| Message ID | 20180508002547.GA16338@bombadil.infradead.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
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;
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; >
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.
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); - state = kmem_cache_alloc(extent_state_cache, mask); + state = kmem_cache_alloc(extent_state_cache, mask & GFP_RECLAIM_MASK); if (!state) return state; state->state = 0;