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?
Otherwise the patch itself LGTM.
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!