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Tue, 12 May 2026 20:10:27 -0700 (PDT) Message-ID: <85de3792-596b-4efb-b2a9-89556e098f01@gmail.com> Date: Wed, 13 May 2026 11:10:19 +0800 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/memory: avoid unnecessary #PF on mTHP allocation race To: "David Hildenbrand (Arm)" , linux-mm@kvack.org, linux-kernel@vger.kernel.org Cc: akpm@linux-foundation.org, ljs@kernel.org, liam@infradead.org, vbabka@kernel.org, rppt@kernel.org, surenb@google.com, mhocko@suse.com References: <20260512095031.1333997-1-chenwandun@lixiang.com> <220524ba-e39a-4d7f-b44f-d4e2e7132397@kernel.org> Content-Language: en-US From: Wandun In-Reply-To: <220524ba-e39a-4d7f-b44f-d4e2e7132397@kernel.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: BD2A32000E X-Stat-Signature: c5ozzjgxpkbthi7bkpjsx56wffedgpgf X-HE-Tag: 1778641828-416248 X-HE-Meta: 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 4hO39ZQE 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 5/12/26 18:35, David Hildenbrand (Arm) wrote: > On 5/12/26 11:50, Wandun Chen wrote: >> When an mTHP folio is allocated in do_anonymous_page() and the target >> pte range is not fully empty, current code would release the folio >> and return. >> >> This results an illusion that a page fault has already been processed >> even if the fact is vmf->address itself is still pte_none(). Another >> page fault will be triggered again. > Yes. Why is that a problem? Honestly, the only data I have is the reproducer; I haven't been able to show a measurable impact on a real workload. The motivation was "we did the work of allocatingan mTHP folio and then throw it away just to redo it from #PF, and this #PF can be avoided by adding a small check + retry". But as you point out, the second fault path can handle this case properly, the behaviour is already correct.Please drop the patch. I'll come back with numbers if I run into a workload where this race is actually hot.Thanks for taking a look. Best regards, Wandun >