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b=y7Z0xcF0WSAb1tRIWCPPRT9vYYKkGOQ58+/GAuPZNz2slmC4A7hKorrKTaMwoAizuFO2B7 ca04qXHliynZ6wCOynjd71439EZlJf2ZkRnXnYEl4vCExcI3Yeada8+ojjGevLMZu/Hq2S IfaiYxUCLYIUocqkjr9EvAQW0/fPWSo= X-Stat-Signature: hn8pzojqctz41ufgx8ta3w1pkjzxsgyk X-Rspamd-Queue-Id: DFED14001A X-Rspam-User: Authentication-Results: imf17.hostedemail.com; dkim=pass header.d=Nvidia.com header.s=selector2 header.b=ZIgdOO9p; dmarc=pass (policy=reject) header.from=nvidia.com; arc=pass ("microsoft.com:s=arcselector9901:i=1"); spf=pass (imf17.hostedemail.com: domain of jhubbard@nvidia.com designates 40.107.237.67 as permitted sender) smtp.mailfrom=jhubbard@nvidia.com X-Rspamd-Server: rspam09 X-HE-Tag: 1667076148-267156 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: On 10/29/22 13:30, Linus Torvalds wrote: >> I can think of three options: >> >> (a) filesystems just deal with it >> >> (b) we could move the "page_remove_rmap()" into the "flush-and-free" path too >> >> (c) we could actually add a spinlock (hashed on the page?) for this >> >> I think (a) is basically our current expectation. > > Side note: anybody doing gup + set_page_dirty() won't be fixed by b/c > anyway, so I think (a) is basically the only thing. > > And that's true even if you do a page pinning gup, since the source of > the gup may be actively unmapped after the gup. I was just now writing a response that favored (c) over (b), precisely because of that, yes. :) > > So a filesystem that thinks that only write, or a rmap-accessible mmap > can turn the page dirty really seems to be fundamentally broken. > > And I think that has always been the case, it's just that filesystem > writers may not have been happy with it, and may not have had > test-cases for it. > > It's not surprising that the filesystem people then try to blame users. > > Linus Yes, lots of unhappy debates about this over the years. However, I remain intrigued by (c), because if we had a "dirty page lock" that is looked up by page (much like looking up the ptl), it seems like a building block that would potentially help solve the whole thing. The above points about "file system needs to coordinate with mm about what's allowed to be dirtied, including gup/dma cases", those are still true and not yet solved, yes. But having a solid point of synchronization for this, definitely looks interesting. Of course, without working through this more thoroughly, it's not fair to impose this constraint on the current discussion, understood. :) thanks, -- John Hubbard NVIDIA