Messages in this thread | | | Date | Mon, 14 Sep 2020 20:28:51 -0300 | From | Jason Gunthorpe <> | Subject | Re: [PATCH 1/4] mm: Trial do_wp_page() simplification |
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On Mon, Sep 14, 2020 at 03:59:31PM -0700, Linus Torvalds wrote: > On Mon, Sep 14, 2020 at 3:55 PM Jason Gunthorpe <jgg@nvidia.com> wrote: > > > > Just as an aside, the RDMA stuff is also supposed to set MADV_DONTFORK > > on these regions, so I'm a bit puzzled what is happening here. > > Did the fork perhaps happen _before_ , so the pages are shared when > you do the pin?
Looking at the progam, it seems there are a number of forks for exec before and after pin_user_pages_fast(), but the parent process always does waitpid() after the fork.
> MADV_DONTFORK doesn't mean COW doesn't happen. It just means that the > next fork() won't be copying that memory area.
Yes, this stuff does pin_user_pages_fast() and MADV_DONTFORK together. It sets FOLL_FORCE and FOLL_WRITE to get an exclusive copy of the page and MADV_DONTFORK was needed to ensure that a future fork doesn't establish a COW that would break the DMA by moving the physical page over to the fork. DMA should stay with the process that called pin_user_pages_fast() (Is MADV_DONTFORK still needed with recent years work to GUP/etc? It is a pretty terrible ancient thing)
> That said, it's possible that the test cases do something invalid - or > maybe we've broken MADV_DONTFORK - and it all just happened to work > before.
Hmm. If symptoms stop with this patch should we investigate MADV_DONTFORK?
Thanks, Jason
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