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Fri, 12 Sep 2025 10:35:30 -0400 (EDT) Date: Fri, 12 Sep 2025 15:35:28 +0100 From: Kiryl Shutsemau To: David Hildenbrand Cc: Nico Pache , 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 Subject: Re: [PATCH v11 00/15] khugepaged: mTHP support Message-ID: References: <20250912032810.197475-1-npache@redhat.com> <43f42d9d-f814-4b54-91a6-3073f7c7cedf@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Queue-Id: 41F21160003 X-Stat-Signature: 7iie78y4jbef1wf4p3wwca8utja1wb5x X-Rspam-User: X-Rspamd-Server: rspam01 X-HE-Tag: 1757687735-258425 X-HE-Meta: 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 gHA66S3Y ro1gQnMPdmBouPZGa2przy78Nb+A4JQyvWW2lkCNT0aAqwirugeio2zXrOFDrygR4MCLLaWvUbqtTW9al/OfBYQkdyBj6GeerD+ghu+CW8jzRxUbQAnBZhPWY1BX/iRQFoXxZvOjdmIxGDe6jGwh8fvbfO52ENDAmV4dlTbBhl0WpHSXuW6WTpmTv7FovxkQqbGG9XPKPcp1+dWEhPpc8FD/RqBFn60LDhAralIg0DLJ6VpL2TuWD9YUJDJ4MhhHnHPLJ8RCpqrGHnPdCVoRZdvf+YFTRRI5qIiAE4cQwNQfLhoTZe+/FXGHvBvCrOEbZPzgU6HOWEkvT/IuUcNBJKAjn9KLD+CiKH3Zb 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 04:28:09PM +0200, David Hildenbrand wrote: > On 12.09.25 15:47, 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 collapse > > > > anonymous memory regions to mTHPs. > > > > > > > > To achieve this we generalize the khugepaged functions to no longer depend > > > > on PMD_ORDER. Then during the PMD scan, we use a bitmap to track individual > > > > pages that are occupied (!none/zero). After the PMD scan is done, we do > > > > 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 scaled > > > > by the attempted collapse order to determine how full a mTHP must be to be > > > > eligible for the collapse to occur. If a mTHP collapse is attempted, but > > > > 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 size > > > > to be enabled. > > > > > > > > When enabling (m)THP sizes, if max_ptes_none >= HPAGE_PMD_NR/2 (255 on > > > > 4K page size), it will be automatically capped to HPAGE_PMD_NR/2 - 1 for > > > > mTHP collapses to prevent collapse "creep" behavior. This prevents > > > > constantly promoting mTHPs to the next available size, which would occur > > > > because a collapse introduces more non-zero pages that would satisfy the > > > > 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. > > Aaand, thinking again from the other direction, this would mean that just > because pages became zero after some time that we would no longer collapse > because none_or_zero would then be higher. Hm .... > > How I hate all of this so very very much :) This is not new. Shrinker has the same problem: it cannot distinguish between hot 4k that happened to be zero from the 4k that is there just because of we faulted in 2M a time. -- Kiryl Shutsemau / Kirill A. Shutemov