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Thu, 21 Aug 2025 08:26:09 -0700 (PDT) X-Google-Smtp-Source: AGHT+IEA0f6PQCn0aQQ73DO5x40Ev9z93mEJijQRhHTNYum23X09DqH0vh6LBZKUrLyYzx7jARY0Uu5ScQ6B+4yOKuI= X-Received: by 2002:a05:6902:100b:b0:e93:b92c:ee20 with SMTP id 3f1490d57ef6-e95088e66fdmr2698871276.6.1755789968301; Thu, 21 Aug 2025 08:26:08 -0700 (PDT) MIME-Version: 1.0 References: <20250819134205.622806-1-npache@redhat.com> <38b37195-28c8-4471-bd06-951083118efd@arm.com> <0d9c6088-536b-4d7a-8f75-9be5f0faa86f@lucifer.local> In-Reply-To: <0d9c6088-536b-4d7a-8f75-9be5f0faa86f@lucifer.local> From: Nico Pache Date: Thu, 21 Aug 2025 09:25:42 -0600 X-Gm-Features: Ac12FXxfRB0-YWz05cxs6Jud7daM5CQrb2gQwUVzVsT0dS5nUzi2PQuSgYs9--g Message-ID: Subject: Re: [PATCH v10 00/13] khugepaged: mTHP support To: Lorenzo Stoakes Cc: Dev Jain , linux-mm@kvack.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org, david@redhat.com, ziy@nvidia.com, baolin.wang@linux.alibaba.com, Liam.Howlett@oracle.com, ryan.roberts@arm.com, corbet@lwn.net, rostedt@goodmis.org, mhiramat@kernel.org, mathieu.desnoyers@efficios.com, akpm@linux-foundation.org, baohua@kernel.org, willy@infradead.org, peterx@redhat.com, wangkefeng.wang@huawei.com, usamaarif642@gmail.com, sunnanyong@huawei.com, vishal.moola@gmail.com, thomas.hellstrom@linux.intel.com, yang@os.amperecomputing.com, kirill.shutemov@linux.intel.com, aarcange@redhat.com, raquini@redhat.com, anshuman.khandual@arm.com, catalin.marinas@arm.com, tiwai@suse.de, will@kernel.org, dave.hansen@linux.intel.com, jack@suse.cz, cl@gentwo.org, jglisse@google.com, surenb@google.com, zokeefe@google.com, hannes@cmpxchg.org, rientjes@google.com, mhocko@suse.com, rdunlap@infradead.org, hughd@google.com X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: dQL4gjRCCTU-MfXNlxV9EAuUawf3_tp5ynirTWFmmtY_1755789970 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: E411840007 X-Stat-Signature: hoqgmw3spfz4os1q3s3bq3znmi5w9cag X-Rspam-User: X-HE-Tag: 1755789971-363648 X-HE-Meta: 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 00x5AC1K DNRZRhpjge5UnyJFaEU80QmcreCuXh+6hKcHNAidG4ahkaaArfSxWUbwxWEem8D9nshVBF1HH1fF0RCmZ4r7BK85GGLAj2F4AYUJmOt9X6izJJVsdnGzU/U+KOyaZFDQ/Csyt4iyBdC9O09fFiZp30fc5M1lLzmF069wGLXIyE5TZBZZQifKfMJ4oQ7CZVx8Fqj2B+BMubp5p1OdNiiZXKHVtxrJ0yAtxch2T+a/tzTla0MpZTui+JgFalCcw69gZUHLOIFrmIpQInop2PPRNM6RERivJCys6Guo78RSBX0zqBXuVs1TTA0bD8ozu0+U6B1OzA1vyxBb7LqTcMsJRyFR56sKS/I+FCMWoHd32I+WpyMfANlJfXqw6ds9DRDNBZd6aNnvRUpPBKTgGz78kuo3t+xgYg123QrdhaVmRfqW2F3t1Ht+a7N7nBgEXtK7QNemjKCBPfjW03+Q= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Thu, Aug 21, 2025 at 9:20=E2=80=AFAM Lorenzo Stoakes wrote: > > On Thu, Aug 21, 2025 at 08:43:18PM +0530, Dev Jain wrote: > > > > On 21/08/25 8:31 pm, Lorenzo Stoakes wrote: > > > OK so I noticed in patch 13/13 (!) where you change the documentation= that you > > > essentially state that the whole method used to determine the ratio o= f PTEs to > > > collapse to mTHP is broken: > > > > > > khugepaged uses max_ptes_none scaled to the order of the enabled > > > mTHP size to determine collapses. When using mTHPs it's recommend= ed > > > to set max_ptes_none low-- ideally less than HPAGE_PMD_NR / 2 (25= 5 > > > on 4k page size). This will prevent undesired "creep" behavior th= at > > > leads to continuously collapsing to the largest mTHP size; when w= e > > > collapse, we are bringing in new non-zero pages that will, on a > > > subsequent scan, cause the max_ptes_none check of the +1 order to > > > always be satisfied. By limiting this to less than half the curre= nt > > > order, we make sure we don't cause this feedback > > > loop. max_ptes_shared and max_ptes_swap have no effect when > > > collapsing to a mTHP, and mTHP collapse will fail on shared or > > > swapped out pages. > > > > > > This seems to me to suggest that using > > > /sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_none as some = means > > > of establishing a 'ratio' to do this calculation is fundamentally fla= wed. > > > > > > So surely we ought to introduce a new sysfs tunable for this? Perhaps > > > > > > /sys/kernel/mm/transparent_hugepage/khugepaged/mthp_max_ptes_none_rat= io > > > > > > Or something like this? > > > > > > It's already questionable that we are taking a value that is expresse= d > > > essentially in terms of PTE entries per PMD and then use it implicitl= y to > > > determine the ratio for mTHP, but to then say 'oh but the default val= ue is > > > known-broken' is just a blocker for the series in my opinion. > > > > > > This really has to be done a different way I think. > > > > > > Cheers, Lorenzo > > > > FWIW this was my version of the documentation patch: > > https://lore.kernel.org/all/20250211111326.14295-18-dev.jain@arm.com/ > > > > The discussion about the creep problem started here: > > https://lore.kernel.org/all/7098654a-776d-413b-8aca-28f811620df7@arm.co= m/ > > > > and the discussion continuing here: > > https://lore.kernel.org/all/37375ace-5601-4d6c-9dac-d1c8268698e9@redhat= .com/ > > > > ending with a summary I gave here: > > https://lore.kernel.org/all/8114d47b-b383-4d6e-ab65-a0e88b99c873@arm.co= m/ > > > > This should help you with the context. > > > > > > Thanks and I"ll have a look, but this series is unmergeable with a broken > default in > /sys/kernel/mm/transparent_hugepage/khugepaged/mthp_max_ptes_none_ratio > sorry. > > We need to have a new tunable as far as I can tell. I also find the use o= f > this PMD-specific value as an arbitrary way of expressing a ratio pretty > gross. The first thing that comes to mind is that we can pin max_ptes_none to 255 if it exceeds 255. It's worth noting that the issue occurs only for adjacently enabled mTHP sizes. ie) if order!=3DHPAGE_PMD_ORDER && khugepaged_max_ptes_none > 255 temp_max_ptes_none =3D 255; > > Thanks, Lorenzo >