Re: [BUG?] possible recursive locking detected

[Nick Piggin <nickpiggin@yahoo.com.au>]

Anton Altaparmakov wrote:
> On Thu, 2006-07-27 at 19:18 +1000, Nick Piggin wrote:
>
> > Anton Altaparmakov wrote:

> > > I beg to differ.  It is a bug.  You cannot reenter the file system when
> > > the file system is trying to allocate memory.  Otherwise you can never
> > > allocate memory with any locks held or you are bound to introduce an
> > > A->B B->A deadlock somewhere.
> >
> > I don't think it is a bug in general. It really depends on the allocation:
> >
> > - If it is a path that might be required in order to writeout a page, then
> > yes GFP_NOFS is going to help prevent deadlocks.
> >
> > - If it is a path where you'll take the same locks as page reclaim requires,
> > then again GFP_NOFS is required.
> >
> > For NTFS case, it seems like holding i_mutex on the write path falls foul
> > of the second problem. But I agree with Andrew that this is a critical case
> > where we do have to enter the fs. GFP_NOFS is too big a hammer to use.
> >
> > I guess you'd have to change NTFS to do something sane privately, or come
> > up with a nice general solution that doesn't harm the common filesystems
> > that apparently don't have a problem here... can you just add GFP_NOFS to
> > NTFS's mapping_gfp_mask to start with?
>
>
> I don't think NTFS has a problem either.  It is a theoretical problem

No, I mean: *really* doesn't have a problem. If Andrew says ext2 doesn't
need i_mutex in reclaim, then I tend to believe him.

> with an extremely small chance of being hit.  I am happy to have such a
> problem for now.  There are more pressing problems to solve.  The only
> thing that needs to happen is for the messages to stop so people stop
> complaining / getting worried about them...

I guess the memory deadlock issue is probably mostly theoretical, although
it is still nice to get them fixed. I'd imagine a deadlock condition -- if
one really exists -- could be hit without much problem though. Page reclaim
will readily get kicked from the write(2) path, and will potentially free
*lots* of stuff from there.

If it isn't a problem for you, I'd suspect it might be due to some other
conditions which happen to mean it is avoided. For example, the inode who's
i_mutex you are holding will have a raised refcount AFAIK, so it will not
get reclaimed and so may get around your problem.

This would be a valid solution IMO. It probably could do with documentation
to outline the issues, though.

--
SUSE Labs, Novell Inc.
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