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t=1731404914; bh=LrtAQhxTOMENsDkUTMA8xkgddOLcwv2DtqWpfLXRdSo=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=izgjO7xQF0rD6dbjle7potQOfb9o9kMeJg2fNMJf8M1tniYhJhTTkLi9xDlxySdyc as71DhG+oYTmbbnKJZi+c1pgLLJn5MIyXyTvXjwDIbn1OM34NrlPw/Y6r2M2AhPptT UpcwHIaPq80Dua/+/TLdHrFwMQti+IV+pwkGVnxOl/HYQeGmgCZN8YiI90MakOj4R7 sWW66AMdHFfwtUScOnZQw9Q2NTCrzvo3ekF+2f4F0H+1PLW/dcKT+c9f+vo5Fxa5FO f5BXMI1vpr7SmkA5NsgJjzx1xz4Dsjw6uWUFTmhKo9eryVEY9wSLk4Tn/Y3LFdVKTZ o6zLkpX+KOdYA== Message-ID: <821d15f9-233e-4b9d-8194-5de1835113c7@asahilina.net> Date: Tue, 12 Nov 2024 18:48:33 +0900 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm: Fix __wp_page_copy_user fallback path for remote mm To: Alistair Popple , David Hildenbrand Cc: Andrew Morton , Sergio Lopez Pascual , linux-mm@kvack.org, linux-kernel@vger.kernel.org, asahi@lists.linux.dev, Dan Williams References: <20241101-mm-remote-pfn-v1-1-080b609270b7@asahilina.net> <2d8380b9-3d03-4263-b5bf-7e0227c83ba9@asahilina.net> <0977a33b-8318-43a5-a5a1-4eb8c93ca270@redhat.com> <64d386e8-6684-4213-8aba-7d1daf94f2cf@asahilina.net> <412298ff-80bc-4111-8c72-29a5263a5d32@redhat.com> <87ttceu0i8.fsf@nvdebian.thelocal> Content-Language: en-US From: Asahi Lina In-Reply-To: <87ttceu0i8.fsf@nvdebian.thelocal> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: 5C0D314001D X-Stat-Signature: w8tj3wo7r6iccbncu8mjkpunxc5dac4g X-Rspam-User: X-HE-Tag: 1731404891-152285 X-HE-Meta: U2FsdGVkX18OGdI+u1HvqxWQoccyN+C1TUz8USzfo67ztsIp9qEK6spnMGpg9mpaRRv7C4Qgk0BjBRele+tzAWIQwaiMPlHqIaHTcnDTqQvvmDdFQT8du/pyEqKkfb1N+bI34d3ochJgo5V0HVxY57wW+fVY1Bkc3qec/x2PnwJT2xfYhnnq7z2NK1KzvyJ/mV7ccm8v0o2r7DVP3obEMIMeFae2NsoKL0OKkZlvaRlOKFeWsNWsI6MYZw3KA+P2InIWlXRIZzwPhjXRTUp9M/1C+pAcvZIW+3JSc0qAhgq/rAN4WAJ4e5CfnQ5olDFoPyND1YRuFQwd99ziVjcDyowuGW3Hfc4X++fx94o1Xd8p8n8pFtODqrBQ1KE9/mzJE7vsl02A2E3tAz8ByJ6geFjtVqLxvjvK7X4/UViDc+p6wsC/UBUOhHASRL4QU1wx2Ptnbb6OhJiY0Y3egfUqkZNoMFJLJRQYADTH4sm5lrADIWf7NCJ5zWUbqW25mO2vNMGO8wYw6kK7G7yw7/KoeKk6I2iIAQHO5lgZ/iMEsDT7LzM+gOaIId13S4KOnIHJhk9KAoKpsw6C62th1fwdgj9ta4vCfx8iaNNxx4wsVkCUgccBTqIk2Fk2GZz66T1yObhnqs7AQ4qIoPQiDORgAsjYLlT8isNIzQ0QtXzKw6FgKUUdfdLmeXzQdrUmCdrunvd3swklEWCCKiSPy+13SQtVF4WUe3SW1BBoWkWBhlY88lMBu8moQ5RUSXlqMUlHgJy9MEKL568nwSIa8K6zNRAphw+vjyB8TKY7NNKAvZgoriVxRvfLr7CIHsz/3xoyp/u6M5fb6GirSy/Vxll6FJ0esd9+CihML7huM/V95jS++qm/YZEhNur8cHpT6YSLz/XtMCdoFTkcxzxl/HqFiZJT3+DegmzcA7wyJ24XvLdLsbuxw8s8CvDCoxr1THzxVO1BBS2nc4zHhd54gGa DTXJJ6cZ bsQg3lpul1E8fNn1OHMVfqfLwhq0+MY3vPGesW6srpgXwnOAKmgl4P1lbenZiB6tkVarLuLNck3GwbflFl0HY5M16R2fXYyoch8xtr6C7uUpLA/uuy62DKJCe2OAb5SRsuvCBn51eJJau+WSFjpEl+sjoT+Nb4A1zvaLlyTIrXWTleJ7o+A9Z+Pb+bkYrBCgRXoVjl4XSTMOVOpmD0SrsKKMx2epi24AhhAOVBW84aE3Gxk9veNYUDSuMYzYnw7jheglnHm0y8tVPHk+90YxcTBMPBl2gQuNi0Q5lmJm6R3xYApk7QrFS2pN2FYdDnNCk17oqJf4Ka/VPh8o1yC8qonTaLbRzWpfZ1zn/ X-Bogosity: Ham, tests=bogofilter, spamicity=0.000002, 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 11/11/24 8:24 AM, Alistair Popple wrote: > > David Hildenbrand writes: > >> On 07.11.24 18:32, Asahi Lina wrote: >>> On 11/8/24 2:14 AM, David Hildenbrand wrote: >>>> I recall that there is still a problem with false-positives on >>>> folio_test_anon() with ZONE_DEVICE pages, so it's maybe not that >>>> easy ... and the whole get_dev_pagemap() stuff is nasty. > > Specifically FS DAX reuses PAGE_MAPPING_ANON in > include/linux/page-flags.h > > /* > * Different with flags above, this flag is used only for fsdax mode. It > * indicates that this page->mapping is now under reflink case. > */ > #define PAGE_MAPPING_DAX_SHARED ((void *)0x1) > > FS DAX pages are never anon though, so you could probably test for > !vma_is_dax() and/or add an implementation of is_fsdax_page(). > >>>> Likely we would have to do what GUP does, and temporarily grab a pgmap >>>> reference. Gah. >>>> >>>> >>>> So if we sort out the pagemap stuff and the possibly wrong >>>> folio_test_anon() on some ZONE_DEVICE pages (but not all, because IIRC >>>> DEVICE_PRIVATE can be anon ...), it might be doable. > > Correct, DEVICE_PRIVATE and DEVICE_COHERENT pages are always anon (at > least for now). > >>>> But it sounds ugly, especially because that code might change soon and >>>> not require messing with ZONE_DEVICE pages on that level. > > Yes, I'm hopoing to get the next version of that series posted this > week. I found a couple of other FS DAX bugs that slowed me down. > > - Alistair > >>>> And then, we'd not be able to handle VM_PFNMAP cleanly ... >>>> If this is all going to be fixed another way soon then I think there's no rush to get a workaround in earlier than that, I just don't want it to fall by the wayside. We have my original patch downstream in libkrunfw (which despite the lockdep complaints does work in practice), so we can keep that in until the proper solution lands upstream. Alistair, can you Cc me on future submissions? So I'm reminded to test this out (FWIW the case I'm interested in is just `gdb /bin/foo` with the whole root filesystem on DAX, though I imagine a statically linked foo on a DAX mount by itself is probably an equivalent repro). ~~ Lina