We don't really need to lock all the pages being returned to protect against truncate. We only need to lock the one at the highest index, and check i_size while that lock is held since truncate_inode_pages_range() will block on any page that is locked.
We're still vulnerable to holepunches, but there's no locking currently between holepunches and truncate, so we're no worse off now. ________________________________________ From: Rik van Riel [[email protected]] Sent: February 18, 2014 5:28 AM To: Linus Torvalds; Kirill A. Shutemov Cc: Andrew Morton; Mel Gorman; Andi Kleen; Wilcox, Matthew R; Dave Hansen; Alexander Viro; Dave Chinner; linux-mm; linux-fsdevel; Linux Kernel Mailing List Subject: Re: [RFC, PATCHv2 0/2] mm: map few pages around fault address if they are in page cache On 02/17/2014 02:01 PM, Linus Torvalds wrote: > - increment the page _mapcount (iow, do "page_add_file_rmap()" > early). This guarantees that any *subsequent* unmap activity on this > page will walk the file mapping lists, and become serialized by the > page table lock we hold. > > - mb_after_atomic_inc() (this is generally free) > > - test that the page is still unlocked and uptodate, and the page > mapping still points to our page. > > - if that is true, we're all good, we can use the page, otherwise we > decrement the mapcount (page_remove_rmap()) and skip the page. > > Hmm? Doing something like this means that we would never lock the > pages we prefault, and you can go back to your gang lookup rather than > that "one page at a time". And the race case is basically never going > to trigger. > > Comments? What would the direct io code do when it runs into a page with elevated mapcount, but for which a mapping cannot be found yet? Looking at the code, it looks like the above scheme could cause some trouble with invalidate_inode_pages2_range(), which has the following sequence: if (page_mapped(page)) { ... unmap page } BUG_ON(page_mapped(page)); In other words, it looks like incrementing _mapcount first could lead to an oops in the truncate and direct IO code. The page lock is used to prevent such races. *sigh* -- All rights reversed -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
