Date: Wed, 31 May 2006 16:09:06 +0100 (BST)
From: Hugh Dickins <hugh@veritas.com>
To: Nick Piggin <nickpiggin@yahoo.com.au>
cc: Jens Axboe <axboe@suse.de>, Andrew Morton <akpm@osdl.org>,
       linux-kernel@vger.kernel.org, linux-mm@kvack.org, mason@suse.com,
       andrea@suse.de, torvalds@osdl.org
Subject: Re: [rfc][patch] remove racy sync_page?
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On Wed, 31 May 2006, Nick Piggin wrote:
> Jens Axboe wrote:
> > 
> > Maybe I'm being dense, but I don't see a problem there. You _should_
> > call the new mapping sync page if it has been migrated.
> 
> But can some other thread calling lock_page first find the old mapping,
> and then run its ->sync_page which finds the new mapping? While it may
> not matter for anyone in-tree, it does break the API so it would be
> better to either fix it or rip it out than be silently buggy.

Splicing a page from one mapping to another is rather worrying/exciting,
but it does look safely done to me.  remove_mapping checks page_count
while page lock and old mapping->tree_lock are held, and gives up if
anyone else has an interest in the page.  And we already know it's
unsafe to lock_page without holding a reference to the page, don't we?

Hugh
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