Date: Wed, 1 Dec 2010 15:36:08 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Trond Myklebust <Trond.Myklebust@netapp.com>,
        Hugh Dickins <hughd@google.com>, Nick Piggin <npiggin@kernel.dk>,
        Nick Bowler <nbowler@elliptictech.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-nfs@vger.kernel.org, Rik van Riel <riel@redhat.com>,
        Christoph Hellwig <hch@lst.de>, Al Viro <viro@zeniv.linux.org.uk>
Subject: Re: [PATCH v2 3/3] NFS: Fix a memory leak in nfs_readdir
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On Wed, 1 Dec 2010 15:31:11 -0800
Linus Torvalds <torvalds@linux-foundation.org> wrote:

> On Wed, Dec 1, 2010 at 2:38 PM, Andrew Morton <akpm@linux-foundation.org> wrote:
> >
> > OK, the stop_machine() plugs a lot of potential race-vs-module-unload
> > things. __But Trond is referring to races against vmscan inode reclaim,
> > unmount, etc.
> 
> So?
> 
> A filesystem module cannot be unloaded while it's still mounted.
> 
> And unmount doesn't succeed until all inodes are gone.
> 
> And getting rid of an inode doesn't succeed until all pages associated
> with it are gone.
> 
> And getting rid of the pages involves locking them (whether in
> truncate or vmscan) and removing them from all lists.
> 
> Ergo: vmscan has a locked page leads to the filesystem being
> guaranteed to not be unmounted.  And that, in turn, guarantees that
> the module won't be unloaded until the machine has gone through an
> idle cycle.

The page isn't attached to the address_space any more:

static int __remove_mapping(struct address_space *mapping, struct page *page)
{
	...
		__remove_from_page_cache(page);
		spin_unlock_irq(&mapping->tree_lock);
	...
}