mlock: avoid dirtying pages and triggering writeback (5ecfda04) · Commits · 戴 / test

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 <walken@google.com> Cc: Hugh Dickins <hughd@google.com> Cc: Rik van Riel <riel@redhat.com> Cc: Kosaki Motohiro <kosaki.motohiro@jp.fujitsu.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Nick Piggin <npiggin@kernel.dk> Cc: Theodore Tso <tytso@google.com> Cc: Michael Rubin <mrubin@google.com> Cc: Suleiman Souhlal <suleiman@google.com> Cc: Dave Chinner <david@fromorbit.com> Cc: Christoph Hellwig <hch@infradead.org> Signed-off-by:

Andrew Morton <akpm@linux-foundation.org> Signed-off-by:

Linus Torvalds <torvalds@linux-foundation.org>

mm/memory.c

+6 −1

Original line number	Diff line number	Diff line
		@@ -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;

mm/mlock.c

+6 −1

Original line number	Diff line number	Diff line
		@@ -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 */

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