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Howlett" , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, stable@vger.kernel.org, Alexander Viro , Christian Brauner , Jan Kara , Matthew Wilcox , Song Liu , Eric Hagberg , Gregg Leventhal Subject: Re: [PATCH stable] mm/khugepaged: write all dirty file folios when collapsing Date: Thu, 02 Jul 2026 13:24:39 -0400 X-Mailer: MailMate (2.0r6290) Message-ID: <2DA84662-F9E4-4ED3-A225-71054FEC3849@nvidia.com> In-Reply-To: <20260702165409.164568-1-pfalcato@suse.de> References: <20260702165409.164568-1-pfalcato@suse.de> Content-Type: text/plain Content-Transfer-Encoding: quoted-printable X-ClientProxiedBy: MN2PR03CA0008.namprd03.prod.outlook.com (2603:10b6:208:23a::13) To IA0PR12MB8374.namprd12.prod.outlook.com (2603:10b6:208:40e::7) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: IA0PR12MB8374:EE_|PH0PR12MB7907:EE_ X-MS-Office365-Filtering-Correlation-Id: cc52229b-75a4-45f8-4426-08ded85eccac X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|23010399003|366016|1800799024|7416014|376014|11063799006|5023799004|56012099006|18002099003|22082099003; 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X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: =?us-ascii?Q?gSxfSJ9vs6hmvqHKts23gXODNfXSxL0XuWGSPlDl5nwYoSUZz/d4XNb/j0Yy?= =?us-ascii?Q?YFOnPltpgnl2wBJE/Z3XDlyCkG06oSICYD1xkiRq9mPd+rstdLlpNGlsIlpN?= =?us-ascii?Q?SSLmrlACfXBsIHPlkeZpTjZiwiJ1Y9sDc5H4FwYV2Ir8ernxtWw3JR4JFRne?= =?us-ascii?Q?JWisJpg71AvQigtbs6BCblBxER8UBP7Z8xdwv2dj2LNrBG6nTmd0Hh2ApsUr?= =?us-ascii?Q?Mi30xTPhF9EWgvYN+iuM8RjRni1QVkXCESXF22P+CxbH5NUFA2KSqiRKj0HD?= =?us-ascii?Q?skZ2ecSMbGJOY/vnDfoKTp511FP6BrR1jeEisSSuePGsXKCzbS6u7hqxdlM0?= =?us-ascii?Q?2vp05+ln2mosHf8frB4dZUS/sPwHNMJ08Gfs3g5eA1wYXeSgB6+vfKJXVERD?= =?us-ascii?Q?NpaL0M3djM8DUAzhZs6RY5UY/Rsxz19wVvCs0ryoR3VQm6IMcBjh5Q6Na8WN?= =?us-ascii?Q?uiSlGIebr1w0DQCV/UnH9jEnnCoaJSYuBGzJnp+FtRiEjy6Hv8sGf12nSQi6?= =?us-ascii?Q?X4NJcxUYYzFhCWWilqxiYsAbFYnnIwYz/DaA/U3cMubmmbO8wWbP3lzKgaYj?= =?us-ascii?Q?mttrBHZ8DCGFeVFYMxXzO7GofigQp8jHvqO5alYRkw8hLQ7oax1o8jQtY6FW?= =?us-ascii?Q?bxBVWEaf6KCDJv1RgPXGQYPCxt+PLVgMgWSdXY7OBnDX6an43HH43In54ldo?= =?us-ascii?Q?mAJ6xMPF+Ks59tzNgQsEPxSULpS2x2GrihrtpJsX9iVwEBuU/Gz+LMKwRFEA?= =?us-ascii?Q?+gFGPXWEawX1/QKNhN2P53Q31WedRtPDAeEcfNdRk+vVe8QYevM4hpWV/y+t?= =?us-ascii?Q?pyijMAYBMw4jdSsr4zm8ADLx2YFc1ZiWqKIwYYogIzSChF9cHWW///wKnEn+?= =?us-ascii?Q?qVLcZeprPFdmvPiu4ebx5JsVDdjWHle3i7PEYDlaF0dltvaozPAxYJ+3NGQB?= =?us-ascii?Q?RBAXOuG6p03yLRpmHaUi8RrFw4Vru5+ILDqmctXZM8JqTjB9LEAnWvHhACH7?= =?us-ascii?Q?TvgLEBVhBDfbEpGZM2t3033tXakocrBUC/Qo8Tgg2B8jA/rKJ7GqDd7gF+br?= =?us-ascii?Q?PrLmP81XVAZ/+r/2WONW02PBxlSQ8VJhb0R11YsKstqx+hijfi05Aq11x+Xn?= =?us-ascii?Q?/I3Me7SkDszC2M8CYY7G0NhbfTkK+pW7SdxerN4bXBNb4IAMRB59PkEvHHdA?= =?us-ascii?Q?p4rFTy1GGLzEkZZoXF5+3XpRVuMJp+xplfC9q0S/oWHtdBr9z95l310uGmH8?= =?us-ascii?Q?+65KmE5kCTF4qNFW5YPyUh1QKuM+ITzAJh+SZGqY6h+LDHFPkpjSs9GvUvWx?= =?us-ascii?Q?WKZedaV8xYu4C1Vt9opT8tBvK3U5tM+ZcWdwWCsQQmYfMOwXwNdjvLqqXhkV?= =?us-ascii?Q?Lcw2mkGdxqbaWOCKQG5L4Jr0cNhprNuD5rUOMmP44QRfYVstWQqB9YGHW6Vu?= =?us-ascii?Q?oQMmqTet01B6M5kIG2/b8BhgmaNdKufUFYlYRjAKlv5uEFFZMcKB5I//geTq?= =?us-ascii?Q?Yr9SzZiuRsrRVHuH8Vfgo1RonTK38sLZ+J+4LKMjl/dwRU3GLMJeG+MPtDX4?= =?us-ascii?Q?pfT4+GnZ/Bktqle2RsVFCsfE+OnStFohS+mregSbqmTK7CK9zUrWKaHEQC/O?= =?us-ascii?Q?Sf2L2jzV44T6DRvWQT9FE6eBWL+J3ZvKxteTekdCdX9PqC1z1hTwi0PiXSQQ?= =?us-ascii?Q?SKZ6zDXl/fZ1Wy5NkYYI1eKOz3vOoSQCQS2x4CpiWi7IlgVZTG5vLPpSeuo0?= =?us-ascii?Q?wl/+JNwDbw=3D=3D?= X-OriginatorOrg: Nvidia.com X-MS-Exchange-CrossTenant-Network-Message-Id: cc52229b-75a4-45f8-4426-08ded85eccac X-MS-Exchange-CrossTenant-AuthSource: IA0PR12MB8374.namprd12.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 02 Jul 2026 17:24:40.7496 (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: rZNeGSHEVW7PBwlF60fhz6cfEBxQ+BusNh4qBjOrzhCVyC2Jf0G+3JicpZpTgEHP X-MS-Exchange-Transport-CrossTenantHeadersStamped: PH0PR12MB7907 On 2 Jul 2026, at 12:54, Pedro Falcato wrote: > As-is, khugepaged and writable-file opening exclude each other. A file > cannot be open writeable and have THPs (because the filesystem is not a= ware > of them). khugepaged will never collapse file pages for files that are > opened writeable. On an open(O_RDWR/O_WRONLY), the page cache for that > particular file is dropped. This is fine because nothing could've been > dirtied. > > However, there is an edge-case: collapse_file() might not be able to > coexist with concurrent writers, but it can coexist with dirty folios > (from previous writers). Therefore, the following can happen: > > open(file, O_RDWR) > write(file) > close(file) > madvise(file_mapping, MADV_COLLAPSE, some non-dirty range) > open(file, O_RDWR) > nr_thps > 0 > truncate_inode_pages() > /* THPs are cleared out, but so are the dirty folios */ > > When this edge-case happens, there is data loss, as the dirty folios ar= e > fully discarded. > > Fix it by fully writing back the page cache (and waiting) when collapsi= ng > file THPs. Doing so provides the guarantee that no dirty folio will be > observed while there are active THPs. To fully ensure this is safe, the= > invalidate_lock needs to be held while doing the writeout, so that > do_dentry_open()'s page cache truncation excludes this write-and-wait. > > Cc: stable@vger.kernel.org > Cc: Alexander Viro > Cc: Christian Brauner > Cc: Jan Kara > Cc: Matthew Wilcox > Cc: Song Liu > Cc: Eric Hagberg > Cc: Zi Yan > Fixes: 99cb0dbd47a1 ("mm,thp: add read-only THP support for (non-shmem)= FS") > Reported-by: Gregg Leventhal > Closes: https://lore.kernel.org/linux-mm/CAFN_u7H_0ECF3jixP=3DT=3DU7AH5= =3DQ3wQNvJMo8an3VqUDMerQfUw@mail.gmail.com/ > Tested-by: Zi Yan > Signed-off-by: Pedro Falcato > --- > This patch is written against 7.1.0 (because the code no longer exists = in mainline). > > Zi, I kept your Tested-by, but I had to move some things around and > use the invalidate lock. Please re-test if you can. Tested it again on top of v6.12 (the patch applied cleanly) and the issue= is gone. My Tested-by still holds. :) > > mm/khugepaged.c | 39 +++++++++++++++++++++++++-------------- > 1 file changed, 25 insertions(+), 14 deletions(-) > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > index b8452dbdb043..0707d719a270 100644 > --- a/mm/khugepaged.c > +++ b/mm/khugepaged.c > @@ -2094,32 +2094,43 @@ static enum scan_result collapse_file(struct mm= _struct *mm, unsigned long addr, > goto xa_unlocked; > } > > - if (!is_shmem) { > +xa_locked: > + xas_unlock_irq(&xas); > +xa_unlocked: > + > + /* > + * If collapse is successful, flush must be done now before copying. > + * If collapse is unsuccessful, does flush actually need to be done? > + * Do it anyway, to clear the state. > + */ > + try_to_unmap_flush(); > + > + if (result =3D=3D SCAN_SUCCEED && !is_shmem) { > + /* > + * invalidate_lock as shared excludes against concurrent opens > + * in do_dentry_open() truncating the page cache. This is > + * particularly important if there are dirty folios in transit. > + */ > + filemap_invalidate_lock_shared(mapping); > 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. > + * file is opened writable. If collapse looks to be successful, > + * flush any dirty pages out the page cache. With the nr_thps > + * incremented, there won't be any new writers (nor new dirties). > */ > smp_mb(); > - if (inode_is_open_for_write(mapping->host)) { > + if (inode_is_open_for_write(mapping->host) || filemap_write_and_wait= (mapping)) { > result =3D SCAN_FAIL; > filemap_nr_thps_dec(mapping); > + filemap_invalidate_unlock_shared(mapping); > + goto rollback; > } > + filemap_invalidate_unlock_shared(mapping); > } > > -xa_locked: > - xas_unlock_irq(&xas); > -xa_unlocked: > - > - /* > - * If collapse is successful, flush must be done now before copying. > - * If collapse is unsuccessful, does flush actually need to be done? > - * Do it anyway, to clear the state. > - */ > - try_to_unmap_flush(); > - > if (result =3D=3D SCAN_SUCCEED && nr_none && > !shmem_charge(mapping->host, nr_none)) > result =3D SCAN_FAIL; > -- = > 2.54.0 Best Regards, Yan, Zi