On Tue, Jul 22, 2025 at 10:23:39AM +0800, Barry Song wrote: > On Tue, Jul 22, 2025 at 9:30 AM Baolin Wang > <baolin.wang@xxxxxxxxxxxxxxxxx> 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 <lorenzo.stoakes@xxxxxxxxxx> > > > --- > > > 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 <size> 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@xxxxxxxxxxxxxxxxx/ > > 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? For now see [0], we have decided at this time that this series should not be applied. I again apologise sincerely to Baolin for this being such a back and forth and him doing so much work here prior to this decision, but overall David and I felt that _at this time_ we didn't want to risk breaking anybody by changing this behaviour. And so as I promised, this patch is my updating the documentation to reflect the current (and I entirely agree - odd) reality of 'never means never'. Cheers, Lorenzo [0]:https://lore.kernel.org/linux-mm/573eb43a-8536-4206-a7c6-d0daa1fd7e70@lucifer.local/ > > Thanks > Barry
