Messages in this thread | | | Date | Tue, 6 Mar 2007 22:36:41 -0800 | From | Andrew Morton <> | Subject | Re: [patch 3/6] mm: fix fault vs invalidate race for linear mappings |
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On Wed, 21 Feb 2007 05:50:05 +0100 (CET) Nick Piggin <npiggin@suse.de> wrote:
> Fix the race between invalidate_inode_pages and do_no_page. > > Andrea Arcangeli identified a subtle race between invalidation of > pages from pagecache with userspace mappings, and do_no_page. > > The issue is that invalidation has to shoot down all mappings to the > page, before it can be discarded from the pagecache. Between shooting > down ptes to a particular page, and actually dropping the struct page > from the pagecache, do_no_page from any process might fault on that > page and establish a new mapping to the page just before it gets > discarded from the pagecache. > > The most common case where such invalidation is used is in file > truncation. This case was catered for by doing a sort of open-coded > seqlock between the file's i_size, and its truncate_count. > > Truncation will decrease i_size, then increment truncate_count before > unmapping userspace pages; do_no_page will read truncate_count, then > find the page if it is within i_size, and then check truncate_count > under the page table lock and back out and retry if it had > subsequently been changed (ptl will serialise against unmapping, and > ensure a potentially updated truncate_count is actually visible). > > Complexity and documentation issues aside, the locking protocol fails > in the case where we would like to invalidate pagecache inside i_size. > do_no_page can come in anytime and filemap_nopage is not aware of the > invalidation in progress (as it is when it is outside i_size). The > end result is that dangling (->mapping == NULL) pages that appear to > be from a particular file may be mapped into userspace with nonsense > data. Valid mappings to the same place will see a different page. > > Andrea implemented two working fixes, one using a real seqlock, > another using a page->flags bit. He also proposed using the page lock > in do_no_page, but that was initially considered too heavyweight. > However, it is not a global or per-file lock, and the page cacheline > is modified in do_no_page to increment _count and _mapcount anyway, so > a further modification should not be a large performance hit. > Scalability is not an issue. > > This patch implements this latter approach. ->nopage implementations > return with the page locked if it is possible for their underlying > file to be invalidated (in that case, they must set a special vm_flags > bit to indicate so). do_no_page only unlocks the page after setting > up the mapping completely. invalidation is excluded because it holds > the page lock during invalidation of each page (and ensures that the > page is not mapped while holding the lock). > > This also allows significant simplifications in do_no_page, because > we have the page locked in the right place in the pagecache from the > start. >
Why was truncate_inode_pages_range() altered to unmap the page if it got mapped again?
Oh. Because the unmap_mapping_range() call got removed from vmtruncate(). Why? (Please send suitable updates to the changelog).
I guess truncate of a mmapped area isn't sufficiently common to worry about the inefficiency of this change.
Lots of memory barriers got removed in memory.c, unchangeloggedly.
Gratuitous renaming of locals in do_no_page() makes the change hard to review. Should have been a separate patch.
In fact, the patch would have been heaps clearer if that renaming had been a separate patch.
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