Re: [PATCH 4/6] xfs: use memalloc_nofs_{save,restore} instead of memalloc_noio*

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Mon, Feb 06, 2017 at 07:47:43PM +0100, Michal Hocko wrote:
> On Mon 06-02-17 10:32:37, Darrick J. Wong wrote:
> > On Mon, Feb 06, 2017 at 06:44:15PM +0100, Michal Hocko wrote:
> > > On Mon 06-02-17 07:39:23, Matthew Wilcox wrote:
> > > > On Mon, Feb 06, 2017 at 03:07:16PM +0100, Michal Hocko wrote:
> > > > > +++ b/fs/xfs/xfs_buf.c
> > > > > @@ -442,17 +442,17 @@ _xfs_buf_map_pages(
> > > > >  		bp->b_addr = NULL;
> > > > >  	} else {
> > > > >  		int retried = 0;
> > > > > -		unsigned noio_flag;
> > > > > +		unsigned nofs_flag;
> > > > >  
> > > > >  		/*
> > > > >  		 * vm_map_ram() will allocate auxillary structures (e.g.
> > > > >  		 * pagetables) with GFP_KERNEL, yet we are likely to be under
> > > > >  		 * GFP_NOFS context here. Hence we need to tell memory reclaim
> > > > > -		 * that we are in such a context via PF_MEMALLOC_NOIO to prevent
> > > > > +		 * that we are in such a context via PF_MEMALLOC_NOFS to prevent
> > > > >  		 * memory reclaim re-entering the filesystem here and
> > > > >  		 * potentially deadlocking.
> > > > >  		 */
> > > > 
> > > > This comment feels out of date ... how about:
> > > 
> > > which part is out of date?
> > > 
> > > > 
> > > > 		/*
> > > > 		 * vm_map_ram will allocate auxiliary structures (eg page
> > > > 		 * tables) with GFP_KERNEL.  If that tries to reclaim memory
> > > > 		 * by calling back into this filesystem, we may deadlock.
> > > > 		 * Prevent that by setting the NOFS flag.
> > > > 		 */
> > > 
> > > dunno, the previous wording seems clear enough to me. Maybe little bit
> > > more chatty than yours but I am not sure this is worth changing.
> > 
> > I prefer to keep the "...yet we are likely to be under GFP_NOFS..."
> > wording of the old comment because it captures the uncertainty of
> > whether or not we actually are already under NOFS.  If someone actually
> > has audited this code well enough to know for sure then yes let's change
> > the comment, but I haven't gone that far.

Ugh, /me hands himself another cup of coffee...

Somehow I mixed up _xfs_buf_map_pages and kmem_zalloc_large in this
discussion.  Probably because they have similar code snippets with very
similar comments to two totally different parts of xfs.

The _xfs_buf_map_pages can be called inside or outside of
transaction context, so I think we still have to memalloc_nofs_save for
that to avoid the lockdep complaints and deadlocks referenced in the
commit that added all that (to _xfs_buf_map_pages) in the first place.
ae687e58b3 ("xfs: use NOIO contexts for vm_map_ram")

My comments about kmem_zalloc_large still stand, even though the part
of the patch you two were discussing was the _xfs_buf_map_pages.  I
probably should have clarified that I think both functions actually
/are/ doing the right thing wrt calling (or not calling)
memalloc_nofs_save().

> I believe we can drop the memalloc_nofs_save then as well because either
> we are called from a potentially dangerous context and thus we are in
> the nofs scope we we do not need the protection at all.

Uh, now that I've muddied up the waters, which part are you referring to?

--D

> -- 
> Michal Hocko
> SUSE Labs
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