Messages in this thread | | | Date | Wed, 2 Dec 2009 22:19:47 +0000 | From | Mel Gorman <> | Subject | Re: [PATCH] hugetlb: Acquire the i_mmap_lock before walking the prio_tree to unmap a page |
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On Wed, Dec 02, 2009 at 10:16:02PM +0000, Mel Gorman wrote: > On Wed, Dec 02, 2009 at 08:13:39PM +0000, Hugh Dickins wrote: > > On Wed, 2 Dec 2009, Mel Gorman wrote: > > > > > When the owner of a mapping fails COW because a child process is holding a > > > reference and no pages are available, the children VMAs are walked and the > > > page is unmapped. The i_mmap_lock is taken for the unmapping of the page but > > > not the walking of the prio_tree. In theory, that tree could be changing > > > while the lock is released although in practice it is protected by the > > > hugetlb_instantiation_mutex. This patch takes the i_mmap_lock properly for > > > the duration of the prio_tree walk in case the hugetlb_instantiation_mutex > > > ever goes away. > > > > > > [hugh.dickins@tiscali.co.uk: Spotted the problem in the first place] > > > Signed-off-by: Mel Gorman <mel@csn.ul.ie> > > > > The patch looks good - thanks for taking care of that, Mel. > > > > But the comment seems wrong to me: hugetlb_instantiation_mutex > > guards against concurrent hugetlb_fault()s; but the structure of > > the prio_tree shifts as vmas based on that inode are inserted into > > (mmap'ed) and removed from (munmap'ed) that tree (always while > > holding i_mmap_lock). I don't see hugetlb_instantiation_mutex > > giving us any protection against this at present. > > > > You're right of course. I'll report without that nonsense included. >
Actually, shouldn't the mmap_sem be protecting against concurrent mmap and munmap altering the tree? The comment is still bogus of course.
-- Mel Gorman Part-time Phd Student Linux Technology Center University of Limerick IBM Dublin Software Lab
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