Re: [PATCH] mm/zswap: fix potential deadlock in zswap_frontswap_store()

[Andrey Ryabinin <aryabinin@virtuozzo.com>]

On 04/03/2017 11:47 AM, Michal Hocko wrote:
> On Fri 31-03-17 10:00:30, Shakeel Butt wrote:
>> On Fri, Mar 31, 2017 at 8:30 AM, Andrey Ryabinin
>> <aryabinin@virtuozzo.com> wrote:
>>> zswap_frontswap_store() is called during memory reclaim from
>>> __frontswap_store() from swap_writepage() from shrink_page_list().
>>> This may happen in NOFS context, thus zswap shouldn't use __GFP_FS,
>>> otherwise we may renter into fs code and deadlock.
>>> zswap_frontswap_store() also shouldn't use __GFP_IO to avoid recursion
>>> into itself.
>>>
>>
>> Is it possible to enter fs code (or IO) from zswap_frontswap_store()
>> other than recursive memory reclaim? However recursive memory reclaim
>> is protected through PF_MEMALLOC task flag. The change seems fine but
>> IMHO reasoning needs an update. Adding Michal for expert opinion.
> 
> Yes this is true.

Indeed, I missed that detail.

> I haven't checked all the callers of
> zswap_frontswap_store but is it fixing any real problem or just trying
> to be overly cautious.
>

zswap_frontswap_store() is called only from swap_writepage().
Given that swap_writepage() is called only during reclaim or swapoff
shouldn't be a real problem.

  
> Btw...
> 
>>> zswap_frontswap_store() call zpool_malloc() with __GFP_NORETRY |
>>> __GFP_NOWARN | __GFP_KSWAPD_RECLAIM, so let's use the same flags for
>>> zswap_entry_cache_alloc() as well, instead of GFP_KERNEL.
>>>
>>> Signed-off-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
>>> ---
>>>  mm/zswap.c | 7 +++----
>>>  1 file changed, 3 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/mm/zswap.c b/mm/zswap.c
>>> index eedc278..12ad7e9 100644
>>> --- a/mm/zswap.c
>>> +++ b/mm/zswap.c
>>> @@ -966,6 +966,7 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
>>>         struct zswap_tree *tree = zswap_trees[type];
>>>         struct zswap_entry *entry, *dupentry;
>>>         struct crypto_comp *tfm;
>>> +       gfp_t gfp = __GFP_NORETRY | __GFP_NOWARN | __GFP_KSWAPD_RECLAIM;
> 
> This doesn't trigger direct reclaim so __GFP_NORETRY is bogus. I suspect
> you didn't want GFP_NOWAIT alternative.
> 
> [...]
>>> @@ -1017,9 +1018,7 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
>>>
>>>         /* store */
>>>         len = dlen + sizeof(struct zswap_header);
>>> -       ret = zpool_malloc(entry->pool->zpool, len,
>>> -                          __GFP_NORETRY | __GFP_NOWARN | __GFP_KSWAPD_RECLAIM,
>>> -                          &handle);
>>> +       ret = zpool_malloc(entry->pool->zpool, len, gfp, &handle);
> 
> and here we used to do GFP_NOWAIT alternative already. What is going on
> here?


I suspect that there was no particular reason to assemble this custom set of gfp flags.
This code probably should have been using GFP_NOWAIT|__GFP_NOWARN from the very beginning.


>>>         if (ret == -ENOSPC) {
>>>                 zswap_reject_compress_poor++;
>>>                 goto put_dstmem;
>>> --
>>> 2.10.2
>>>
> 

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