Re: shmem_getpage_gfp VM_BUG_ON triggered. [3.7rc2]

[Ni zhan Chen]

On 10/26/2012 05:48 AM, Hugh Dickins wrote:
> On Thu, 25 Oct 2012, Johannes Weiner wrote:
> > On Wed, Oct 24, 2012 at 09:36:27PM -0700, Hugh Dickins wrote:
> > > On Wed, 24 Oct 2012, Dave Jones wrote:
> > >
> > > > Machine under significant load (4gb memory used, swap usage fluctuating)
> > > > triggered this...
> > > >
> > > > WARNING: at mm/shmem.c:1151 shmem_getpage_gfp+0xa5c/0xa70()
> > > > Pid: 29795, comm: trinity-child4 Not tainted 3.7.0-rc2+ #49
> > > >
> > > > 1148                         error = shmem_add_to_page_cache(page, mapping, index,
> > > > 1149                                                 gfp, swp_to_radix_entry(swap));
> > > > 1150                         /* We already confirmed swap, and make no allocation */
> > > > 1151                         VM_BUG_ON(error);
> > > > 1152                 }
> > > That's very surprising.  Easy enough to handle an error there, but
> > > of course I made it a VM_BUG_ON because it violates my assumptions:
> > > I rather need to understand how this can be, and I've no idea.
> > Could it be concurrent truncation clearing out the entry between
> > shmem_confirm_swap() and shmem_add_to_page_cache()?  I don't see
> > anything preventing that.
> >
> > The empty slot would not match the expected swap entry this call
> > passes in and the returned error would be -ENOENT.
> Excellent notion, many thanks Hannes, I believe you've got it.
>
> I've hit that truncation problem in swapoff (and commented on it
> in shmem_unuse_inode), but never hit it or considered it here.
> I think of the page lock as holding it stable, but truncation's
> free_swap_and_cache only does a trylock on the swapcache page,
> so we're not secured against that possibility.

Hi Hugh,

Even though free_swap_and_cache only does a trylock on the swapcache 
page, but it doens't call delete_from_swap_cache and the associated 
entry should still be there, I am interested in what you have already 
introduce to protect it?

> So I'd like to change it to VM_BUG_ON(error && error != -ENOENT),
> but there's a little tidying up to do in the -ENOENT case, which

Do you mean radix_tree_insert will return -ENOENT if the associated 
entry is not present? Why I can't find this return value in the function 
radix_tree_insert?

> needs more thought.  A delete_from_swap_cache(page) - though we
> can be lazy and leave that to reclaim for such a rare occurrence -
> and probably a mem_cgroup uncharge; but the memcg hooks are always
> the hardest to get right, I'll have think about that one carefully.
>
> Hugh
>
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