lkml.org 
[lkml]   [2012]   [Oct]   [25]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: shmem_getpage_gfp VM_BUG_ON triggered. [3.7rc2]
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
>
> --
> To unsubscribe, send a message with 'unsubscribe linux-mm' in
> the body to majordomo@kvack.org. For more info on Linux MM,
> see: http://www.linux-mm.org/ .
> Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
>



\
 
 \ /
  Last update: 2012-10-26 05:01    [W:0.045 / U:0.452 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site