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

[Matthew Wilcox <willy@infradead.org>]

On Wed, May 09, 2018 at 11:36:59AM +0200, David Sterba wrote:
> On Mon, May 07, 2018 at 05:25:47PM -0700, Matthew Wilcox wrote:
> > On Mon, May 07, 2018 at 11:25:01PM +0200, David Sterba wrote:
> > > On Mon, May 07, 2018 at 11:44:10AM -0700, Matthew Wilcox wrote:
> > > > But something like btrfs should almost certainly be using ~GFP_ZONEMASK.
> > > 
> > > Agreed, the direct use of __GFP_DMA32 was added in 3ba7ab220e8918176c6f
> > > to substitute GFP_NOFS, so the allocation flags are less restrictive but
> > > still acceptable for allocation from slab.
> > > 
> > > The requirement from btrfs is to avoid highmem, the 'must be acceptable
> > > for slab' requirement is more MM internal and should have been hidden
> > > under some opaque flag mask. There was no strong need for that at the
> > > time.
> > 
> > The GFP flags encode a multiple of different requirements.  There's
> > "What can the allocator do to free memory" and "what area of memory
> > can the allocation come from".  btrfs doesn't actually want to
> > allocate memory from ZONE_MOVABLE or ZONE_DMA either.  It's probably never
> > been called with those particular flags set, but in the spirit of
> > future-proofing btrfs, perhaps a patch like this is in order?
> > 
> > ---- >8 ----
> > 
> > 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.

> > +	state = kmem_cache_alloc(extent_state_cache, mask & GFP_RECLAIM_MASK);
> >  	if (!state)
> >  		return state;
> >  	state->state = 0;
>