lkml.org 
[lkml]   [2010]   [Dec]   [1]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [PATCH v2 3/3] NFS: Fix a memory leak in nfs_readdir
From
Date
On Wed, 2010-12-01 at 14:38 -0800, Andrew Morton wrote:
> 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.
>

Yes, that was my question.

However, Linus' explanation appears to answer one of Hugh's objections:
we can apparently use a preempt-disable() in order to ensure that the
module containing the mapping->a_ops is not unloaded until after the
->freepage() call is complete.

That would imply that ->freepage() cannot sleep, but I don't think that
is too nasty a restriction.

--
Trond Myklebust
Linux NFS client maintainer

NetApp
Trond.Myklebust@netapp.com
www.netapp.com



\
 
 \ /
  Last update: 2010-12-01 23:49    [from the cache]
©2003-2011 Jasper Spaans