linux-kernel - Re: [PATCH] kvm: Faults which trigger IO release the mmap

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Date:	Tue, 16 Sep 2014 20:29:13 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Andres Lagar-Cavilla <andreslc@...gle.com>
CC:	Gleb Natapov <gleb@...hat.com>, Rik van Riel <riel@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Mel Gorman <mgorman@...e.de>,
	Andy Lutomirski <luto@...capital.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	Jianyu Zhan <nasa4836@...il.com>,
	Paul Cassella <cassella@...y.com>,
	Hugh Dickins <hughd@...gle.com>,
	Peter Feiner <pfeiner@...gle.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] kvm: Faults which trigger IO release the mmap_sem

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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