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by smtp.kernel.org (Postfix) with ESMTPSA id 1B065C4CED6; Fri, 10 Jan 2025 20:28:35 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1736540916; bh=Ptzj1/3QIqtF0Doe2VkJY82nu9Rzq5AEXnndywJrJOM=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=GQMhq6GpEVk0Rxm3fVXMgPbBh/TZaU2sIYGyAL41+L68ak3n/2+g+BCq0shQv55iA P69exTSH/Lg4WSG7oX3yhS6qG3Ms/n9EEzS+MnAjYlBPVmrTQk2U/VTCf23E9DOs88 5rBuB/8X7y5ItWVPX35fmWSsRvqBXVEjCMUUCFtCVfFfjHQg5fa9DlX94SVVqPMMTr L9OCCNsT5ixOBVtrP97Hu+qj5P9m1Qau9D7Su8cYwdc0om3+YX0zgR+2MMP8M3qV8L JG5RS3pYN6j0QMGcwvl6UqSPEblFXtBI2S94K5HhtMGXrF1g0BF0YTnCwSa8uqdeIK pDW+4SXBS9gVw== Message-ID: <54ebdef4205781d3351e4a38e5551046482dbba0.camel@kernel.org> Subject: Re: [PATCH v6 4/5] mm/migrate: skip migrating folios under writeback with AS_WRITEBACK_INDETERMINATE mappings From: Jeff Layton To: David Hildenbrand , Shakeel Butt Cc: Miklos Szeredi , Joanne Koong , Bernd Schubert , Zi Yan , linux-fsdevel@vger.kernel.org, jefflexu@linux.alibaba.com, josef@toxicpanda.com, linux-mm@kvack.org, kernel-team@meta.com, Matthew Wilcox , Oscar Salvador , Michal Hocko Date: Fri, 10 Jan 2025 15:28:33 -0500 In-Reply-To: <1fdc9d50-584c-45f4-9acd-3041d0b4b804@redhat.com> References: <0ed5241e-10af-43ee-baaf-87a5b4dc9694@redhat.com> <446704ab-434e-45ac-a062-45fef78815e4@redhat.com> <791d4056-cac1-4477-a8e3-3a2392ed34db@redhat.com> <1fdc9d50-584c-45f4-9acd-3041d0b4b804@redhat.com> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv egyjnSsFt7EGoDjdKqr1TS9syJYFjagYtvWk/UfHlW09X+jOh4vYtfX7iYSx/NfqV3W1D7EDi0PqV T2h6v8i8YqsATFPwO4nuiTmL6I40ZofxVd+9wdRI4Db8yUNA4ZSP2nqLcLtFjClYRBoJvRWvsv4lm 0OX6MYPtv76hka8lW4mnRmZqqx3UtfHX/hF/zH24Gj7A6sYKYLCU3YrI2Ogiu7/ksKcl7goQjpvtV YrOOI5VGLHge0awt7bhMCTM9KAfPc+xL/ZxAMVWd3NCk5SamL2cE99UWgtvNOIYU8m6EjTLhsj8sn 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 Cu/lRcJh ixIp2rYZDK694HY0lqtv8JGEosNy1YJ9o2Q0lJP0eCbfS7X5uLrdzVfW8grgh8pWO6WteNz3lqeKsc2gOeil44OqpzJXNBRoXkLz9TPVWqu3GkRx/UA8vs8F7Hqfe4yG4Zr0DZwzgtzjGq2ZOCDrb67nMP4Bh51zKgXgbnFWy3gIrGkdA0PS0T7651c6t7pMlOiYdg7RNjA14jCNk/9tEf8YunTEnoB2VBnyJpqsZ90TU3NBu3jeKeoVi/rS5LeywqVFtz95iJIDhvoGh4gkO7ZCjUM08Sbq5r5PVRONOmB0jAY79GLBKCeeJ11Lchws6qICM 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 Thu, 2025-01-09 at 12:22 +0100, David Hildenbrand wrote: > On 07.01.25 19:07, Shakeel Butt wrote: > > On Tue, Jan 07, 2025 at 09:34:49AM +0100, David Hildenbrand wrote: > > > On 06.01.25 19:17, Shakeel Butt wrote: > > > > On Mon, Jan 06, 2025 at 11:19:42AM +0100, Miklos Szeredi wrote: > > > > > On Fri, 3 Jan 2025 at 21:31, David Hildenbrand = wrote: > > > > > > In any case, having movable pages be turned unmovable due to pe= rsistent > > > > > > writaback is something that must be fixed, not worked around. L= ikely a > > > > > > good topic for LSF/MM. > > > > >=20 > > > > > Yes, this seems a good cross fs-mm topic. > > > > >=20 > > > > > So the issue discussed here is that movable pages used for fuse > > > > > page-cache cause a problems when memory needs to be compacted. Th= e > > > > > problem is either that > > > > >=20 > > > > > - the page is skipped, leaving the physical memory block unmov= able > > > > >=20 > > > > > - the compaction is blocked for an unbounded time > > > > >=20 > > > > > While the new AS_WRITEBACK_INDETERMINATE could potentially make t= hings > > > > > worse, the same thing happens on readahead, since the new page ca= n be > > > > > locked for an indeterminate amount of time, which can also block > > > > > compaction, right? > > >=20 > > > Yes, as memory hotplug + virtio-mem maintainer my bigger concern is t= hese > > > pages residing in ZONE_MOVABLE / MIGRATE_CMA areas where there *must = not be > > > unmovable pages ever*. Not triggered by an untrusted source, not trig= gered > > > by an trusted source. > > >=20 > > > It's a violation of core-mm principles. > >=20 > > The "must not be unmovable pages ever" is a very strong statement and w= e > > are violating it today and will keep violating it in future. Any > > page/folio under lock or writeback or have reference taken or have been > > isolated from their LRU is unmovable (most of the time for small period > > of time). >=20 > ^ this: "small period of time" is what I meant. >=20 > Most of these things are known to not be problematic: retrying a couple= =20 > of times makes it work, that's why migration keeps retrying. >=20 > Again, as an example, we allow short-term O_DIRECT but disallow=20 > long-term page pinning. I think there were concerns at some point if=20 > O_DIRECT might also be problematic (I/O might take a while), but so far= =20 > it was not a problem in practice that would make CMA allocations easily= =20 > fail. >=20 > vmsplice() is a known problem, because it behaves like O_DIRECT but=20 > actually triggers long-term pinning; IIRC David Howells has this on his= =20 > todo list to fix. [I recall that seccomp disallows vmsplice by default= =20 > right now] >=20 > These operations are being done all over the place in kernel. > > Miklos gave an example of readahead.=20 >=20 > I assume you mean "unmovable for a short time", correct, or can you=20 > point me at that specific example; I think I missed that. >=20 > > The per-CPU LRU caches are another > > case where folios can get stuck for long period of time. >=20 > Which is why memory offlining disables the lru cache. See=20 > lru_cache_disable(). Other users that care about that drain the LRU on= =20 > all cpus. >=20 > > Reclaim and > > compaction can isolate a lot of folios that they need to have > > too_many_isolated() checks. So, "must not be unmovable pages ever" is > > impractical. >=20 > "must only be short-term unmovable", better? >=20 Still a little ambiguous. How short is "short-term"? Are we talking milliseconds or minutes? Imposing a hard timeout on writeback requests to unprivileged FUSE servers might give us a better guarantee of forward-progress, but it would probably have to be on the order of at least a minute or so to be workable. > >=20 > > The point is that, yes we should aim to improve things but in iteration= s > > and "must not be unmovable pages ever" is not something we can achieve > > in one step. >=20 > I agree with the "improve things in iterations", but as > AS_WRITEBACK_INDETERMINATE has the FOLL_LONGTERM smell to it, I think we= =20 > are making things worse. >=20 > And as this discussion has been going on for too long, to summarize my= =20 > point: there exist conditions where pages are short-term unmovable, and= =20 > possibly some to be fixed that turn pages long-term unmovable (e.g.,=20 > vmsplice); that does not mean that we can freely add new conditions that= =20 > turn movable pages unmovable long-term or even forever. >=20 > Again, this might be a good LSF/MM topic. If I would have the capacity I= =20 > would suggest a topic around which things are know to cause pages to be= =20 > short-term or long-term unmovable/unsplittable, and which can be=20 > handled, which not. Maybe I'll find the time to propose that as a topic. >=20 This does sound like great LSF/MM fodder! I predict that this session will run long! ;) --=20 Jeff Layton