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

Paolo Bonzini
2014-09-16T18:29:13Z

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