa5b765ac2fa044b24538feb1844 100644 (file)
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3299,7 +3299,12 @@ int make_pages_present(unsigned long addr, unsigned long end)
        vma = find_vma(current->mm, addr);
        if (!vma)
                return -ENOMEM;
-       write = (vma->vm_flags & VM_WRITE) != 0;
+       /*
+        * We want to touch writable mappings with a write fault in order
+        * to break COW, except for shared mappings because these don't COW
+        * and we would not want to dirty them for nothing.
+        */
+       write = (vma->vm_flags & (VM_WRITE | VM_SHARED)) == VM_WRITE;
        BUG_ON(addr >= end);
        BUG_ON(end > vma->vm_end);
        len = DIV_ROUND_UP(end, PAGE_SIZE) - addr/PAGE_SIZE;

diff --git a/mm/mlock.c b/mm/mlock.c
index b70919ce4f72e6941f67b1a5462f5f270c231536..4f318642fbbe56b2d426dc6c02d74b9599e03f8e 100644 (file)
--- a/mm/mlock.c
+++ b/mm/mlock.c
@@ -171,7 +171,12 @@ static long __mlock_vma_pages_range(struct vm_area_struct *vma,
        VM_BUG_ON(!rwsem_is_locked(&mm->mmap_sem));
 
        gup_flags = FOLL_TOUCH | FOLL_GET;
-       if (vma->vm_flags & VM_WRITE)
+       /*
+        * We want to touch writable mappings with a write fault in order
+        * to break COW, except for shared mappings because these don't COW
+        * and we would not want to dirty them for nothing.
+        */
+       if ((vma->vm_flags & (VM_WRITE | VM_SHARED)) == VM_WRITE)
                gup_flags |= FOLL_WRITE;
 
        /* We don't try to access the guard page of a stack vma */