Re: [PATCH] mm/swap_state: remove unnecessary lru_add_drain() from readahead

[JP Kobryn <jp.kobryn@linux.dev>]

On 6/8/26 9:06 AM, Shakeel Butt wrote:
> On Mon, Jun 08, 2026 at 07:32:42AM -0700, Usama Arif wrote:
>> swap_cluster_readahead() and swap_vma_readahead() end the readahead
>> loop with an explicit lru_add_drain() call. That drain is a leftover
>> from 2.6.12 era code and serves no functional purpose for the callers:
>>
>> - do_swap_page() ignores LRU residency for the readahead folios;
>>   it only needs the target folio it called swapin_readahead() for,
>>   and if the write-fault path needs the target folio on the LRU to count
>>   references accurately, it runs its own lru_add_drain() at the
>>   wp_can_reuse_anon_folio() and do_swap_page() sites.
>>
>> - shmem_swapin_cluster() immediately locks the  returned folio, waits
>>   for writeback, then operates on it - LRU residency of either the target
>>   or the readahead folios is irrelevant.
>>
>> - try_to_unuse() likewise locks the folio and calls unuse_pte() without
>>   depending on LRU presence.
>>
>> Folios newly added to the swap cache by the readahead loop sit in
>> the per-CPU LRU folio_batch and will be drained naturally as the
>> batch fills (FOLIO_BATCH_SIZE),by the next reclaim/compaction
>> lru_add_drain_all() and so on.  The unconditional drain only
>> synchronously flushes a partial batch and forces contention on
>> lruvec_lock.
>>
>> On a 176-CPU production host running a memory-pressured workload, this
>> path was observed to call folio_batch_move_lru() from
>> swap_cluster_readahead() ~28K/min, a very large source of LRU lock
>> traffic.
>>
>> This is a direct continuation of the cleanup started in commit
>> 1aa43598c03b ("mm: remove unnecessary calls to lru_add_drain") which
>> removed the equivalent drain from free_pages_and_swap_cache() with
>> the same rationale. A detailed reasoning for this is present in [1].
>>
>> Remove both drains.
>>
>> [1] https://lore.kernel.org/all/dca2824e8e88e826c6b260a831d79089b5b9c79d.camel@surriel.com/T/#u
>>
>> Signed-off-by: Usama Arif <usama.arif@linux.dev>
> 
> Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
> 
> Thanks for pushing this. JP was also looking into LRU lock contention sources.
> Particularly we lack visibiluty into the lru_add_drain_all() callers. The idea
> was to add tracepoints to tracks such callers. (Just nudging you towards it :P)

Yup, I'll send a patch for that one.