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DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1756140875; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=2dZmzyywwnY7lchKOLgGY8doykdyDHwJJ4akbIOpMOs=; b=mqpvkaV+gR6VKDvX93iA5JZH4kv7rLy11Zbppcwm9x5N37j+I+IAD1ZEfrWxnee6mjf5pW fLJw2lyngoXCxU/07dF1HNmsODE42v0IGMe61UUL+hXzMJjUhUJQgyz7c5Hz6+3ktTRBld uealQshO024kKDTe1zIJigJAgbeNZHg= From: Roman Gushchin To: Mateusz Guzik Cc: Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org, "Matthew Wilcox (Oracle)" , Jan Kara Subject: Re: [PATCH] mm: readahead: improve mmap_miss heuristic for concurrent faults In-Reply-To: (Mateusz Guzik's message of "Mon, 25 Aug 2025 14:27:58 +0200") References: <20250815183224.62007-1-roman.gushchin@linux.dev> Date: Mon, 25 Aug 2025 09:54:29 -0700 Message-ID: <87plcjxry2.fsf@linux.dev> MIME-Version: 1.0 Content-Type: text/plain X-Migadu-Flow: FLOW_OUT X-Rspam-User: X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: 070D0A0008 X-Stat-Signature: wfhsa4mh18s99ntcmch3cd39811dmh75 X-HE-Tag: 1756140876-228901 X-HE-Meta: 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 P0gZti9+ F5VhKGJDT287/VeWMgQaz+/DnHfI7NfjLGJThPpIhC/qxFy5Y+GMFHF5fwWOJVRnY2dXTPx5j1Or7BLvmtllq7zbBIHylXxmtvwUVA1wajHqY7isF7Im/Thk8xlAhCKGBpWPP2Qr7PC/yF2sbyFu6ybWhwltskEEKKlxcswsgq2Qni/dm/XhUaueh3Ht3L3/by7cinKRUABJcJHmKx0Oi0mxibdR6DpYne3iaD8Ea4pMFAnIMuuOehCg1PFdVM/BimTvuqAaQ1RPm0h1vmXH1+U2FFc4QyhsGhhf2kmVrsNgMLoe0PT9UFtLsYLH6UZ+SbB+YBDavTAiLaSRB5FgQ8Y5PPA== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Mateusz Guzik writes: > On Fri, Aug 15, 2025 at 11:32:24AM -0700, Roman Gushchin wrote: >> If two or more threads of an application faulting on the same folio, >> the mmap_miss counter can be decreased multiple times. It breaks the >> mmap_miss heuristic and keeps the readahead enabled even under extreme >> levels of memory pressure. >> >> It happens often if file folios backing a multi-threaded application >> are getting evicted and re-faulted. >> >> Fix it by skipping decreasing mmap_miss if the folio is locked. >> >> This change was evaluated on several hundred thousands hosts in Google's >> production over a couple of weeks. The number of containers being >> stuck in a vicious reclaim cycle for a long time was reduced several >> fold (~10-20x), as well as the overall fleet-wide cpu time spent in >> direct memory reclaim was meaningfully reduced. No regressions were >> observed. >> >> Signed-off-by: Roman Gushchin >> Cc: Matthew Wilcox (Oracle) >> Cc: Jan Kara >> Cc: linux-mm@kvack.org >> --- >> mm/filemap.c | 14 +++++++++++--- >> 1 file changed, 11 insertions(+), 3 deletions(-) >> >> diff --git a/mm/filemap.c b/mm/filemap.c >> index c21e98657e0b..983ba1019674 100644 >> --- a/mm/filemap.c >> +++ b/mm/filemap.c >> @@ -3324,9 +3324,17 @@ static struct file *do_async_mmap_readahead(struct vm_fault *vmf, >> if (vmf->vma->vm_flags & VM_RAND_READ || !ra->ra_pages) >> return fpin; >> >> - mmap_miss = READ_ONCE(ra->mmap_miss); >> - if (mmap_miss) >> - WRITE_ONCE(ra->mmap_miss, --mmap_miss); >> + /* >> + * If the folio is locked, we're likely racing against another fault. >> + * Don't touch the mmap_miss counter to avoid decreasing it multiple >> + * times for a single folio and break the balance with mmap_miss >> + * increase in do_sync_mmap_readahead(). >> + */ >> + if (likely(!folio_test_locked(folio))) { >> + mmap_miss = READ_ONCE(ra->mmap_miss); >> + if (mmap_miss) >> + WRITE_ONCE(ra->mmap_miss, --mmap_miss); >> + } > > I'm not an mm person. > > The comment implies the change fixes the race, but it is not at all > clear to me how. > > Does it merely make it significantly less likely? It's not fixing any race, it's fixing the imbalance in the upward and downward pressure on the mmap_miss variable. This improves the readahead behavior under very special circumstances: a multi-threaded application under very heavy memory pressure. There should be no visible difference in behavior in other cases. Thanks!