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Tue, 24 Jun 2025 22:08:44 +0800 Message-ID: <6771bdca-b489-42f3-b2fe-5449879e8687@linux.alibaba.com> Date: Tue, 24 Jun 2025 22:08:43 +0800 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 1/2] mm: huge_memory: disallow hugepages if the system-wide THP sysfs settings are disabled From: Baolin Wang To: Dev Jain , akpm@linux-foundation.org, hughd@google.com, david@redhat.com Cc: ziy@nvidia.com, lorenzo.stoakes@oracle.com, Liam.Howlett@oracle.com, npache@redhat.com, ryan.roberts@arm.com, baohua@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <8912e179-601a-4677-b2f6-14f40d488d98@arm.com> In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: DA198C0017 X-Stat-Signature: wsstt3p6phwekcuxh1xh15x96qpn7mqq X-Rspam-User: X-HE-Tag: 1750774132-206792 X-HE-Meta: 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 GdvlJLlN ekmiJGQhefb/x+fixarvjGK4wgmgW3dNiQgLH1QyyHZF6uF+CoU3AwK02vDNBbSLI7TmOsBG63vuLKNJ80NHtkSXl1yGf9kdcJvlv65syQ/bh4j16SV1QUdTsR/DX34epzOdRojv5iFis6clmCl86+5TKQZMBGOeZ40bTQANtJFtIAe7SOIufMRxVbdxsdSBVimvnfDIsBPgNMQzyhkPS3jZU7z5DoIcJBq8IDCQcIriXFeSou6esH5mwmCIf/Qs61k33PEZ0cw1VgIzFrjxKDrp8jtgIIm41SzFD5VRWJv0Wdu+FZcnrepDAA/vXhaJwjeVBP417IzVU4is3x4WZDUJcFFzq/whYg02tMw0CIaFB48vXig53VLKQpQIO4GSeSpbekhBmo9i8x3g= 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 2025/6/24 17:57, Baolin Wang wrote: > > > On 2025/6/24 16:41, Dev Jain wrote: >> >> On 23/06/25 1:58 pm, Baolin Wang wrote: >>> When invoking thp_vma_allowable_orders(), the TVA_ENFORCE_SYSFS flag >>> is not >>> specified, we will ignore the THP sysfs settings. Whilst it makes >>> sense for the >>> callers who do not specify this flag, it creates a odd and surprising >>> situation >>> where a sysadmin specifying 'never' for all THP sizes still observing >>> THP pages >>> being allocated and used on the system. >>> >>> The motivating case for this is MADV_COLLAPSE. The MADV_COLLAPSE will >>> ignore >>> the system-wide Anon THP sysfs settings, which means that even though >>> we have >>> disabled the Anon THP configuration, MADV_COLLAPSE will still attempt >>> to collapse >>> into a Anon THP. This violates the rule we have agreed upon: never >>> means never. >>> >>> Currently, besides MADV_COLLAPSE not setting TVA_ENFORCE_SYSFS, there >>> is only >>> one other instance where TVA_ENFORCE_SYSFS is not set, which is in the >>> collapse_pte_mapped_thp() function, but I believe this is reasonable >>> from its >>> comments: >>> >>> " >>> /* >>>   * If we are here, we've succeeded in replacing all the native pages >>>   * in the page cache with a single hugepage. If a mm were to fault-in >>>   * this memory (mapped by a suitably aligned VMA), we'd get the >>> hugepage >>>   * and map it by a PMD, regardless of sysfs THP settings. As such, >>> let's >>>   * analogously elide sysfs THP settings here. >>>   */ >>> if (!thp_vma_allowable_order(vma, vma->vm_flags, 0, PMD_ORDER)) >> >> So the behaviour now is: First check whether THP settings converge to >> never. >> Then, if enforce_sysfs is not set, return immediately. So in this >> khugepaged >> code will it be better to call __thp_vma_allowable_orders()? If the sysfs >> settings are changed to never before hitting collapse_pte_mapped_thp(), >> then right now we will return SCAN_VMA_CHECK from here, whereas, the >> comment >> says "regardless of sysfs THP settings", which should include "regardless >> of whether the sysfs settings say never". > > Sounds reasonable to me. Thanks. > > I will change thp_vma_allowable_order() to __thp_vma_allowable_orders() > in the collapse_pte_mapped_thp() function to maintain consistency with > the original logic. > > Lorenzo and David, how do you think? Thanks. After thinking more, since collapse_pte_mapped_thp() is only used for file/shmem collapse, changing to __thp_vma_allowable_orders() has no effect. So I prefer to leave it as is.