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b=ZF1jpp6v/dXCeWr7H1p7HtA++QO5V5PbD/834AWaM1fA5wfB5NOwDk70hyhVOoyJs ciupynr8vusnykDt6q/Xl2hxtnA/TLwYsL8xtFB6w0WVWhqkVahhEgywOv5lVpQ/yj Me4eyQsUzWJWqEWFgDLzQFPgLQvsuskD1QicKh+dCqivZBb8Igy5YB3fAlk8FQZNlg 6PUqHLqlwO1KbLESLAntW2eYdlMQHFlFDxnIS3oGGn0XLPxi6+IhWD5wwI9kF7VWH1 YletVPTwk0H1Kkqkjd+VxK/+F3pC5locj+t7miBietqXB6RidJJ89yPA8aN7vdi3C1 1BIN16E1qzTog== Message-ID: Date: Tue, 9 Jun 2026 11:25:49 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH mm-unstable v19 11/14] mm/khugepaged: Introduce mTHP collapse support To: Nico Pache , Lance Yang Cc: linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-trace-kernel@vger.kernel.org, aarcange@redhat.com, akpm@linux-foundation.org, anshuman.khandual@arm.com, apopple@nvidia.com, baohua@kernel.org, baolin.wang@linux.alibaba.com, byungchul@sk.com, catalin.marinas@arm.com, cl@gentwo.org, corbet@lwn.net, dave.hansen@linux.intel.com, dev.jain@arm.com, gourry@gourry.net, hannes@cmpxchg.org, hughd@google.com, jack@suse.cz, jackmanb@google.com, jannh@google.com, jglisse@google.com, joshua.hahnjy@gmail.com, kas@kernel.org, liam@infradead.org, ljs@kernel.org, mathieu.desnoyers@efficios.com, matthew.brost@intel.com, mhiramat@kernel.org, mhocko@suse.com, peterx@redhat.com, pfalcato@suse.de, rakie.kim@sk.com, raquini@redhat.com, rdunlap@infradead.org, richard.weiyang@gmail.com, rientjes@google.com, rostedt@goodmis.org, rppt@kernel.org, ryan.roberts@arm.com, shivankg@amd.com, sunnanyong@huawei.com, surenb@google.com, thomas.hellstrom@linux.intel.com, tiwai@suse.de, usamaarif642@gmail.com, vbabka@suse.cz, vishal.moola@gmail.com, wangkefeng.wang@huawei.com, will@kernel.org, willy@infradead.org, yang@os.amperecomputing.com, ying.huang@linux.alibaba.com, ziy@nvidia.com, zokeefe@google.com References: <20260605161422.213817-12-npache@redhat.com> <20260606102800.26940-1-lance.yang@linux.dev> <2553caae-9e0e-42a7-8b61-d1216f1e81fa@kernel.org> 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-Server: rspam10 X-Rspam-User: X-Stat-Signature: rw686mnhi1xsxoiqkund1p5rupt4tjht X-Rspamd-Queue-Id: 0E6A240002 X-HE-Tag: 1780997164-92731 X-HE-Meta: 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 59qe6Ak+ 28cbg8T+T+LqQBLRBfTKPgh+AVQ9qrvfdfC8bSaT7c8j0B47AwOE7oz0Va2j54UvGu44D3fZQPZA2DsfR5wxKfblOyL3mBQJa6k9QFF4iTqi2Jr75w8x7nJRtJo42ARWf4JD4gzRzDb5kJR0VEra2qQaVxzdDnmBYQEz80MnkMmD6+X1QACMhhBhW893WZSEFAAo14ueZOSWeicxqO1ZR6POUl4sdqd0PsD9LIBJGwW7bqhyoX9XBqnlFrWktjiTU/+K9CflzrT3H6dp1N8Eb5XbtI3jlw1ifia662+4XVsdNDB8hYO5M2yYbXTDCNIOddo2/Zb4//PXRoOTmEXvqDmZ+WGB4701pqUEHe1dkIPbWbFGA0/2MAwsmjqPWong06ZT5 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 6/9/26 11:06, Nico Pache wrote: > On Mon, Jun 8, 2026 at 8:57 AM David Hildenbrand (Arm) wrote: >> >> On 6/6/26 12:28, Lance Yang wrote: >>> >>> >>> Looks broken for swap PTEs in PMD collapse ... >>> >>> collapse_scan_pmd() allows them up to max_ptes_swap and record them in >>> unmapped, but they don't get a bit in mthp_present_ptes. And then >>> mthp_collapse() does the check above: >> >> Right. I assumed this is implicitly handled by the optimization in collapse_scan_pmd: >> >> if (enabled_orders != BIT(HPAGE_PMD_ORDER)) >> max_ptes_none = KHUGEPAGED_MAX_PTES_LIMIT; >> >> But we perform the check a second time. >> >>> >>> nr_occupied_ptes >= nr_ptes - max_ptes_none >>> >>> So max_ptes_none=0 + 511 present PTEs + one allowed swap PTE won't even >>> call collapse_huge_page() for PMD order. >>> >>> Shouldn't we account for them in the PMD-order check? Something like: >>> >>> if (is_pmd_order(order)) >>> nr_occupied_ptes += unmapped; > > This solution seems good for a temporary fixup. but longterm we may > want something else. I'm still not sure how we plan on supporting > swapin without causing creep. So I'd be ok with adding a fix for > legacy PMD behavior until we know how to handle mTHP creep correctly. > >> As an alternative, we could either 1) skip the check there for >> pmd order (as the check was already done); or 2) introduce+maintain >> a bitmap that tracks non-present PTEs. >> >> @@ -1475,7 +1477,9 @@ static enum scan_result mthp_collapse(struct mm_struct *mm, >> nr_occupied_ptes = bitmap_weight_from(cc->mthp_present_ptes, offset, >> offset + nr_ptes); >> >> - if (nr_occupied_ptes >= nr_ptes - max_ptes_none) { >> + /* Check was already done in the caller. */ >> + if (is_pmd_order(order) || >> + nr_occupied_ptes >= nr_ptes - max_ptes_none) { >> enum scan_result ret; >> >> collapse_address = address + offset * PAGE_SIZE; >> >> 2) would probably be cleanest long-term. > > That would be best for future swapin support in mTHP, but I still > don't think it solves the creep issue. It wouldn't, we'd simply maintain the state we collect + rely on in separate bitmaps. On swapin, we'd have to update/refresh bitmaps I guess. > Perhaps we could combine the > two bitmaps to determine if it would make the future collapse eligible > again? Not sure but ill start thinking about it. > > Should I send a fixup for this using Lance's solution? Or does Lance > want to send a patch out with the fixes tag? If Lance could send a fixup, explaining the situation, that would be nice. -- Cheers, David