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 EFCD23F9A10 for ; Thu, 27 Aug 2026 15:56:20 +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=1787846182; cv=none; b=GX6/1YIuO/IoyMbsAT9BNiS8WIUr8mDOJy5m5lZ2vya3cBSItkMru/GGJF7QrTvNP045YVzFXR+xGLNysJhAl9o14jg9Bds+plDhBp+4Xd8UoWEq+lIAsOx9mW8CE4Ie+8K+IRJ+Eo2aucfcz9hJBQrPd+RnVEH6QQj86DFp5HQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1787846182; c=relaxed/simple; bh=ehVPW6rnCFBXvFyEn0Swl8fpWo4ZcUiImOIeXUr+HiE=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=YzS3S/98TaVJeWZu3glTJRZJjfWtGfVK8WeZtoq4Kim7hekzm3Zn9Uurv6p6/jldPLjVf16UdvggG6AvOsbx0AjoxIb8IzivtR5I+jSbKI0lIw2jupYLutWK9JgBwZnMmBPp8Ct4D5laYQnfFZ6ORdJj3ASEL3q73W2wFSvu7V0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=RnBLMIbo; 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="RnBLMIbo" Received: by smtp.kernel.org (Postfix) with ESMTPSA id E8C921F00A3A; Thu, 27 Aug 2026 15:56:15 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1787846180; bh=CFZM7kipCqrjfRTX1FI+8Lmju/ujloZNsIaYeTnQJuk=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=RnBLMIboNmK700droCbUYEK/N7su/5Jky1PqrEzotOBgEhcEDvs/aF6DyjsyZqUL8 GPmZCWEbSz+eL9ZubOaprfSPNPTu0UMBEIzCeizzPhq1NFgRP+FGe6lXVciYY0RdHJ dIweo1AO72T2rBqQEZhXFyw7MuhadOnzas4g4US/2Wl4Jmg94z1cup8pHAOrz3bmGr Py6ifCIyvB5+fc4Ea3FtX4+OA0RXwrJ3ytAV/L2q//D0NTfFK+qP0GojGxAM0Me3iS g2Xm2gRl75KQqAYhVgHB5KTjh25bOzS+J+imxOEVGjleGsHSp1T53nvcCwNc+VxkGo NBJAMZmPPAr2g== Message-ID: <81085d1e-90b1-429e-9521-a0c7176c6d0d@kernel.org> Date: Thu, 27 Aug 2026 17:56:14 +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 1/5] mm/huge_memory: do not touch frozen folios in deferred_split_isolate() To: Zi Yan Cc: Kiryl Shutsemau , akpm@linux-foundation.org, ljs@kernel.org, hannes@cmpxchg.org, usama.arif@linux.dev, lance.yang@linux.dev, 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, linux-mm@kvack.org, linux-kernel@vger.kernel.org, "Kiryl Shutsemau (Meta)" References: <20260826162101.1314941-1-kirill@shutemov.name> <20260826162101.1314941-2-kirill@shutemov.name> <50FDE99F-3E71-47B8-BD88-05AD771C4BDF@nvidia.com> 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 8/27/26 17:38, Zi Yan wrote: > On 27 Aug 2026, at 11:23, David Hildenbrand (Arm) wrote: > >> On 8/26/26 18:20, 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: >> >> Yes. >> >>> >>> 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(). >>> >>> Reported-by: Lance Yang >>> Link: https://lore.kernel.org/all/20260824131224.73344-1-lance.yang@linux.dev/ >> >> Fixes? >> >>> Assisted-by: Claude-Code:claude-opus-5 >>> Signed-off-by: Kiryl Shutsemau (Meta) >>> --- >>> mm/huge_memory.c | 19 ++++--------------- >>> 1 file changed, 4 insertions(+), 15 deletions(-) >>> >>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c >>> index ced400f72d43..6281ed993243 100644 >>> --- a/mm/huge_memory.c >>> +++ b/mm/huge_memory.c >>> @@ -4590,22 +4590,11 @@ static enum lru_status deferred_split_isolate(struct list_head *item, >>> struct folio *folio = container_of(item, struct folio, _deferred_list); >>> struct list_head *freeable = cb_arg; >>> >>> - if (folio_try_get(folio)) { >>> - list_lru_isolate_move(lru, item, freeable); >>> - return LRU_REMOVED; >>> - } >>> + /* Lost race to folio_put() or the folio is under folio_ref_freeze() */ >>> + if (!folio_try_get(folio)) >>> + return LRU_SKIP; >>> >>> - /* >>> - * We lost race with folio_put(). Read folio state before the >>> - * isolate: folio_unqueue_deferred_split() checks list_empty() >>> - * locklessly, so once removed the folio can be freed any time. >>> - */ >>> - if (folio_test_partially_mapped(folio)) { >>> - folio_clear_partially_mapped(folio); >>> - mod_mthp_stat(folio_order(folio), >>> - MTHP_STAT_NR_ANON_PARTIALLY_MAPPED, -1); >>> - } >>> - list_lru_isolate(lru, item); >>> + list_lru_isolate_move(lru, item, freeable); >>> return LRU_REMOVED; >>> } >>> >> >> Who will clean up the stats that we used to clean up? That should be mentioned >> in the commit log, otherwise it looks like some piece of the puzzle is missing. > > It is kinda implied in the last paragraph in the commit log (copied below), > but I agree that stating it explicitly is better. > > |> 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(). yes, I read that but was not convinced by it that all existing freezing code would handle it correctly. So spelling that out more clearly would be best. -- Cheers, David