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docs: transhuge: document process level THP controls

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This includes the PR_SET_THP_DISABLE/PR_GET_THP_DISABLE pair of prctl
calls as well the newly introduced PR_THP_DISABLE_EXCEPT_ADVISED flag for
the PR_SET_THP_DISABLE prctl call.

Link: https://lkml.kernel.org/r/20250815135549.130506-5-usamaarif642@gmail.com
Signed-off-by: Usama Arif <usamaarif642@gmail.com>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Jann Horn <jannh@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Mariano Pache <npache@redhat.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Rik van Riel <riel@surriel.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: SeongJae Park <sj@kernel.org>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Yafang <laoar.shao@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Usama Arif
2025-09-13 16:55:05 -07:00
committed by Andrew Morton
parent 8cdc4d2701
commit 7de854910b
1 changed files with 36 additions and 0 deletions
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Documentation/admin-guide/mm/transhuge.rst
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@@ -225,6 +225,42 @@ to "always" or "madvise"), and it'll be automatically shutdown when
PMD-sized THP is disabled (when both the per-size anon control and the
top-level control are "never")
process THP controls
--------------------
A process can control its own THP behaviour using the ``PR_SET_THP_DISABLE``
and ``PR_GET_THP_DISABLE`` pair of prctl(2) calls. The THP behaviour set using
``PR_SET_THP_DISABLE`` is inherited across fork(2) and execve(2). These calls
support the following arguments::
prctl(PR_SET_THP_DISABLE, 1, 0, 0, 0):
This will disable THPs completely for the process, irrespective
of global THP controls or madvise(..., MADV_COLLAPSE) being used.
prctl(PR_SET_THP_DISABLE, 1, PR_THP_DISABLE_EXCEPT_ADVISED, 0, 0):
This will disable THPs for the process except when the usage of THPs is
advised. Consequently, THPs will only be used when:
- Global THP controls are set to "always" or "madvise" and
madvise(..., MADV_HUGEPAGE) or madvise(..., MADV_COLLAPSE) is used.
- Global THP controls are set to "never" and madvise(..., MADV_COLLAPSE)
is used. This is the same behavior as if THPs would not be disabled on
a process level.
Note that MADV_COLLAPSE is currently always rejected if
madvise(..., MADV_NOHUGEPAGE) is set on an area.
prctl(PR_SET_THP_DISABLE, 0, 0, 0, 0):
This will re-enable THPs for the process, as if they were never disabled.
Whether THPs will actually be used depends on global THP controls and
madvise() calls.
prctl(PR_GET_THP_DISABLE, 0, 0, 0, 0):
This returns a value whose bits indicate how THP-disable is configured:
Bits
1 0 Value Description
|0|0| 0 No THP-disable behaviour specified.
|0|1| 1 THP is entirely disabled for this process.
|1|1| 3 THP-except-advised mode is set for this process.
Khugepaged controls
-------------------