Il 16/09/2014 18:52, Andres Lagar-Cavilla ha scritto:

Was this:

        down_read(&mm->mmap_sem);
        npages = get_user_pages(NULL, mm, addr, 1, 1, 0, NULL, NULL);
        up_read(&mm->mmap_sem);

the intention rather than get_user_pages_fast?

I meant the intention of the original author, not yours.

By that point in the call chain I felt comfortable dropping the _fast.
All paths that get there have already tried _fast (and some have tried
_NOWAIT).

Yes, understood.

    I think a first patch should introduce kvm_get_user_page_retry ("Retry a
    fault after a gup with FOLL_NOWAIT.") and the second would add
    FOLL_TRIED ("This properly relinquishes mmap semaphore if the
    filemap/swap has to wait on page lock (and retries the gup to completion
    after that").

That's not what FOLL_TRIED does. The relinquishing of mmap semaphore is
done by this patch minus the FOLL_TRIED bits. FOLL_TRIED will let the
fault handler (e.g. filemap) know that we've been there and waited on
the IO already, so in the common case we won't need to redo the IO.

Yes, that's not what FOLL_TRIED does.  But it's the difference between
get_user_pages and kvm_get_user_page_retry, right?

Paolo

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