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

[Hillf Danton <dhillf@gmail.com>]

On Sat, Jan 14, 2012 at 7:39 AM, Andrew Morton
<akpm@linux-foundation.org> wrote:
> 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?
>

Hi Andrew

The case of no page, in the fault path, is handled after acquiring
hugetlb_instantiation_mutex, and on ohter hand, kmalloc is called
if new region required, so no race to check reservation needed but
after spinning page_table_lock.

Thanks
Hillf

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