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 4F3B0399011; Wed, 16 Sep 2026 06:25:06 +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=1789539907; cv=none; b=manxoMo5SvlEshQTfq2JiiEeiwIx9A2AWfCncQFmqciVmuqoSuyKXxco3BsUJjrDHr7SQERBoEHSaDIE3iiZ+jBlcv1LsQtFX15MeQLeDnh4SYge35jK7TsmN0R2L9xEdSFZJgj6r8bdV1PxvDo2iy/F9shw5ed+wVGZBZ5Do/s= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789539907; c=relaxed/simple; bh=6e0c/U8DotYKPyZ1BxuhomQMELSOBuXE4EzfatbRGHE=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=H9GhJxKdc8Se8c/0dzZ3Vd2VjjbcKpVgmeuNqxvqQV6N2/mCR2I1Il3KMSokdZ/3e+zCtCXKZC86xxCZkiP1K8L3gigp5UJc4kyYzxg98sAQC8lD/9qcu+fSpaAcekWRJm1Yb7ttHXAyzxEVmNclIFJWRD0NNmS11D+taLE0QLU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=PWlWiHbo; 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="PWlWiHbo" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 1279B1F000FF; Wed, 16 Sep 2026 06:25:03 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1789539906; bh=WSAK4OewrbYk1s8dXTIrNF8g5vHL0nZd3Hyf6R+tOGI=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=PWlWiHbozNTNPVYdWeMwfblXPsRUHvZ2LxCu4OsQAnqDtZHViQh9tAFFL5FLlmvFd A65abK9Fy838XvqboO6LTsjyNPDDws1PJJtUQkHbvhFxwi/oaEqBBz+0TMc5mPGQyi jlTCNEhOtIZJAyU25bUL0ceA9oQCZFT7aS0PHJj+txmfMM2IOChCjT8Qx9GFnjR+FF fuuyznnPmKDNkXP0SxhiwW39J7+FtF2xD3g/x9SH57za1goAA0kR9kQJ/CAIS/Mgt0 iDoMcXS33MJIclWt1XP6vWMN/GEF9wtzTeEgsmGOK4JIYT5y2PRxAJ6eO+22Xaof72 j7CHcFhNy6w+Q== Message-ID: <2f63b25e-71ca-4af6-83d2-70be2388a55a@kernel.org> Date: Wed, 16 Sep 2026 08:25:01 +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: [RESEND PATCH] mm/madvise: use folio_trylock() in the cold/pageout PMD split To: Gregory Price , linux-mm@kvack.org Cc: linux-kernel@vger.kernel.org, kernel-team@meta.com, akpm@linux-foundation.org, liam@infradead.org, ljs@kernel.org, vbabka@kernel.org, jannh@google.com, sashiko-bot , stable@vger.kernel.org References: <20260912110540.3203010-1-gourry@gourry.net> 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 9/12/26 13:05, Gregory Price wrote: > MADV_COLD or MADV_PAGEOUT over part of a PMD splits the THP in > madvise_cold_or_pageout_pte_range(). Two threads doing that to > the same THP create spurious failures. > > CPU0 CPU1 > ---- ---- > folio_get() > spin_unlock(ptl) > folio_lock() > folio_get() > spin_unlock(ptl) > folio_lock() <- blocks, keeps its ref > split_folio() > folio_expected_ref_count(folio) != folio_ref_count(folio) - 1 > -EAGAIN > > CPU1 cannot drop its reference until it gets the lock CPU0 holds, so CPU0's > split always fails. folio_trylock() makes CPU1 leave without ever taking a > reference. The PTE branch of this same function already does this, as do > madvise_free_pte_range() and madvise_free_huge_pmd(). > > Reproducer: 400 rounds of eight threads calling MADV_COLD on half of each > of eight THPs, re-formed with MADV_COLLAPSE between rounds. From > /proc/vmstat: > > thp_split_page thp_split_page_failed > before 3186 860 > after 3200 0 > > The short before count is rounds where every thread failed and the > advice was dropped for that THP entirely. While the split now succeeds, one of both calls will just effectively skip processing the page table. SO while CPU0 will succeed with the split, CPU1 would just skip the page table. And what happened before? Split on CPU0 failed and it would skip the page table. Split on CPU1, however, would likely have succeeded? So this is all far from perfect. ... but this matches what we do further down in the PTE scenario. Acked-by: David Hildenbrand (Arm) -- Cheers, David