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s=arc-20220608; d=hostedemail.com; t=1762484962; a=rsa-sha256; cv=none; b=ej7CtK4um+CK0k2FXPoCC6DxS5de47djQdYjpk7XHW+Bh7Jc33ecZ8DV5rSG7NPrxKzl2t ighvnKdnFx5grh+rDCsnf79WG/IWHZnjN80tSgdp6nu0pZuu80yKF6RV2pNtGoXRSvvpvg g46eM5nmSKJGW+IeZFWdfYAQ7jSVrls= ARC-Authentication-Results: i=1; imf24.hostedemail.com; dkim=none; dmarc=pass (policy=none) header.from=arm.com; spf=pass (imf24.hostedemail.com: domain of dev.jain@arm.com designates 217.140.110.172 as permitted sender) smtp.mailfrom=dev.jain@arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id A435514BF; Thu, 6 Nov 2025 19:09:12 -0800 (PST) Received: from [10.164.136.34] (unknown [10.164.136.34]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 316A13F63F; Thu, 6 Nov 2025 19:09:05 -0800 (PST) Message-ID: <1c05c28a-a4f7-4562-8799-6f08a03bd3eb@arm.com> Date: Fri, 7 Nov 2025 08:39:03 +0530 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v12 mm-new 07/15] khugepaged: generalize collapse_huge_page for mTHP collapse To: Lorenzo Stoakes , Nico Pache Cc: linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org, linux-mm@kvack.org, linux-doc@vger.kernel.org, david@redhat.com, ziy@nvidia.com, baolin.wang@linux.alibaba.com, Liam.Howlett@oracle.com, ryan.roberts@arm.com, corbet@lwn.net, rostedt@goodmis.org, mhiramat@kernel.org, mathieu.desnoyers@efficios.com, akpm@linux-foundation.org, baohua@kernel.org, willy@infradead.org, peterx@redhat.com, wangkefeng.wang@huawei.com, usamaarif642@gmail.com, sunnanyong@huawei.com, vishal.moola@gmail.com, thomas.hellstrom@linux.intel.com, yang@os.amperecomputing.com, kas@kernel.org, aarcange@redhat.com, raquini@redhat.com, anshuman.khandual@arm.com, catalin.marinas@arm.com, tiwai@suse.de, will@kernel.org, dave.hansen@linux.intel.com, jack@suse.cz, cl@gentwo.org, jglisse@google.com, surenb@google.com, zokeefe@google.com, hannes@cmpxchg.org, rientjes@google.com, mhocko@suse.com, rdunlap@infradead.org, hughd@google.com, richard.weiyang@gmail.com, lance.yang@linux.dev, vbabka@suse.cz, rppt@kernel.org, jannh@google.com, pfalcato@suse.de References: <20251022183717.70829-1-npache@redhat.com> <20251022183717.70829-8-npache@redhat.com> <7b6b5c6b-0298-47c1-ae48-17d603401bac@lucifer.local> Content-Language: en-US From: Dev Jain In-Reply-To: <7b6b5c6b-0298-47c1-ae48-17d603401bac@lucifer.local> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Stat-Signature: mzofdp3jqgjpqftmi1tw4go1efqjuma3 X-Rspam-User: X-Rspamd-Queue-Id: 87DBB180015 X-Rspamd-Server: rspam10 X-HE-Tag: 1762484961-278151 X-HE-Meta: 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 ES5mWRdp BDBsT2etIVDc6rtJpksBmwW14XJy+WxdKnB/71Ha6S3T78apQN6JUXoe7RPiN+WNkZOf0Zo8JpdWZEOokKaDd64LhOaLEQAbiOmgL2/To6bGfBgtA7PUpDlfAPoFqidupuT0xrDkhzpXXIFfXmOGnZPjeTisdnEBJw4JxBqrk6c2XcBUQBue6TFzb9TL2R2DYaG23oDvjgHv7elmeB03M92VH/yt8BPVXP05wbei2iVJW9NF3x8aAyq+o0iGwsc7m/MoTUUdXLuKWHsg= 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: > ----------[snip]------------ > >> + >> + spin_lock(pmd_ptl); > We're duplicating this in both branches, why not do outside if/else? > >> + WARN_ON_ONCE(!pmd_none(*pmd)); > Hmm so the PMD entry will still always be empty on mTHP collapse? Surely we > could be collapsing more than one mTHP into an existing PTE table no? I may > be missing something here/confused :) After this code path isolates the PTE table, we don't want any other code path doing "Hey, I see an empty PMD, let's install a PTE table here". One of the reasons why all the heavy locking is required here. Also, I want to ask a question about WARN vs BUG_ON: suppose that the race I described above occurs. After khugepaged isolates the PTE table, someone faults in a PTE table there, and eventually writes data in the underlying folios. Then the buggy khugepaged nukes out that table and installs a new one, installing an mTHP folio which had old data. How do we decide whether such a condition is worthy of a BUG_ON (leading to system crash) vs letting this pass with WARN? > > ------------[snip]---------- >