Re: [TOPIC] Last iput() from flusher thread, last fput() from munmap()...

[Steven Whitehouse <swhiteho@redhat.com>]

Hi,

On Wed, 2012-03-28 at 03:38 +0100, Al Viro wrote:
> On Tue, Mar 27, 2012 at 11:08:58PM +0200, Jan Kara wrote:
> >   Hello,
> > 
> >   maybe the name of this topic could be "How hard should be life of
> > filesystems?" but that's kind of broad topic and suggests too much of
> > bikeshedding. I'd like to concentrate on concrete possible pain points
> > between filesystems & VFS (possibly writeback or even generally MM).
> > Lately, I've myself came across the two issues in $SUBJECT:
> > 1) dropping of last file reference can happen from munmap() and in that
> >    case mmap_sem will be held when ->release() is called. Even more it
> >    could be held when ->evict_inode() is called to delete inode because
> >    inode was unlinked.
> 
> Yes, it can.
> 
> > 2) since flusher thread takes inode reference when writing inode out, the
> >    last inode reference can be dropped from flusher thread. Thus inode may
> >    get deleted in the flusher thread context. This does not seem that
> >    problematic on its own but if we realize progress of memory reclaim
> >    depends (at least from a longterm perspective) on flusher thread making
> >    progress, things start looking a bit uncertain. Even more so when we
> >    would like avoid ->writepage() calls from reclaim and let flusher thread
> >    do the work instead. That would then require filesystems to carefully
> >    design their ->evict_inode() routines so that things are not
> >    deadlockable.
> 
> You mean "use GFP_NOIO for allocations when holding fs-internal locks"?
> 
> >   Both these issues should be avoidable (we can postpone fput() after we
> > drop mmap_sem; we can tweak inode refcounting to avoid last iput() from
> > flusher thread) but obviously there's some cost in the complexity of generic
> > layer. So the question is, is it worth it?
> 
> I don't thing it is.  ->i_mutex in ->release() is never needed; existing
> cases are racy and dropping preallocation that way is simply wrong.  And
> ->evict_inode() is a non-issue, since it has no reason whatsoever to take
> *any* locks in mutex - the damn thing is called when nobody has references
> to struct inode anymore.  Deadlocks with flusher... that's what NOIO and
> NOFS are for.
> 
For cluster filesystems, we have to take locks (cluster wide) in
->evict_inode() in order to establish for certain whether we are the
last opener of the inode. Just because there are no references on the
local node, doesn't mean that a remote node doesn't hold the file open
still.

We do always use GFP_NOFS when allocating memory while holding such
locks, so I'm not quite sure from the above whether or not that will be
an issue,

Steve.


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