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Howlett" , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Jonathan Corbet , linux-mm@kvack.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org References: <20250721155530.75944-1-lorenzo.stoakes@oracle.com> From: Baolin Wang In-Reply-To: <20250721155530.75944-1-lorenzo.stoakes@oracle.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: EC2B9180004 X-Stat-Signature: zo97fdqkmrm7xwktita5g9yzxjtq1a44 X-Rspam-User: X-Rspamd-Server: rspam11 X-HE-Tag: 1753147823-943899 X-HE-Meta: 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 yTrA80lz cKQhbkexk2pY370Or+Fgef40P9JJsaUFUvFOcoxSKjo5mtm+D+zmRyKyCnVCgzzRIatagk4304rn/Nn+1MtkVS+kviNoCx+l3uO+JAOx6Z5+55Cw2Vdl8MWSVoe2NiEyBOCIxOyRzh7diYTeWKIdPNwY6MuO8JsklUb1wBk2FmDBSr158ExF8RyP0UcfoA7FDydEJ5N0+rj1FZy8QUXzsJa2fGYde1rmcvOBzZ+LcRqjnxTE6qfpcyoop0l3l+I/wftLE2FX2jKSg+4jhH8D/nQO9nsns7zc2zl/Yzi5tmZB5ws5qnTEyHYbSTAClh/ILA7GWPS4jFHKCypurZOiaw9j/7LwxklSUZQBT 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 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.