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X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 25 Jun 2025 06:50:06.8252 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 81b2cc42-8053-420a-2d52-08ddb3b48559 X-MS-Exchange-CrossTenant-Id: f34e5979-57d9-4aaa-ad4d-b122a662184d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=f34e5979-57d9-4aaa-ad4d-b122a662184d;Ip=[4.158.2.129];Helo=[outbound-uk1.az.dlp.m.darktrace.com] X-MS-Exchange-CrossTenant-AuthSource: AM4PEPF00027A67.eurprd04.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: AM8PR08MB6385 X-Rspamd-Server: rspam11 X-Rspam-User: X-Rspamd-Queue-Id: DA1921A0005 X-Stat-Signature: fdufq8ph9r7wmum7dw97hsiy11kz8a7b X-HE-Tag: 1750834210-570482 X-HE-Meta: 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 oJuCOEHw 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 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 25/06/25 11:56 am, Baolin Wang wrote: > > > On 2025/6/25 13:53, Hugh Dickins wrote: >> On Wed, 25 Jun 2025, Baolin Wang wrote: >> >>> When invoking thp_vma_allowable_orders(), if 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. And the MADV_COLLAPSE is an >>> example of >>> such a case, that means it will not set TVA_ENFORCE_SYSFS when calling >>> thp_vma_allowable_orders(). >>> >>> As we discussed in the previous thread [1], the MADV_COLLAPSE will >>> ignore >>> the system-wide anon/shmem THP sysfs settings, which means that even >>> though >>> we have disabled the anon/shmem THP configuration, MADV_COLLAPSE >>> will still >>> attempt to collapse into a anon/shmem THP. This violates the rule we >>> have >>> agreed upon: never means never. >>> >>> For example, system administrators who disabled THP everywhere must >>> indeed very >>> much not want THP to be used for whatever reason - having individual >>> programs >>> being able to quietly override this is very surprising and likely to >>> cause headaches >>> for those who desire this not to happen on their systems. >>> >>> This patch set will address the MADV_COLLAPSE issue. >>> >>> Test >>> ==== >>> 1. Tested the mm selftests and found no regressions. >>> 2. With toggling different Anon mTHP settings, the allocation and >>> madvise collapse for >>> anonymous pages work well. >>> 3. With toggling different shmem mTHP settings, the allocation and >>> madvise collapse for >>> shmem work well. >>> 4. Tested the large order allocation for tmpfs, and works as expected. >>> >>> [1] >>> https://lore.kernel.org/all/1f00fdc3-a3a3-464b-8565-4c1b23d34f8d@linux.alibaba.com/ >>> >>> Changes from v3: >>>   - Collect reviewed tags. Thanks. >>>   - Update the commit message, per David. >>> >>> Changes from v2: >>>   - Update the commit message and cover letter, per Lorenzo. Thanks. >>>   - Simplify the logic in thp_vma_allowable_orders(), per Lorenzo >>> and David. Thanks. >>> >>> Changes from v1: >>>   - Update the commit message, per Zi. >>>   - Add Zi's reviewed tag. Thanks. >>>   - Update the shmem logic. >>> >>> Baolin Wang (2): >>>    mm: huge_memory: disallow hugepages if the system-wide THP sysfs >>>      settings are disabled >>>    mm: shmem: disallow hugepages if the system-wide shmem THP sysfs >>>      settings are disabled >>> >>>   include/linux/huge_mm.h                 | 51 >>> ++++++++++++++++++------- >>>   mm/shmem.c                              |  6 +-- >>>   tools/testing/selftests/mm/khugepaged.c |  8 +--- >>>   3 files changed, 43 insertions(+), 22 deletions(-) >>> >>> -- >>> 2.43.5 >> >> Sorry for chiming in so late, after so much effort: but I beg you, >> please drop these. > > Thanks Hugh for your input. (yes, we put in a lot of effort on > discussion and testing:( ). > >> I did not want to get into a fight, and had been hoping a voice of >> reason would come from others, before I got around to responding. >> >> And indeed Ryan understood correctly at the start; and he, Usama >> and Barry, perhaps others I've missed, have raised appropriate >> concerns but not prevailed. >> >> If we're sloganeering, I much prefer "never break userspace" to >> "never means never", attractive though that over-simplification is. > > Yes, agree. we should not break userspace, however, I suspect whether > this can really break userspace. We can set > '/sys/kernel/mm/transparent_hugepage/enabled' to 'madvise' to allow > MADV_COLLAPSE. Additionally, I really doubt that when the system-wide > THP settings are set to 'never', userspace would still expect to > collapse into THP using MADV_COLLAPSE. After this patch, will a user still be able to use MADV_COLLAPSE and ensure no interference from khugepaged? > > Moreover, what makes this issue particularly frustrating is that when > we introduce mTHP collapse[1], MADV_COLLAPSE complicates matters > further. That is, when the system only enables 64K mTHP, MADV_COLLAPSE > still allows collapsing into PMD-sized THP. This really breaks the > user's settings. This issue will still be there without this patch right? > > [1] > https://lore.kernel.org/all/20250515032226.128900-1-npache@redhat.com/ > >> Seldom has a feature been so thorougly documented as MADV_COLLAPSE, >> in its 6.1 commits and in the "man 2 madvise" page: which are >> explicit about MADV_COLLAPSE providing a way to get THPs where the >> sysfs setting governing automatic behaviour does not insert them. >> >> We would all prefer a less messy world of THP tunables.  I certainly >> find plenty to dislike there too; and wish that a less assertive name >> than "never" had been chosen originally for the default off position. >> >> But please don't break the accepted and documented behaviour of >> MADV_COLLAPSE now. >> >> If you want to exclude all possibility of THPs, then please use the >> prctl(PR_SET_THP_DISABLE); or shmem_enabled=deny (I think it was me >> who insisted that be respected by MADV_COLLAPSE back then). > > Yes, that will prevent MADV_COLLAPSE. > >> Add a "deny" option to /sys/kernel/mm/transparent_hugepage/enabled >> if you like.  (But in these days of filesystem large folios, adding >> new protections against them seems a few years late.) >> >> If Andrew decides that these patches should go in, then I'll have to >> scrutinize them more carefully than I've done so far: but currently >> I'm hoping to avoid that. >> >> Hugh >