On Tue, Jul 22, 2025 at 09:30:18AM +0800, 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 <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. Thanks, will send a fix-patch to add it there too!
