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

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

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