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

Il 16/09/2014 20:42, Andres Lagar-Cavilla ha scritto:

On Tue, Sep 16, 2014 at 11:29 AM, Paolo Bonzini [off-list ref] wrote:

quoted

quoted

    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).

Yeah, that's how it is in your patch.  I can see that.

What I'm saying is that your patch is two changes in one:

1) do not use gup_fast in hva_to_pfn_slow, instead use gup as in
async_pf_execute.  This change can already introduce a function called
kvm_get_user_page_retry, and can already use it in async_pf_execute and
hva_to_pfn_slow

2) introduce the two-phase RETRY + TRIED mechanism in
kvm_get_user_page_retry, so that the mmap semaphore is relinquished
properly if the filemap or swap has to wait on the page lock.

I would prefer to split it in two patches.  Is it clearer now?

Paolo

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