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[80.187.83.189]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-46fab3d2c1asm54109495e9.1.2025.10.10.00.40.29 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 10 Oct 2025 00:40:31 -0700 (PDT) Message-ID: <1d7476e6-5c6d-4175-b28d-3622222ea8e6@redhat.com> Date: Fri, 10 Oct 2025 09:40:29 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] Revert "mm, hugetlb: remove hugepages_treat_as_movable sysctl" To: Gregory Price Cc: Michal Hocko , linux-mm@kvack.org, corbet@lwn.net, muchun.song@linux.dev, osalvador@suse.de, akpm@linux-foundation.org, hannes@cmpxchg.org, laoar.shao@gmail.com, brauner@kernel.org, mclapinski@google.com, joel.granados@kernel.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, Mel Gorman , Alexandru Moise <00moses.alexander00@gmail.com>, Mike Kravetz , David Rientjes References: <20251007214412.3832340-1-gourry@gourry.net> <402170e6-c49f-4d28-a010-eb253fc2f923@redhat.com> <271f9af4-695c-4aa5-9249-2d21ad3db76e@redhat.com> <83e33641-8c42-4341-8e6e-5c75d00f93b9@redhat.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 8CE0DC000A X-Rspamd-Server: rspam11 X-Rspam-User: X-Stat-Signature: mtgih3wa8erubtaymw53m65bgo3dt8ds X-HE-Tag: 1760082035-844716 X-HE-Meta: U2FsdGVkX1/PyE+rAGYCce7UqXqaPESgGznZJQHvmyb+/h8UX5pGC8CKeGlOFJPWZ1Qkxme8FzfcPZsQptjikNtzHjjfCtD2iMscAjRBJIzpLVANSTlSuvIcdTfUgYRn1PMkfpFa8gX2nMkcep+gUucGxEhHWaw6zZU4c231w0DfmgkWmznH2uZRZ1vPf8M7GRD3+KARcfhKLSUxr8PclP+K7LmUVJn/1+1Xtsfilkwo8r+99E78mRc/B+HaS15CaClMCE0sLIYpOE56dxkSu3E3E6mdco7dk4uILdfJGZAHDjqMHb2WBTluxCLdOWd9OVAFHCSUFsiERudI3a+pQG0/0EEOrqZyVz0PqTdROE3jdCUIV61ETYN16t4ETaUw5rsn96Kh4k73QtZFqHWDo7Zs7NPfCQnjUCd35eDZTEb0GwlN/H7ZGsnBO/lwf9q83rkXKAOCjrtDKNz2haHQLSXdTuBfigx8Nnd9Ou13VgDNn+pEd2qdKKJeqd6/uiN6aK2KWNKMsYzAgherjsRyGcjrvcZnwdIhmqyzDDNjGUbumJfGnupsNhYI5YmW8StVUKC862erYy2cC8f/b3bExBx676ix22YZt0CXT3oWJ2Svo+jv/u3Xm402hUOdiE5vozQIEQc6dyv6rD6v22QjA9Avifc0KdbQmeSe/6hGdyfxrcknwmHgLuoDcKqZ8/EIiyAC9UiXXrf57+QlzLOokld3QiBArWDpZSSc8+C5/xvSnfbB3BMk9gprRZHpgjDnGLtW6lmybSVovNNRmopKgZoZgdnJcKiPltNgS6PbVX5ldux8e/wngN2nx9MxAS8jQawXq9D72dEjtNQuZzYvA8apEgLZPszKjfOmDaLf8mxGvjrbstjsCtuZOg2BVkECr1Ht21DZGKCqEE3mXbrOImF8hqXqylUT0PQiJxE9wJiwAzsF5lSgPbdRKO1A+Q6uQ82/JQ3Krbs69rKU8Y+ sWSoXwAl 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 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 09.10.25 23:31, Gregory Price wrote: > On Thu, Oct 09, 2025 at 08:51:54PM +0200, David Hildenbrand wrote: >> On 09.10.25 17:29, Gregory Price wrote: >> Or would it be sufficient to selectively enable (explicit opt-in) some user >> pages to end up on ZONE_MOVABLE? IOW, change the semantics of the zone by an >> admin. >> >> Like, allowing longterm pinning on ZONE_MOVABLE. >> >> Sure, it would degrade memory hotunplug (until the relevant applications are >> shut down) and probably some other things. >> >> Further, I am not so sure about the value of having ZONE_MOVABLE sprinkled >> with small unmovable allocations (same concern regarding any such zone that >> allows for unmovable things). Kind of against the whole concept. >> >> But I mean, if the admin decides to do that (opt in), so he is to blame. >> > > For what it's worth, this patch (or the new one i posted as an RFC), I > was able to allocate gigantic pages and migrate them back and forth > between nodes even after they were allocated for KVM instances. > > I was surprised this did not cause pinning. KVM does not end up longterm-pinning pages (what we care about regarding migration) when mapping stuff into the guest MMU, so KVM in general is not a problem. The problem shows up once you would try to use something like vfio, liburing fixed buffers etc, where we will longterm-pin pages. > > This was all while running the QEMU machine actively eating ~2GB of > memory. So this seems... acceptable? My primary use case was VM > hugepages, but it doesn't even seem like these have been pinned. > > I think the confidential-compute / guest_memfd path would have an > issue, because those are pinned and/or entirely unmapped from the > host, but that just seems like a known quantity and a reason to leave > this off by default (make them read the docs :]). guest_memfd allocates folios without GFP_MOVABLE, because they are ... unmovable. So they would never end up on ZONE_MOVABLE. There are prototypes / ideas to support migration of guest_memfd pages, so it would be solvable. At least for some scenarios. > > Seems like this is pretty stable tbh. Obviously if you hack off the > node0 hugepages migration fails - but I feel like you're signing up for > that when you turn the bit on. Right, just needs to be documented thoroughly IMHO. -- Cheers David / dhildenb