Re: [External] Re: [PATCH 2/3] include/linux/gfp.h: use unsigned int in gfp_zone

[David Sterba <dsterba@suse.cz>]

On Tue, May 15, 2018 at 04:54:04AM -0700, Matthew Wilcox wrote:
> > > Subject: btrfs: Allocate extents from ZONE_NORMAL
> > > From: Matthew Wilcox <mawilcox@microsoft.com>
> > > 
> > > If anyone ever passes a GFP_DMA or GFP_MOVABLE allocation flag to
> > > allocate_extent_state, it will try to allocate memory from the wrong zone.
> > > We just want to allocate memory from ZONE_NORMAL, so use GFP_RECLAIM_MASK
> > > to get what we want.
> > 
> > Looks good to me.
> > 
> > > Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com>
> > > 
> > > diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
> > > index e99b329002cf..4e4a67b7b29d 100644
> > > --- a/fs/btrfs/extent_io.c
> > > +++ b/fs/btrfs/extent_io.c
> > > @@ -216,12 +216,7 @@ static struct extent_state *alloc_extent_state(gfp_t mask)
> > >  {
> > >  	struct extent_state *state;
> > >  
> > > -	/*
> > > -	 * The given mask might be not appropriate for the slab allocator,
> > > -	 * drop the unsupported bits
> > > -	 */
> > > -	mask &= ~(__GFP_DMA32|__GFP_HIGHMEM);
> > 
> > I've noticed there's GFP_SLAB_BUG_MASK that's basically open coded here,
> > but this would not filter out the placement flags.
> > 
> > > -	state = kmem_cache_alloc(extent_state_cache, mask);
> > 
> > I'd prefer some comment here, it's not obvious why the mask is used.
> 
> Sorry, I dropped the ball on this.  Would you prefer:
> 
>         /* Allocate from ZONE_NORMAL */
>         state = kmem_cache_alloc(extent_state_cache, mask & GFP_RECLAIM_MASK);
> 
> or
> 
> 	/*
> 	 * Callers may pass in a mask which indicates they want to allocate
> 	 * from a special zone, so clear those bits here rather than forcing
> 	 * each caller to do it.  We only want to use their mask to indicate
> 	 * what strategies the memory allocator can use to free memory.
> 	 */
>         state = kmem_cache_alloc(extent_state_cache, mask & GFP_RECLAIM_MASK);
> 
> I tend to lean towards being more terse, but it's not about me, it's
> about whoever reads this code next.

I prefer the latter variant, it's clear that it's some MM stuff. Thanks.