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Fri, 12 Sep 2025 16:35:32 -0700 (PDT) X-Google-Smtp-Source: AGHT+IFf6FRXt0puU/GkbiNMmkdOBMh3PmcUJa8ItdRIcV21DzC27xRjlQRyXSqK3+grheHlKb8t0P44+F+NVaC7sWY= X-Received: by 2002:a05:690c:6ac2:b0:729:677e:b7b with SMTP id 00721157ae682-73065fa0c40mr46614957b3.47.1757720131865; Fri, 12 Sep 2025 16:35:31 -0700 (PDT) MIME-Version: 1.0 References: <20250912032810.197475-1-npache@redhat.com> <43f42d9d-f814-4b54-91a6-3073f7c7cedf@redhat.com> In-Reply-To: <43f42d9d-f814-4b54-91a6-3073f7c7cedf@redhat.com> From: Nico Pache Date: Fri, 12 Sep 2025 17:35:05 -0600 X-Gm-Features: Ac12FXwW-JyuDr21ldb1iQAOW2JaSV_ia9I1rYO-9C_34jUWMMoQfpgn4vajl84 Message-ID: Subject: Re: [PATCH v11 00/15] khugepaged: mTHP support To: David Hildenbrand Cc: Kiryl Shutsemau , linux-mm@kvack.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org, ziy@nvidia.com, baolin.wang@linux.alibaba.com, lorenzo.stoakes@oracle.com, Liam.Howlett@oracle.com, ryan.roberts@arm.com, dev.jain@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, 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, richard.weiyang@gmail.com, lance.yang@linux.dev, vbabka@suse.cz, rppt@kernel.org, jannh@google.com, pfalcato@suse.de X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: b-s-r8LhejN3CRoeoPDYYp2SEh1QCpJnnGUe9fF4zDA_1757720132 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: C68B940003 X-Stat-Signature: b8515st1n4ebsgerjzorz5i37g9iq1pn X-Rspam-User: X-HE-Tag: 1757720134-143902 X-HE-Meta: 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 xZ4dj5Uv FhUlag8Qk63BN8QlfbU8Uo8NCulJElWgi3mhBdDpUAazn88TUbk4LKfObTFN9yrm0jfHjp65QD+V9lZQx1sOMnXh+iXqaDeILPVsrFc+VCngPCWqjtQD3/alaCFOok7wI6sTRIrJC30btZoeavJQNkMJTrlyhYjVmEfXL8WbgirCV/RdepOLRWSKdIIhpmbhsd6FYoDBoQCFidi3q2+7WtryYcRfY1U096JV3KvTqoCi5nvLerDbJC3vpkr+cMEyYLHE1+2JLEBdYUkqWDsvyYczbE4i1Nu43H/SCBlMp5cCf5se78qaLJLPr7cPI3oQ/kU1yDrzvsOcbk887Y2gpJz0rPB3Dqb3PzWZ/ 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 Fri, Sep 12, 2025 at 7:48=E2=80=AFAM David Hildenbrand wrote: > > On 12.09.25 14:19, Kiryl Shutsemau wrote: > > On Thu, Sep 11, 2025 at 09:27:55PM -0600, Nico Pache wrote: > >> The following series provides khugepaged with the capability to collap= se > >> anonymous memory regions to mTHPs. > >> > >> To achieve this we generalize the khugepaged functions to no longer de= pend > >> on PMD_ORDER. Then during the PMD scan, we use a bitmap to track indiv= idual > >> pages that are occupied (!none/zero). After the PMD scan is done, we d= o > >> binary recursion on the bitmap to find the optimal mTHP sizes for the = PMD > >> range. The restriction on max_ptes_none is removed during the scan, to= make > >> sure we account for the whole PMD range. When no mTHP size is enabled,= the > >> legacy behavior of khugepaged is maintained. max_ptes_none will be sca= led > >> by the attempted collapse order to determine how full a mTHP must be t= o be > >> eligible for the collapse to occur. If a mTHP collapse is attempted, b= ut > >> contains swapped out, or shared pages, we don't perform the collapse. = It is > >> now also possible to collapse to mTHPs without requiring the PMD THP s= ize > >> to be enabled. > >> > >> When enabling (m)THP sizes, if max_ptes_none >=3D HPAGE_PMD_NR/2 (255 = on > >> 4K page size), it will be automatically capped to HPAGE_PMD_NR/2 - 1 f= or > >> mTHP collapses to prevent collapse "creep" behavior. This prevents > >> constantly promoting mTHPs to the next available size, which would occ= ur > >> because a collapse introduces more non-zero pages that would satisfy t= he > >> promotion condition on subsequent scans. > > > > Hm. Maybe instead of capping at HPAGE_PMD_NR/2 - 1 we can count > > all-zeros 4k as none_or_zero? It mirrors the logic of shrinker. > > BTW, I thought further about this and I agree: if we count zero-filled > pages towards none_or_zero one we can avoid the "creep" problem. > > The scanning-for-zero part is rather nasty, though. IIRC me and David have discussed this in the past and decided to avoid this approach at the moment because it would be complicated and "nasty". > > -- > Cheers > > David / dhildenb >