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a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1766134044; bh=TGs5B+GMZUdb5OMX5/di7bdBSNlBgR8ieYIBhM34LpU=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=Sdb8bgc0r+gRok+GaEPKKkfkD2nx5RJeivKTW3N5XzX4VHnCwB48u98bWG720dTGs 2yrotDf6UVBVC6WxvH+agX8qXAU5d3bS+Ya6C+Tlr0C6oIen2mEgLfCBl4UvQAbNiZ QYbPWhc9MYKRZa4yAbrg488watiZq2oOyky2YEc9b40QnhfhIZDrbsy/7Axry6Sipk 4r71ztnv0Wf+Ytf+5khWuBDf5EnuDsqYGkzQriRsHFACvk25OwWJWYt9lJIlOyB+Oh IaxdwJaeiQm8/roE93h3BlAHkiQFBkBG87R47tDq3hDJgS65Ke+RnoyQBeVoFtUL6V pz33/e6xh2vbg== Message-ID: Date: Fri, 19 Dec 2025 09:47:17 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 2/2] mm: avoid unnecessary PTE table lock during initial swap folio scan To: Wei Yang , akpm@linux-foundation.org, lorenzo.stoakes@oracle.com, Liam.Howlett@oracle.com, vbabka@suse.cz, rppt@kernel.org, surenb@google.com, mhocko@suse.com Cc: linux-mm@kvack.org, hanchuanhua@oppo.com, v-songbaohua@oppo.com References: <20251216075943.29593-1-richard.weiyang@gmail.com> <20251216075943.29593-3-richard.weiyang@gmail.com> From: "David Hildenbrand (Red Hat)" Content-Language: en-US In-Reply-To: <20251216075943.29593-3-richard.weiyang@gmail.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 31F42180002 X-Rspamd-Server: rspam03 X-Stat-Signature: g9b8ypngyhh3u68xnadk6mfnagdxsb1z X-Rspam-User: X-HE-Tag: 1766134045-680774 X-HE-Meta: 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 scPqtvh2 RtV50yfOtkdm/v4maXPnIJDVAzeopPaiybyPUufNu58BfrxqScZOQzYoYqQKE2SKKoMb8OQs500bLyU9ZQU551/TnUTk0iwotdmxNkxen6Dj0WzcU3gzXvUCQbOUUS+4pAJO2V0zrzshM6BlNZ+FIDc0Pa0hDXLPP6xdz5fPp5RqnM/WQ/5fe7ffhMStmzq8O7nDs3UI/7v8kFubC5cfha+gmzjVPa4s38+7Wt8HeXEWLvo2t0QwSl1CMeGS8w1NHoK9CgwG4rX5MN+BvXZSj1nHGtQ8v5wZBABjnMPfjPDlvxlTOKUx3lXklYSpvgzaLJ1ay+fBuMBQ9X7zjAb76H1ZZbEvsjjJoaTHJqlDD0gXbHGRgzCBAn2ZtJw== 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: On 12/16/25 08:59, Wei Yang wrote: > The alloc_swap_folio() function performs an initial scan of the PTE > table solely to determine the potential size (order) of the folio > content that needs to be swapped in. > > Locking the PTE table during this initial read is unnecessary for two > reasons: > > * We are not writing to the PTE table at this stage. > > * The code will re-check and lock the table again immediately before > any actual modification is attempted. > > This commit refactors the initial scan to map the PTE table without > acquiring the lock. This reduces contention and overhead, improving > performance of the swap-in path. > > Signed-off-by: Wei Yang > Cc: Chuanhua Han > Cc: Barry Song > --- > mm/memory.c | 6 ++---- > 1 file changed, 2 insertions(+), 4 deletions(-) > > diff --git a/mm/memory.c b/mm/memory.c > index 1b8ef4f0ea60..f8d6adfa83d7 100644 > --- a/mm/memory.c > +++ b/mm/memory.c > @@ -4529,7 +4529,6 @@ static struct folio *alloc_swap_folio(struct vm_fault *vmf) > struct folio *folio; > unsigned long addr; > softleaf_t entry; > - spinlock_t *ptl; > pte_t *pte; > gfp_t gfp; > int order; > @@ -4563,8 +4562,7 @@ static struct folio *alloc_swap_folio(struct vm_fault *vmf) > if (!orders) > goto fallback; > > - pte = pte_offset_map_lock(vmf->vma->vm_mm, vmf->pmd, > - vmf->address & PMD_MASK, &ptl); > + pte = pte_offset_map(vmf->pmd, vmf->address & PMD_MASK); > if (unlikely(!pte)) Can can_swapin_thp() deal with concurrent unmap and possible freeing of pages+swap? We have some code that depends on swap entries stabilizing the swap device etc; the moment you allow for that concurrently to go away you open a can of worns. -- Cheers David