Re: [PATCH] mm: hugetlb: undo change to page mapcount in fault handler

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 11 Jan 2012 20:06:30 +0800
Hillf Danton <dhillf@gmail.com> wrote:

> On Thu, Jan 5, 2012 at 7:16 AM, Andrew Morton <akpm@linux-foundation.org> wrote:
> > On Fri, 23 Dec 2011 21:00:41 +0800
> > Hillf Danton <dhillf@gmail.com> wrote:
> >
> >> Page mapcount should be updated only if we are sure that the page ends
> >> up in the page table otherwise we would leak if we couldn't COW due to
> >> reservations or if idx is out of bounds.
> >
> > It would be much nicer if we could run vma_needs_reservation() before
> > even looking up or allocating the page.
> >
> > And afaict the interface is set up to do that: you run
> > vma_needs_reservation() before allocating the page and then
> > vma_commit_reservation() afterwards.
> >
> > But hugetlb_no_page() and hugetlb_fault() appear to have forgotten to
> > run vma_commit_reservation() altogether. __Why isn't this as busted as
> > it appears to be?
> 
> Hi Andrew
> 
> IIUC the two operations, vma_{needs, commit}_reservation, are folded in
> alloc_huge_page(), need to break the pair?

Looking at it again, it appears that the vma_needs_reservation() calls
are used to predict whether a subsequent COW attempt is going to fail.

If that's correct then things aren't as bad as I first thought. 
However I suspect the code in hugetlb_no_page() is a bit racy: the
vma_needs_reservation() call should happen after we've taken
page_table_lock.  As things stand, another thread could sneak in there
and steal the reservation which this thread thought was safe.

What do you think?

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