Messages in this thread | | | From | Jann Horn <> | Date | Mon, 21 Sep 2020 23:55:06 +0200 | Subject | Re: [PATCH 4/5] mm: Do early cow for pinned pages during fork() for ptes |
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On Mon, Sep 21, 2020 at 11:20 PM Peter Xu <peterx@redhat.com> wrote: > This patch is greatly inspired by the discussions on the list from Linus, Jason > Gunthorpe and others [1]. > > It allows copy_pte_range() to do early cow if the pages were pinned on the > source mm. Currently we don't have an accurate way to know whether a page is > pinned or not. The only thing we have is page_maybe_dma_pinned(). However > that's good enough for now. Especially, with the newly added mm->has_pinned > flag to make sure we won't affect processes that never pinned any pages.
To clarify: This patch only handles pin_user_pages() callers and doesn't try to address other GUP users, right? E.g. if task A uses process_vm_write() on task B while task B is going through fork(), that can still race in such a way that the written data only shows up in the child and not in B, right?
I dislike the whole pin_user_pages() concept because (as far as I understand) it fundamentally tries to fix a problem in the subset of cases that are more likely to occur in practice (long-term pins overlapping with things like writeback), and ignores the rarer cases ("short-term" GUP).
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