Re: [BUGFIX][mm][PATCH] fix migration race in rmap_walk

[KAMEZAWA Hiroyuki]

On Fri, 23 Apr 2010 16:58:01 +0100
Mel Gorman <mel@xxxxxxxxx> wrote:

> > I had considered this idea as well as it is vaguely similar to how zones get
> > resized with a seqlock. I was hoping that the existing locking on anon_vma
> > would be usable by backing off until uncontended but maybe not so lets
> > check out this approach.
> > 
> 
> A possible combination of the two approaches is as follows. It uses the
> anon_vma lock mostly except where the anon_vma differs between the page
> and the VMAs being walked in which case it uses the seq counter. I've
> had it running a few hours now without problems but I'll leave it
> running at least 24 hours.
> 
ok, I'll try this, too.


> ==== CUT HERE ====
>  mm,migration: Prevent rmap_walk_[anon|ksm] seeing the wrong VMA information by protecting against vma_adjust with a combination of locks and seq counter
> 
> vma_adjust() is updating anon VMA information without any locks taken.
> In constract, file-backed mappings use the i_mmap_lock. This lack of
> locking can result in races with page migration. During rmap_walk(),
> vma_address() can return -EFAULT for an address that will soon be valid.
> This leaves a dangling migration PTE behind which can later cause a
> BUG_ON to trigger when the page is faulted in.
> 
> With the recent anon_vma changes, there is no single anon_vma->lock that
> can be taken that is safe for rmap_walk() to guard against changes by
> vma_adjust(). Instead, a lock can be taken on one VMA while changes
> happen to another.
> 
> What this patch does is protect against updates with a combination of
> locks and seq counters. First, the vma->anon_vma lock is taken by
> vma_adjust() and the sequence counter starts. The lock is released and
> the sequence ended when the VMA updates are complete.
> 
> The lock serialses rmap_walk_anon when the page and VMA share the same
> anon_vma. Where the anon_vmas do not match, the seq counter is checked.
> If a change is noticed, rmap_walk_anon drops its locks and starts again
> from scratch as the VMA list may have changed. The dangling migration
> PTE bug was not triggered after several hours of stress testing with
> this patch applied.
> 
> [kamezawa.hiroyu@xxxxxxxxxxxxxx: Use of a seq counter]
> Signed-off-by: Mel Gorman <mel@xxxxxxxxx>

I think this patch is nice!

Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>



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