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Howlett" , Nico Pache , Ryan Roberts , Dev Jain , Jonathan Corbet , linux-mm@kvack.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, Hugh Dickins Subject: Re: [PATCH] docs: update THP documentation to clarify sysfs "never" setting Message-ID: References: <20250721155530.75944-1-lorenzo.stoakes@oracle.com> <35df32ae-dc95-48e1-bdb1-90f17bfd4d5c@linux.alibaba.com> Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-ClientProxiedBy: LO4P123CA0144.GBRP123.PROD.OUTLOOK.COM (2603:10a6:600:193::23) To DM4PR10MB8218.namprd10.prod.outlook.com (2603:10b6:8:1cc::16) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DM4PR10MB8218:EE_|SJ0PR10MB4765:EE_ X-MS-Office365-Filtering-Correlation-Id: 393abd38-9a81-4a5d-a038-08ddc8e0b42d X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|366016|376014|7416014|1800799024; X-Microsoft-Antispam-Message-Info: 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List-Unsubscribe: +cc Hugh since we're mentioning him here, and not-trimming for context - TL;DR I am updating the docs to reflect the sysfs never 'doesn't mean never' behaviour for THP. On Tue, Jul 22, 2025 at 11:37:07AM +0800, Barry Song wrote: > On Tue, Jul 22, 2025 at 10:33 AM Baolin Wang > wrote: > > > > > > > > On 2025/7/22 10:23, Barry Song wrote: > > > On Tue, Jul 22, 2025 at 9:30 AM Baolin Wang > > > wrote: > > >> > > >> > > >> > > >> On 2025/7/21 23:55, Lorenzo Stoakes wrote: > > >>> Rather confusingly, setting all Transparent Huge Page sysfs settings to > > >>> "never" does not in fact result in THP being globally disabled. > > >>> > > >>> Rather, it results in khugepaged being disabled, but one can still obtain > > >>> THP pages using madvise(..., MADV_COLLAPSE). > > >>> > > >>> This is something that has remained poorly documented for some time, and it > > >>> is likely the received wisdom of most users of THP that never does, in > > >>> fact, mean never. > > >>> > > >>> It is therefore important to highlight, very clearly, that this is not the > > >>> ase. > > >>> > > >>> Signed-off-by: Lorenzo Stoakes > > >>> --- > > >>> Documentation/admin-guide/mm/transhuge.rst | 11 +++++++++-- > > >>> 1 file changed, 9 insertions(+), 2 deletions(-) > > >>> > > >>> diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst > > >>> index dff8d5985f0f..182519197ef7 100644 > > >>> --- a/Documentation/admin-guide/mm/transhuge.rst > > >>> +++ b/Documentation/admin-guide/mm/transhuge.rst > > >>> @@ -107,7 +107,7 @@ sysfs > > >>> Global THP controls > > >>> ------------------- > > >>> > > >>> -Transparent Hugepage Support for anonymous memory can be entirely disabled > > >>> +Transparent Hugepage Support for anonymous memory can be disabled > > >>> (mostly for debugging purposes) or only enabled inside MADV_HUGEPAGE > > >>> regions (to avoid the risk of consuming more memory resources) or enabled > > >>> system wide. This can be achieved per-supported-THP-size with one of:: > > >>> @@ -119,6 +119,11 @@ system wide. This can be achieved per-supported-THP-size with one of:: > > >>> where is the hugepage size being addressed, the available sizes > > >>> for which vary by system. > > >>> > > >>> +.. note:: Setting "never" in all sysfs THP controls does **not** disable > > >>> + Transparent Huge Pages globally. This is because ``madvise(..., > > >>> + MADV_COLLAPSE)`` ignores these settings and collapses ranges to > > >>> + PMD-sized huge pages unconditionally. > > >>> + > > >>> For example:: > > >>> > > >>> echo always >/sys/kernel/mm/transparent_hugepage/hugepages-2048kB/enabled > > >>> @@ -187,7 +192,9 @@ madvise > > >>> behaviour. > > >>> > > >>> never > > >>> - should be self-explanatory. > > >>> + should be self-explanatory. Note that ``madvise(..., > > >>> + MADV_COLLAPSE)`` can still cause transparent huge pages to be > > >>> + obtained even if this mode is specified everywhere. > > >> > > >> I hope this part of the explanation is also copy-pasted into the > > >> 'Hugepages in tmpfs/shmem' section. Otherwise look good to me. Thanks. > > > > > > Apologies if this is a silly question, but regarding this patchset: > > > https://lore.kernel.org/linux-mm/cover.1750815384.git.baolin.wang@linux.alibaba.com/ > > > > > > It looks like the intention is to disable hugepages even for > > > `MADV_COLLAPSE` when the user has set the policy to 'never'. However, > > > based on Lorenzo's documentation update, it seems we still want to allow > > > hugepages for `MADV_COLLAPSE` even if 'never' is set? > > > > > > Could you clarify what the intended behavior is? It seems we've decided > > > to keep the existing behavior unchanged—am I understanding that > > > correctly? > > > > Yes, Hugh has already explicitly opposed the current changes to the > > MADV_COLLAPSE logic[1], although there are still some disagreements that > > cannot be resolved. > > > > At least we reached the consensus to update the documentation to reflect > > the current sysfs THP control logic first, to avoid the misunderstanding > > that 'sysfs THP controls can disable Transparent Huge Pages globally'. > > Nice, thanks! Personally, I prefer this approach as well. Updating the > man page feels a bit odd, since it's something people are already > familiar with and may have memorized. Indeed, Hugh's input was important here and gave pause for thought. This was not an easy decision, and I ended up changing my mind from initially supporting this chnage... :) We may return to it later, but for the time being this is the rather conservative approach we've decided upon. Re: man page - I _do_ intend to update the man page as I find it far too vague on this topic currently, so that patch will be coming soon. I will cc- the THP folks on that patch when I send it. > > > > > [1] > > https://lore.kernel.org/linux-mm/75c02dbf-4189-958d-515e-fa80bb2187fc@google.com/ > > Best regards > Barry Cheers, Lorenzo