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Wed, 10 Jun 2026 09:51:53 -0700 (PDT) Received: from localhost ([2603:7001:f100:500:365a:60ff:fe62:ff29]) by smtp.gmail.com with ESMTPSA id d75a77b69052e-51775e2e16dsm222674371cf.31.2026.06.10.09.51.52 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 10 Jun 2026 09:51:53 -0700 (PDT) Date: Wed, 10 Jun 2026 12:51:52 -0400 From: Johannes Weiner To: Usama Arif Cc: Andrew Morton , riel@surriel.com, david@kernel.org, baohua@kernel.org, baoquan.he@linux.dev, chrisl@kernel.org, kasong@tencent.com, linux-mm@kvack.org, shakeel.butt@linux.dev, nphamcs@gmail.com, shikemeng@huaweicloud.com, youngjun.park@lge.com, linux-kernel@vger.kernel.org, kernel-team@meta.com Subject: Re: [PATCH] mm/swap_state: remove unnecessary lru_add_drain() from readahead Message-ID: References: <20260608143242.2869392-1-usama.arif@linux.dev> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260608143242.2869392-1-usama.arif@linux.dev> X-Rspamd-Queue-Id: 017AB20011 X-Stat-Signature: kx5pfzhgco667i3yze3p6xdiqonhcf8h X-Rspam-User: X-Rspamd-Server: rspam12 X-HE-Tag: 1781110314-485697 X-HE-Meta: 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 dyLq0WNl 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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 Acked-by: Johannes Weiner