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X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: =?us-ascii?Q?lFv9O6iXyZ+Aq8RR5z7y1tSgo5OpudlTMUrHR5srdVagb3PsZF1J+My9o77T?= =?us-ascii?Q?HSimyCVJF26/jbQB3GgEd19E3zynpNvH+OHpK6p/3vfpHyYmzIQO+LMWZNBi?= =?us-ascii?Q?HIhDFctdLVIib/89P8L2dOkC4NgDzcbXlYRa/eoGTynClsjL4JVnkYg4rHeY?= =?us-ascii?Q?k2/GGa93MLVcEexN0YRAltpQQuEPEtIJmJV9ywPz89JJ/EHvmIqAVWDDKOEi?= =?us-ascii?Q?pE9Z3kqhqTf3FrT6YGqXcPnKk8AFzokUK3iunzsCYy74pRsLckfl8cyd396P?= =?us-ascii?Q?eDai/MGsmUM23V04vfNz0h2GsFvg0jYtAN6F4j39CPf6y0nUuDyIzUlp9khs?= =?us-ascii?Q?kaDQ/mWgWRWRJ4es7sXFzoF4wGuUoTs6XFFYD71p6HENV/w7Uv4zxZG+MWzr?= =?us-ascii?Q?6rmyZ7sHm9O8yiUuKAM1UUyZz9Qifz8ErEsx0JOfC0nwk7g1/2imGHJUMVZI?= =?us-ascii?Q?DO9kxVdXN8JUi3UD1fUuvfJs4IdUISoUkowWdncZVG27mLp+ehabBWVuef8s?= =?us-ascii?Q?LOQHOv8/3lIz5ziEssw7IbpZiPXMtKbJ0DF2n6unKIv0TogVFdrVRUXew7IC?= =?us-ascii?Q?dHnG8XFui2og24XjARMxa/aIn6rlRIBHDJsBUA8JQOagyR7fMrbQDDa63JnI?= =?us-ascii?Q?v3urJUzchO5Cm9E99RcSzohVVmNLwGNEFthE06KsJOcuG7vO4Vm1hHinKgOG?= =?us-ascii?Q?Auw9z4TPYAypO+wY3tczWQkLWGWim2+bkEC8chSvw/0fojYqvpLWAwvWt8eO?= =?us-ascii?Q?YbER50ASYgl6OKhuUUtoGlUAtEUUGqBIOBOnMdjkOpNnLUv0Z33hCWHmjFqE?= =?us-ascii?Q?rIxJt3R1viMNfhN+ihYozXib/WDR4WGxTVNa3t0dJ7B0johjHhqxLgMKd1y/?= =?us-ascii?Q?vF29LWSepu1WYm/enZVBbXfWyzLT+6tSyQ3RmsE7+w1+iOq53EOlYZQKxf4V?= =?us-ascii?Q?Txio3G0rtK/0Tdwe+mGje7tC8p0jWD2iFiCTzze7tSkE3KmPdFOgZOQKCU2H?= =?us-ascii?Q?ueXCt8ggQOScs8HLZr0Gp+FbRkcLNhV18+e2vxAglVEL8ZNt1Fy2vygRaphZ?= =?us-ascii?Q?9Rn5AZ9U7tZYfmAleqbAfcxfAd4TiMviHf0/xZuwYECVIuinuax2VS1RpCty?= =?us-ascii?Q?fuCcviBHvBQPpKaWxafM6Gz5To5bfSQf4hif/f18GKlG1M4nn7BYkIUNOyGQ?= =?us-ascii?Q?wjBZ73TPSHNoNSj+/x8Q3SM2pFEg4xfYFrQ37XCd7e+VsAWRuM4XKRq0KvXz?= =?us-ascii?Q?hSbZ5gGgh7jhMvvGBLXFogbHE1qPkUwLx/vcmPEdxntOyoBvB2vhtRsgYJpQ?= =?us-ascii?Q?nSguUqYOCPjvrzMz8DjBsAsuebsyz7w5Q9D+Q0zyU8d6GZYU3Pz1K2RWEnoJ?= =?us-ascii?Q?BufSq+z57OhPlKXgKJefP2ImxVoICwWMkLU1U264w8q5UeHO4MkJ832Oz7J3?= =?us-ascii?Q?RNX/tjyT63pkAeoTN063RcX0S8zh9teRUf8I6wiV2Rrg8JfB3jJiLuH3ERsS?= =?us-ascii?Q?XwYpVqKUfUVJpkZrixz4HE2sDSDTPylGYHUB9y6bbLrxKC04QmEe2SZEBHKT?= =?us-ascii?Q?FbUgIIMNWGXZV2CywMrSFmimd08CJr8PmNXYSkEm/P9Wqt2MgsjGjozeY5Lk?= =?us-ascii?Q?jBKQeGsPb/bJQdkwrofZnPvMlJ4NmdOL5YYS94nJ++uqYKE5bOSQT5CVUj8I?= =?us-ascii?Q?X6PXOmGq8VTZb66egIPaCKgeuEhQHseltEJC1jKycntCEJowaIFFPpZxryGt?= =?us-ascii?Q?52HKPW84sg=3D=3D?= X-OriginatorOrg: Nvidia.com X-MS-Exchange-CrossTenant-Network-Message-Id: 6952ca18-a873-4abb-8c9c-08de8c1244a0 X-MS-Exchange-CrossTenant-AuthSource: DS7PR12MB9473.namprd12.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 27 Mar 2026 15:05:22.3786 (UTC) X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-CrossTenant-Id: 43083d15-7273-40c1-b7db-39efd9ccc17a X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: Ak/8TxoZzlOCxt2vKyis6qFhn/05zLYGVqT7Zzj5+09VQ1gVwhZvl9czDqgkM1cC X-MS-Exchange-Transport-CrossTenantHeadersStamped: MW4PR12MB7312 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