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    SubjectRe: [PATCH 1/4] mm/gup: Add FOLL_INTERRUPTIBLE
    On Tue, Jun 28, 2022 at 05:31:43PM -0700, John Hubbard wrote:
    > On 6/28/22 15:33, Peter Xu wrote:
    > > > The key point is the connection between "locked" and killable. If the comment
    > > > explained why "locked" means "killable", that would help clear this up. The
    > > > NOWAIT sentence is also confusing to me, and adding "mostly NOWAIT" does not
    > > > clear it up either... :)
    > >
    > > Sorry to have a comment that makes it feels confusing. I tried to
    > > explicitly put the comment to be after setting FAULT_FLAG_KILLABLE but
    > > obviously I didn't do my job well..
    > >
    > > Maybe that NOWAIT thing adds more complexity but not even necessary.
    > >
    > > Would below one more acceptable?
    > >
    > > /*
    > > * We'll only be able to respond to signals when "locked !=
    > > * NULL". When with it, we'll always respond to SIGKILL
    > > * (as implied by FAULT_FLAG_KILLABLE above), and we'll
    > > * respond to non-fatal signals only if the GUP user has
    > > * specified FOLL_INTERRUPTIBLE.
    > > */
    > It looks like part of this comment is trying to document a pre-existing
    > concept, which is that faultin_page() only ever sets FAULT_FLAG_KILLABLE
    > if locked != NULL.

    I'd say that's not what I wanted to comment.. I wanted to express that
    INTERRUPTIBLE should rely on KILLABLE, that's also why I put the comment to
    be after KILLABLE, not before. IMHO it makes sense already to have
    "interruptible" only if "killable", no matter what's the pre-requisite for
    KILLABLE (in this case it's having "locked" being non-null).

    > The problem I am (personally) having is that I don't yet understand why
    > or how those are connected: what is it about having locked non-NULL that
    > means the process is killable? (Can you explain why that is?)

    Firstly RETRY_KILLABLE relies on ALLOW_RETRY, because if we don't allow
    retry at all it means we'll never wait in handle_mm_fault() anyway, then no
    need to worry on being interrupted by any kind of signal (fatal or not).

    Then if we allow retry, we need some way to know "whether mmap_sem is
    released or not" during the process for the caller (because the caller
    cannot see VM_FAULT_RETRY). That's why we added "locked" parameter, so
    that we can set *locked=false to tell the caller we have released mmap_sem.

    I think that's why we have "locked" defined as "we allow this page fault
    request to retry and wait, during wait we can always allow fatal signals".
    I think that's defined throughout the gup call interfaces too, and
    faultin_page() is the last step to talk to handle_mm_fault().

    To make this whole picture complete, NOWAIT is another thing that relies on
    ALLOW_RETRY but just to tell "oh please never release the mmap_sem at all".
    For example, when we want to make sure no vma will be released after
    faultin_page() returned.

    > If that were clear, I think I could suggest a good comment wording.

    IMHO it's a little bit weird to explain "locked" here, especially after
    KILLABLE is set, that's why I didn't try to mention "locked" in my 2nd
    attempt. There are some comments for "locked" above the definition of
    faultin_page(), I think that'll be a nicer place to enrich explanations for
    "locked", and it seems even more suitable as a separate patch?


    Peter Xu

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