+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 > <baolin.wang@xxxxxxxxxxxxxxxxx> wrote: > > > > > > > > On 2025/7/22 10:23, 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? > > > > 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@xxxxxxxxxx/ > > Best regards > Barry Cheers, Lorenzo
