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Howlett" , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Shuah Khan , linux-btrfs@vger.kernel.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kselftest@vger.kernel.org Subject: Re: [PATCH v1 03/10] mm: fs: remove filemap_nr_thps*() functions and their users Date: Fri, 27 Mar 2026 11:05:19 -0400 X-Mailer: MailMate (2.0r6290) Message-ID: <25899F67-955D-44DC-935E-D7F234BD335A@nvidia.com> In-Reply-To: <53cf6157-58b1-4539-a276-2486e8796c57@lucifer.local> References: <20260327014255.2058916-1-ziy@nvidia.com> <20260327014255.2058916-4-ziy@nvidia.com> <53cf6157-58b1-4539-a276-2486e8796c57@lucifer.local> Content-Type: text/plain X-ClientProxiedBy: MN0P220CA0011.NAMP220.PROD.OUTLOOK.COM (2603:10b6:208:52e::23) To DS7PR12MB9473.namprd12.prod.outlook.com (2603:10b6:8:252::5) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: 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Do9DCkIk2WygHHPCs4AZjGonrN+k2n6SA34C6Aw/4nXoDv7oiCtmGVgHTR6zyMoQuC9grrg4dYKmj9nRXVma6Lq/HLXAt9Qk8GwXgXAOapYjBme6aHJnyvtbo1i9BQfWMJZjBLNWpaZ0XzBRGb9kYO+ejKf1qSJe32Dp5iN180IlXzoOuLCR+vvEGnbnXpBvqXicYfjwb+VFoRVpQb5C+pEhd2rTCYNyRw9cB4jpMTguDZJ10rvwXmukg8zDL9LZhGbxTPLM7nBTZbkPtybN2tjPM4NCpACrez1xX2FalllJ4HUPBN30DY4YjOeD9zDTi6xplQaoM5Xj8k+ZxSQUgwGPPJZnG7jiK2uWL0Z7JjTbzux36lVkCmP3NeBHvMpb4e7a/tXQnqLLqiNU= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 27 Mar 2026, at 10:23, Lorenzo Stoakes (Oracle) wrote: > On Fri, Mar 27, 2026 at 02:58:12PM +0100, David Hildenbrand (Arm) wrote: >> On 3/27/26 13:23, Lorenzo Stoakes (Oracle) wrote: >>> On Thu, Mar 26, 2026 at 09:42:48PM -0400, Zi Yan wrote: >>>> They are used by READ_ONLY_THP_FOR_FS to handle writes to FSes without >>>> large folio support, so that read-only THPs created in these FSes are not >>>> seen by the FSes when the underlying fd becomes writable. Now read-only PMD >>>> THPs only appear in a FS with large folio support and the supported orders >>>> include PMD_ORDRE. >>> >>> Typo: PMD_ORDRE -> PMD_ORDER >>> >>>> >>>> Signed-off-by: Zi Yan >>> >>> This looks obviously-correct since this stuff wouldn't have been invoked for >>> large folio file systems before + they already had to handle it separately, and >>> this function is only tied to CONFIG_READ_ONLY_THP_FOR_FS (+ a quick grep >>> suggests you didn't miss anything), so: >> >> There could now be a race between collapsing and the file getting opened >> r/w. >> >> Are we sure that all code can really deal with that? >> >> IOW, "they already had to handle it separately" -- is that true? >> khugepaged would have never collapse in writable files, so I wonder if >> all code paths are prepared for that. > > OK I guess I overlooked a part of this code... :) see below. > > This is fine and would be a no-op anyway > > - if (f->f_mode & FMODE_WRITE) { > - /* > - * Depends on full fence from get_write_access() to synchronize > - * against collapse_file() regarding i_writecount and nr_thps > - * updates. Ensures subsequent insertion of THPs into the page > - * cache will fail. > - */ > - if (filemap_nr_thps(inode->i_mapping)) { > > But this: > > - if (!is_shmem) { > - filemap_nr_thps_inc(mapping); > - /* > - * Paired with the fence in do_dentry_open() -> get_write_access() > - * to ensure i_writecount is up to date and the update to nr_thps > - * is visible. Ensures the page cache will be truncated if the > - * file is opened writable. > - */ > - smp_mb(); > > We can drop barrier > > - if (inode_is_open_for_write(mapping->host)) { > - result = SCAN_FAIL; > > But this is a functional change! > > Yup missed this. But I added + if (!is_shmem && inode_is_open_for_write(mapping->host)) + result = SCAN_FAIL; That keeps the original bail out, right? > > - filemap_nr_thps_dec(mapping); > - } > - } > > For below: > > - /* > - * Undo the updates of filemap_nr_thps_inc for non-SHMEM > - * file only. This undo is not needed unless failure is > - * due to SCAN_COPY_MC. > - */ > - if (!is_shmem && result == SCAN_COPY_MC) { > - filemap_nr_thps_dec(mapping); > - /* > - * Paired with the fence in do_dentry_open() -> get_write_access() > - * to ensure the update to nr_thps is visible. > - */ > - smp_mb(); > - } > > Here is probably fine to remove if barrier _only_ for nr_thps. > >> >> -- >> Cheers, >> >> David > > Sorry Zi, R-b tag withdrawn... :( I missed that 1 functional change there. > > Cheers, Lorenzo Best Regards, Yan, Zi