From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 549F14EBAFF for ; Mon, 7 Sep 2026 13:48:29 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788788916; cv=none; b=mKfwiqo9bOHGWVAKl577rLda/N7qW+JoJ0KQ+BLCUTsic7zzEizvMXnwg9rBXKLbhHm5hgPyDhz6xonIa1vGLhvLK+ELWqY0NEXbKGDggXEScxWqpLnEKRje8kwVGnwa+b/5wDqE3DrnnfOhZdvSEm6b7h9ADsv7GiZV/3k3Gv0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788788916; c=relaxed/simple; bh=jBAuPDYQ7P4hjMQO29h3hOO33d2+OXimgXMuTfGFDpc=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=uzh4PkrPpB6svr6566rXfWsYqu3oV4fJ9CU/ELfQ4b/oro5q7BLGam6uFrY8ZNy7viuEbGT/E950U1nkV+b7j54Y+jLZKVBZ0wuiyVgDr1yWG2sTvud0O9VULqN8oT0yOB3/R+/hOFlojLoJEaYGwHkKJhB8A1p2aM/esbN44IU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=CiavmS91; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="CiavmS91" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 293F61F00A3A; Mon, 7 Sep 2026 13:48:21 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1788788906; bh=j5Sb5qERZJvW54tBhlgjobR1+hnd79QPkfb2XhBE6Dc=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=CiavmS911DInJ74DVkXRLrW9SLVwTuAZ4qk6H0/Tn4FYAfwg0DXscwmHSHp3bftH0 QajgYDGFeWru4s99Hjwu0ziahN10DVli0avm8Ifs3F88Q2VeB1wiJTNnks1K5o20Pv WNqrIkNDf7yGZfhmDNZRCyGfz7ZZXuYRaYlCFPCVwKLseu4mwTtf87amLeNMmqZStJ aYV6DFu19BIZ8zXVZHADlw+TmibOeSzBHG1HqQSFaG7/ImLDHvmLnM+8cgce5RaZAA YAXLAMzhcLMrQlvNf3P+fb0THc8/CsXvL6IOEH5TQl6V9BONalTh6/xXxZE2hS7Va8 TrCflNjG2Emkg== Message-ID: Date: Mon, 7 Sep 2026 15:48:19 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 1/2] mm/huge_memory: do not touch frozen folios in deferred_split_isolate() To: Kiryl Shutsemau , akpm@linux-foundation.org, ljs@kernel.org, hannes@cmpxchg.org, usama.arif@linux.dev Cc: lance.yang@linux.dev, ziy@nvidia.com, kasong@tencent.com, hughd@google.com, baolin.wang@linux.alibaba.com, baohua@kernel.org, liam@infradead.org, nico.pache@linux.dev, dev.jain@arm.com, ryan.roberts@arm.com, balbirs@nvidia.com, kas@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20260831091514.1879786-1-kirill@shutemov.name> <20260831091514.1879786-2-kirill@shutemov.name> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 8/31/26 11:15, Kiryl Shutsemau wrote: > From: "Kiryl Shutsemau (Meta)" > > deferred_split_isolate() probes each queued folio with folio_try_get(). > folio_try_get() failure is treated as a lost race with folio_put(): clear > PG_partially_mapped, correct MTHP_STAT_NR_ANON_PARTIALLY_MAPPED, take > the folio off the queue. > > The folio_put() race is the most common case for !folio_try_get(), but > it is not the only option. Another scenario is folio_ref_freeze(). > > A zero refcount in such cases does not mean the folio is going away. It > means "don't touch me" and current deferred_split_isolate() doesn't > respect it. It can lead to unqueueing folios from the deferred list for > no reason: > > CPU 0 CPU 1 > --------------------------- ------------------------------ > freeze a mapped folio deferred_split_scan() > folio_ref_freeze() folio_try_get() fails > folio_clear_partially_mapped() > NR_ANON_PARTIALLY_MAPPED-- > folio off the queue > give up, put it back > folio_ref_unfreeze() > > The folio is still partially mapped, but it is no longer a split candidate. > Nothing queues it again until part of it is unmapped once more. > > Skip the folio instead: whoever freezes the folio, owns it and owner is > responsible for its fate. It also covers the folio_put() case: > __folio_put() unqueues the folio via folio_unqueue_deferred_split(). > > Nothing is lost by skipping. Everything that frees a queued folio > unqueues it first, and folio_unqueue_deferred_split() clears > PG_partially_mapped and brings MTHP_STAT_NR_ANON_PARTIALLY_MAPPED down > on the way: > > __folio_put(), folios_put_refs() mm/folio.c > __folio_migrate_mapping() mm/migrate.c > shrink_folio_list() mm/vmscan.c > > __folio_freeze_and_split_unmapped() does the same by hand, under the > list_lru lock it holds across the freeze. A freeze that ends in > folio_ref_unfreeze() leaves a folio that is still partially mapped and > still belongs on the queue. > > Fixes: 8422acdc97ed ("mm: introduce a pageflag for partially mapped folios") > Reported-by: Lance Yang > Closes: https://lore.kernel.org/all/20260824131224.73344-1-lance.yang@linux.dev/ > Assisted-by: Claude-Code:claude-opus-5 > Signed-off-by: Kiryl Shutsemau (Meta) > Reviewed-by: Zi Yan > Reviewed-by: Johannes Weiner > --- Acked-by: David Hildenbrand (Arm) -- Cheers, David