Messages in this thread | | | Date | Mon, 24 Apr 2023 11:17:55 +0100 | From | Lorenzo Stoakes <> | Subject | Re: [PATCH v2] mm/gup: disallow GUP writing to file-backed mappings by default |
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On Mon, Apr 24, 2023 at 02:43:56AM -0700, Christoph Hellwig wrote: > I'm pretty sure DIRECT I/O reads that write into file backed mappings > are out there in the wild. > > So while I wish we had never allowed this, the exercise seems futile and > instead we need to work on supporting this usecase, with the FOLL_PIN > infrastructure being a big step toward that.
It's not entirely futile, there's at least one specific use case, which is io_uring which is currently open coding an equivalent check themselves. By introducing this change we prevent them from having to do so and provide a means by which other callers who implicitly need this do not have to do so either.
In addition, this change frees up a blocked patch series intending to clean up GUP which should help open the door to further improvements across the system.
So I would argue certainly not futile.
In addition, I think it's useful to explicitly document that this is a broken case and, through use of the flag, highlight places which are problematic (although perhaps not exhaustively).
I know Jason is keen on fixing this at a fundamental level and this flag is ultimately his suggestion, so it certainly doesn't stand in the way of this work moving forward.
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