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dkim=pass header.d=Nvidia.com header.s=selector2 header.b=gXCpPFyy; spf=none (imf19.hostedemail.com: domain of jhubbard@nvidia.com has no SPF policy when checking 40.107.237.79) smtp.mailfrom=jhubbard@nvidia.com; dmarc=pass (policy=reject) header.from=nvidia.com X-HE-Tag: 1646272040-409894 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 3/2/22 12:38, David Hildenbrand wrote: ... > BUT, once we actually write to the private mapping via the page table, > the GUP pin would go out of sync with the now-anonymous page mapped into > the page table. However, I'm having a hard time answering what's > actually expected? > > It's really hard to tell what the user wants with MAP_PRIVATE file > mappings and stumbles over a !anon page (no modifications so far): > > (a) I want a R/O pin to observe file modifications. > (b) I want the R/O pin to *not* observe file modifications but observe > my (eventual? if any) private modifications, > On this aspect, I think it is easier than trying to discern user intentions. Because it is less a question of what the user wants, and more a question of how mmap(2) is specified. And the man page clearly indicates that the user has no right to expect to see file modifications. Here's the excerpt: "MAP_PRIVATE Create a private copy-on-write mapping. Updates to the mapping are not visible to other processes mapping the same file, and are not carried through to the underlying file. It is unspecified whether changes made to the file after the mmap() call are visible in the mapped region. " > Of course, if we already wrote to that page and now have an anon page, > it's easy: we are already no longer following file changes. Yes, and in fact, I've always thought that the way this was written means that it should be treated as a snapshot of the file contents, and no longer reliably connected in either direction to the page(s). thanks, -- John Hubbard NVIDIA