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[2003:d8:2f2b:b200:607d:d3d2:3271:1be0]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-459dfa64da0sm48813325e9.3.2025.08.05.03.24.53 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 05 Aug 2025 03:24:54 -0700 (PDT) Message-ID: <7828753c-98fc-4f96-bf7c-0d94f3b99e4b@redhat.com> Date: Tue, 5 Aug 2025 12:24:52 +0200 Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 4/6] docs: transhuge: document process level THP controls To: Usama Arif , Andrew Morton , linux-mm@kvack.org Cc: linux-fsdevel@vger.kernel.org, corbet@lwn.net, rppt@kernel.org, surenb@google.com, mhocko@suse.com, hannes@cmpxchg.org, baohua@kernel.org, shakeel.butt@linux.dev, riel@surriel.com, ziy@nvidia.com, laoar.shao@gmail.com, dev.jain@arm.com, baolin.wang@linux.alibaba.com, npache@redhat.com, lorenzo.stoakes@oracle.com, Liam.Howlett@oracle.com, ryan.roberts@arm.com, vbabka@suse.cz, jannh@google.com, Arnd Bergmann , sj@kernel.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, kernel-team@meta.com References: <20250804154317.1648084-1-usamaarif642@gmail.com> <20250804154317.1648084-5-usamaarif642@gmail.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 04.08.25 17:40, Usama Arif wrote: > 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. > > Signed-off-by: Usama Arif > --- > Documentation/admin-guide/mm/transhuge.rst | 38 ++++++++++++++++++++++ > 1 file changed, 38 insertions(+) > > diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst > index 370fba113460..a36a04394ff5 100644 > --- a/Documentation/admin-guide/mm/transhuge.rst > +++ b/Documentation/admin-guide/mm/transhuge.rst > @@ -225,6 +225,44 @@ 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. These calls support the > +following arguments:: > + Not sure if we really want to talk about MMF_ internals. > + prctl(PR_SET_THP_DISABLE, 1, 0, 0, 0): > + This will set the MMF_DISABLE_THP_COMPLETELY mm flag which will > + result in no THPs being faulted in or collapsed, irrespective > + of global THP controls. This flag and hence the behaviour is > + inherited across fork(2) and execve(2). " This will disable THPs completely for the process, irrespective of global THP controls or MADV_COLLAPSE. " ? > + > + prctl(PR_SET_THP_DISABLE, 1, PR_THP_DISABLE_EXCEPT_ADVISED, 0, 0): > + This will set the MMF_DISABLE_THP_EXCEPT_ADVISED mm flag which > + will result in THPs being faulted in or collapsed only for > + the following cases: Maybe: 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 the area either has VM_HUGEPAGE set (e.g., due do MADV_HUGEPAGE) or MADV_COLLAPSE is used. - Global THP controls are set to "never" and 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 VM_NOHUGEPAGE is set on an area. ... > + or MADV_COLLAPSE. > + - Global THP controls are set to "always" or "madvise" and > + the process has madvised the region with either MADV_HUGEPAGE > + or MADV_COLLAPSE. > + - Global THP controls is set to "never" and the process has > + madvised the region with MADV_COLLAPSE. > + This flag and hence the behaviour is inherited across fork(2) > + and execve(2). > + > + prctl(PR_SET_THP_DISABLE, 0, 0, 0, 0): > + This will clear the MMF_DISABLE_THP_COMPLETELY and > + MMF_DISABLE_THP_EXCEPT_ADVISED mm flags. The process will > + behave according to the global THP controls. This behaviour > + will be inherited across fork(2) and execve(2). "This will re-enabled THPs for the process, as if they would never have been disabled. Whether THPs will actually be used depends on global THP controls." ? > + > + prctl(PR_GET_THP_DISABLE, 0, 0, 0, 0): > + This will return the THP disable mm flag status of the process > + that was set by prctl(PR_SET_THP_DISABLE, ...). i.e. > + - 1 if MMF_DISABLE_THP_COMPLETELY flag is set if PR_SET_THP_DISABLE was called without any flags > + - 3 if MMF_DISABLE_THP_EXCEPT_ADVISED flag is set if PR_SET_THP_DISABLE was called with PR_THP_DISABLE_EXCEPT_ADVISED > + - 0 otherwise. > + > Khugepaged controls > ------------------- > -- Cheers, David / dhildenb