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[2003:cb:c739:8e00:7a46:1b8c:8b13:d3d]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-381eda0411csm14784067f8f.95.2024.11.12.02.00.28 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 12 Nov 2024 02:00:28 -0800 (PST) Message-ID: <50578e9d-6562-4f0f-af7c-cabfb875bbb7@redhat.com> Date: Tue, 12 Nov 2024 11:00:27 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm: Fix __wp_page_copy_user fallback path for remote mm To: Asahi Lina , Alistair Popple 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> <821d15f9-233e-4b9d-8194-5de1835113c7@asahilina.net> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 12.11.24 10:48, Asahi Lina wrote: > > > 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) I assume it's sufficient to deadlock when a writer pops up after you succeeded with the first read-locking, and before you start the second read-locking. IIRC, rwsem is a fair lock, so read-locking when already-read-locked is not guaranteed to work. That's why lockdep complains. -- Cheers, David / dhildenb