Re: [PATCH v2 3/3] NFS: Fix a memory leak in nfs_readdir

[Trond Myklebust]

On Wed, 2010-12-01 at 13:38 -0800, Andrew Morton wrote:
> On Wed, 1 Dec 2010 13:15:07 -0800 (PST)
> Hugh Dickins <hughd@xxxxxxxxxx> wrote:
> 
> > On Wed, 1 Dec 2010, Hugh Dickins wrote:
> > > On Wed, 1 Dec 2010, Andrew Morton wrote:
> > > > On Wed, 01 Dec 2010 15:10:50 -0500
> > > > Trond Myklebust <Trond.Myklebust@xxxxxxxxxx> wrote:
> > > > 
> > > > > --- a/include/linux/fs.h
> > > > > +++ b/include/linux/fs.h
> > > > > @@ -602,6 +602,7 @@ struct address_space_operations {
> > > > >  	sector_t (*bmap)(struct address_space *, sector_t);
> > > > >  	void (*invalidatepage) (struct page *, unsigned long);
> > > > >  	int (*releasepage) (struct page *, gfp_t);
> > > > > +	void (*freepage)(struct page *);
> > > > >  	ssize_t (*direct_IO)(int, struct kiocb *, const struct iovec *iov,
> > > > >  			loff_t offset, unsigned long nr_segs);
> > > > >  	int (*get_xip_mem)(struct address_space *, pgoff_t, int,
> > > > 
> > > > It would be good to think about and then clearly spell out exactly what
> > > > state the page is in here.  It is locked, and I assume clean and not
> > > > under writeback.  What about its refcount, freezedness status and
> > > > eligibility for lookups?
> > > > 
> > > > And as Hugh pointed out, some callees might needs the address_space*
> > > > although we can perhaps defer that until such a callee turns up. 
> > > > If/when that happens we might have a problem though: if this locked
> > > > page is no longer attached to the address_space then what now pins the
> > > > address_space, protecting it from inode reclaim?
> > > 
> > > That's an excellent point and trumps mine: it would be actively wrong
> > > to provide the struct address_space *mapping arg I was asking for.
> > > (Bet someone then tries stashing it away via page->private though.)
> > 
> > Hmm, thinking further along the same lines: can we even guarantee that
> > the filesystem module is still loaded at that point?  i.e. might
> > mapping->freepage now be pointing off into the garbage heap?
> 
> I don't see anything on the VFS side which would prevent a module
> unload.  Or, more realistically, a concurrent unmount, freeing of the
> superblock and everything associated with it.  All we have here is a
> page*.
> 
> 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()?

I'm clearly missing whatever mechanism prevents iput_final() from racing
with vmscan if the latter clears out the last page from the mapping.

-- 
Trond Myklebust
Linux NFS client maintainer

NetApp
Trond.Myklebust@xxxxxxxxxx
www.netapp.com

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