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b=J+U2e8kT9BYiBG+FapB4TOMQjAdnkPeYBWJ8i9g/qghLw1NgV/iMSxiPAZ0WuVLhZ fial1QCXeMyNJ1DwplSFrrdCqAVEeoNMz9SAknqE9czp4JRMQZlG7gPBicTakhXyYQ Obx5yaMgbp2NuBoZoJm8YGxnP2QaV4XEFCl6cZtRyq+fWvDqaVBzMofAGGne7zrHkJ jO5duelZY/kLy9X32rFOWZiLdi6F11K58SCvQuxDkcqTYkQpF2vH17LYCxoc3hBa6N Tquq+wRPRXtbIr6xzfFuvDdvmUPW5g9+VKgjpXYyU4SKo6HLX2NXdpgvVZZfZmmFLW BtUqGbdSQOKNw== Message-ID: Date: Tue, 9 Jun 2026 14:59:32 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH mm-unstable v1 2/3] mm/migrate.c: Prevent folio splitting from interacting with KSM To: Nico Pache , xu.xin16@zte.com.cn Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, usamaarif642@gmail.com, yuzhao@google.com, aarcange@redhat.com, akpm@linux-foundation.org, chengming.zhou@linux.dev, ljs@kernel.org, ziy@nvidia.com, baolin.wang@linux.alibaba.com, liam@infradead.org, ryan.roberts@arm.com, dev.jain@arm.com, baohua@kernel.org, lance.yang@linux.dev, matthew.brost@intel.com, joshua.hahnjy@gmail.com, rakie.kim@sk.com, byungchul@sk.com, gourry@gourry.net, ying.huang@linux.alibaba.com, apopple@nvidia.com References: <20260609114619.144416-3-npache@redhat.com> <20260609201202615oT6yd_LtIoKkGLksFaCha@zte.com.cn> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: 750D418000A X-Rspam-User: X-Stat-Signature: j6je3cyd7pnjyzanq7eaxkxqd85hftyn X-Rspamd-Server: rspam09 X-HE-Tag: 1781009982-808997 X-HE-Meta: U2FsdGVkX1+Q39pl6s0S1A1YbH8wbJ7rwYVd+yF15SD9RFTvE1EPTH/WBjJkqMXZCrT2N8mIe78VkJQAmzZYCFj2KIs48jJnV9wLYcA0OeVwJwNJpz93la1oVSqDKhjn2nwqW2utFxVoGZXUyJRZB3H6OjUSbUnKNwe4sqH/v6RDJyOfB1heXAzrBGvxeHqF7aT69+uefZr1G4X1UaZtYUroO62CsxpgH98mdNk+bdAuCuP3wiisBCzmXgxPFSFuPN1l2cUUGQwjcS1jw4SOfre/Y96er0KUStzmZ/UXIlTx8wt0SEkbvRMPbwhPm14fNtQRP+Y7vV+SuZiXlEnZmbPjWhZSMjJEF57bWIXGP5m7TYz7w2P4dknvqiyqG8B2R1rion7AskGQFGgbZRB7e8BaG/F2rQP1y+K9/w/W+OJR8D1SmvY4vRaDJnRtmP+4RAT52lbl1a+JtwHAMUszfFnRR7AhDaRcEFbrKSvqm/GaOhT47fiEHv3hZlfig5ngN6DZmUUMss8HvFmBOj1qATJOxb1q9YdZ92JyYRyZxywu++fhM6uXh3/6ZdZxGa13YJgdG4BzmI2Yt1vACHRqOYxxg4o1oaKEbbHwmYzB+/pcHOZYpBdEtc5KsFg6akmdTxcFDKwH3Zp3pJL8/Biu6gy/LL26T9NdJbWyd5tjprP3A00bDd1U9UfjOMUJqbMMRRnfixwwt1LxXtoxc62/XqraZk9tKcA/Q2zgEJ++qJ1dFfsckX/7Il3sQfTglyw8tDMGfmO4w4lIgI+TpEthp4lDu9I9E6pC6KBEsXbW29WWXaD5K7UrvgM8O6MTQaKiFna1DplFv/CmePY5vftzp6cUEhUUiOzeJqVrU6OVogmb1sGgqTg5Jc6abava6DGR4ixsO8wesM89HPOz1WHivstGWxNv6MymyGIkBhhL/qFArMhIbT7ipFbw++RT3t/6+66yowgvVYTcvz8/Uyf Z2p6PZNG guzTv81yijdF17Ss6JDm07PnyhQzIIcVpu+mNeULhTFDZCVrieL19M2e2UmWCq5BT8nYMlcErOaoHpmT/aFUOwhAw//JY7JV64B9fYunMrUzeVukdo4v57PJIusP4kGPD38eJEobEI1M+HHGArRw+ZFtaIlT/XJm7hrvo+mcHflSASDv49jy/DwdtpIN9bwGiBzhBUWU46NOAKupkRZEIs6lh97AbaduN+izC3rz6DqpOhjb1TUNczLRLlZGfurdQEkWE+RZEokDEcL1KCfNwV63T2TZImdn/rrX6UUrxnFc325I/mv5tHssn3Kmt3j++u/wT+1ZEJz7qiYHIiFZDtl7NDVWkhx8l7BAChNqd2l/ntF6CDW86/8VCIPtvX95j2HZftRrx+a792AmaTDON3PM3SyUIHQw/9dwcoTRFQ2Qni4isIGqGa9HBolxsi2714big Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 6/9/26 14:57, Nico Pache wrote: > On Tue, Jun 9, 2026 at 6:12 AM wrote: >> >>> Since commit b1f202060afe ("mm: remap unused subpages to shared zeropage >>> when splitting isolated thp"), splitting an anonymous THP remaps all >>> zero-filled subpages to the shared zeropage via TTU_USE_SHARED_ZEROPAGE. >>> This flag is set unconditionally for every anonymous folio split, >>> including splits triggered by KSM. >>> >>> When KSM is enabled with THP=always, this causes two regressions: >>> >>> 1. use_zero_pages=1: KSM calls try_to_merge_one_page() which triggers >>> split_huge_page(). The split remaps all 512 zero-filled subpages to >>> the shared zeropage at once, freeing the entire 2MB THP when KSM only >>> intended to process a single 4KB page. This bypasses KSM's >>> pages_to_scan rate limiting, causing ~1GB to be freed almost >>> instantly. >>> >> >> Why do you see it as regressions? > > Since the zero-page remapping was introduced our test has shown the > following behavior changes: > > With use_zero_pages=0, the merge rate drops from 60MB/s to ~6 MB/s > even after raising pages_to_scan. The KSM merging is now much slower > and CPU utilization has increased. > > With use_zero_pages=1, ~1 GB is freed almost instantly, and it no > longer respects the pages_to_scan behavior. > > Even with just this patch (1 & 2) or the RFC linked in the cover > letter, the issue no longer occurs. > >> >> AFAIU, KSM and THP do often conflict with each other. THP tries hard to collapse >> a huge page (which may contain many zero pages). If KSM is enabled and part of >> that huge page is mergeable, it can easily be split by KSM, rendering THP's >> efforts futile. >> >> Therefore, in our actual production environment, we typically avoid making the >> same region both KSM mergeable and THP always. > > THP=always is a global setting used in many production environments, > so these features now interact very poorly together. Red Hat documents, though, that both things in combination are shaky: "As KSM can reduce the occurrence of transparent huge pages, you may want to disable it before enabling THP." [1] (for RHEL 6, but nothing should have changed in that regard) But yeah, having KSM and THP enabled at the same time is not uncommon. [1] https://docs.redhat.com/en/documentation/red_hat_enterprise_linux/6/html/virtualization_tuning_and_optimization_guide/sect-virtualization_tuning_optimization_guide-memory-huge_pages -- Cheers, David