Date: Wed, 1 Dec 2010 14:38:56 -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 14:24:42 -0800
Linus Torvalds <torvalds@linux-foundation.org> wrote:

> On Wed, Dec 1, 2010 at 2:13 PM, Andrew Morton <akpm@linux-foundation.org> wrote:
> > On Wed, 01 Dec 2010 16:51:12 -0500
> > Trond Myklebust <Trond.Myklebust@netapp.com> wrote:
> >
> >> On Wed, 2010-12-01 at 13:38 -0800, Andrew Morton wrote:
> >> > Probably on most call paths we'll be OK - if a process is in the middle
> >> > of a file truncate, holdin a file* ref which holds an inode ref then
> >> > nobody will be unmounting that fs and hence nobody will be unloading
> >> > that module.
> >> >
> >> > However on the random_code->alloc_page->vmscan->releasepage path, none
> >> > of that applies.
> >>
> >> Just out of interest, what ensures that the mapping is still around for
> >> the 'spin_unlock_irq(&mapping->tree_lock);' in __remove_mapping()?
> >
> > Nothing, afacit.
> 
> No, we're good.
> 
> Module unload has to go through a "stop_machine()" cycle, and that in
> turn requires an idle period for everything. And just a preemption
> reschedule isn't enough for that.
> 
> So what is sufficient is that
> 
>  - we had the page locked and on the mapping
> 
>    This implies that we had an inode reference to the module, and the
> page lock means that the inode reference cannot go away (because it
> will involve invalidate-pages etc)
> 
>  - we're not sleeping after __remove_mapping, so unload can't happen afterwards.
> 
> A _lot_ of the module races depend on that latter thing. We have
> almost no cases that are strictly about actual reference counts etc.
> 

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.