Re: [PATCH] kvm: Faults which trigger IO release the mmap_sem

Andres Lagar-Cavilla
2014-09-16T18:42:17Z

On Tue, Sep 16, 2014 at 11:29 AM, Paolo Bonzini <pbonzini@redhat.com> wrote:
> 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.

Yes, in all likelihood. I hope!

>
>> 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?

Unfortunately get_user_pages does not expose the param (int
*nonblocking) that __gup will use to set FAULT_FLAG_ALLOW_RETRY. So
that's one difference. The second difference is that kvm_gup_retry
will call two times if necessary (the second without _RETRY but with
_TRIED).

Thanks
Andres
>
> Paolo