Re: [PATCH 2/6] mm/zswap: invalidate zswap entry when swap entry free

[Chengming Zhou]

On 2024/2/2 08:11, Yosry Ahmed wrote:
> On Thu, Feb 01, 2024 at 03:49:02PM +0000, Chengming Zhou wrote:
>> During testing I found there are some times the zswap_writeback_entry()
>> return -ENOMEM, which is not we expected:
>>
>> bpftrace -e 'kr:zswap_writeback_entry {@[(int32)retval]=count()}'
>> @[-12]: 1563
>> @[0]: 277221
>>
>> The reason is that __read_swap_cache_async() return NULL because
>> swapcache_prepare() failed. The reason is that we won't invalidate
>> zswap entry when swap entry freed to the per-cpu pool, these zswap
>> entries are still on the zswap tree and lru list.
>>
>> This patch moves the invalidation ahead to when swap entry freed
>> to the per-cpu pool, since there is no any benefit to leave trashy
>> zswap entry on the tree and lru list.
>>
>> With this patch:
>> bpftrace -e 'kr:zswap_writeback_entry {@[(int32)retval]=count()}'
>> @[0]: 259744
>>
>> Note: large folio can't have zswap entry for now, so don't bother
>> to add zswap entry invalidation in the large folio swap free path.
> 
> This makes me slightly nervous. Should we add a comment somewhere just
> in case this is missed if someone adds large folio support?

Ok, will add this comment:

+       /* Large folio swap slot is not covered. */
	zswap_invalidate(entry);

> 
> Otherwise the patch itself LGTM.

Thanks!