lkml.org 
[lkml]   [2013]   [Jan]   [7]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH] mm: thp: Acquire the anon_vma rwsem for lock during split
On Fri, Jan 04, 2013 at 01:28:09PM -0800, Hugh Dickins wrote:
> I've added Alexander, Hillf and Alex to the Cc.
>
> On Fri, 4 Jan 2013, Mel Gorman wrote:
> > Zhouping, please test this patch.
> >
> > Andrea and Hugh, any comments on whether this could be improved?
>
> Your patch itself looks just right to me, no improvement required;
> and it's easy to understand how the bug crept in, from a blanket
> rwsem replacement of anon_vma mutex meeting the harmless-looking
> anon_vma_interval_tree_foreach in __split_huge_page, which looked
> as if it needed only the readlock provided by the usual method.
>

Indeed. Thanks Hugh for taking a look over it.

> But I'd fight shy myself of trying to describe all the THP locking
> conventions in the commit message: I haven't really tried to work
> out just how right you've got all those details.
>

I thought it was risky myself but it was the best way of getting Andrea
to object if I missed some subtlety! If I had infinite time I would
follow up with a patch to Documentation/vm/transhuge.txt explaining how
the anon_vma lock is used by THP.

> The actual race in question here was just two processes (one or both
> forked) doing split_huge_page() on the same THPage at the same time,
> wasn't it? (Though of course we only see the backtrace from one of
> them.) Which would be very confusing, and no surprise that the
> pmd_trans_splitting test ends up skipping pmds already updated by
> the racing process, so the mapcount doesn't match what's expected.
> Of course we need exclusive lock against that, which you give it.
>

Ok, thanks. Will resend to Andrew with some changelog edits.

--
Mel Gorman
SUSE Labs


\
 
 \ /
  Last update: 2013-01-07 16:21    [W:0.387 / U:0.816 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site