linux-kernel - Re: [PATCH] mm: Downgrade mmap_sem before locking or populating on mmap

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Message-ID: <20121214144927.GS4939@ZenIV.linux.org.uk>
Date:	Fri, 14 Dec 2012 14:49:27 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Michel Lespinasse <walken@...gle.com>,
	Hugh Dickins <hughd@...gle.com>, J??rn Engel <joern@...fs.org>
Subject: Re: [PATCH] mm: Downgrade mmap_sem before locking or populating on
 mmap

On Fri, Dec 14, 2012 at 03:14:50AM -0800, Andy Lutomirski wrote:

> > Wait a minute.  get_user_pages() relies on ->mmap_sem being held.  Unless
> > I'm seriously misreading your patch it removes that protection.  And yes,
> > I'm aware of execve-related exception; it's in special circumstances -
> > bprm->mm is guaranteed to be not shared (and we need to rearchitect that
> > area anyway, but that's a separate story).
> 
> Unless I completely screwed up the patch, ->mmap_sem is still held for
> read (it's downgraded from write).  It's just not held for write
> anymore.

Huh?  I'm talking about the call of get_user_pages() in aio_setup_ring().
With your patch it's done completely outside of ->mmap_sem, isn't it?
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