Re: [PATCH 1/11] Add __GFP_MOVABLE flag and update callers

[Hugh Dickins <hugh@veritas.com>]

On Sat, 25 Nov 2006, Linus Torvalds wrote:
> On Fri, 24 Nov 2006, Hugh Dickins wrote:
> > 
> > You need to add in something like the patch below (mutatis mutandis
> > for whichever approach you end up taking): tmpfs uses highmem pages
> > for its swap vector blocks, noting where on swap the data pages are,
> > and allocates them with mapping_gfp_mask(inode->i_mapping); but we
> > don't have any mechanism in place for reclaiming or migrating those.
> 
> I think this really just points out that you should _not_ put MOVABLE into 
> the "mapping_gfp_mask()" at all.
> 
> The mapping_gfp_mask() should really just contain the "constraints" on 
> the allocation, not the "how the allocation is used". So things like "I 
> need all my pages to be in the 32bit DMA'able region" is a constraint on 
> the allocator, as is something like "I need the allocation to be atomic". 
> 
> But MOVABLE is really not a constraint on the allocator, it's a guarantee 
> by the code _calling_ the allocator that it will then make sure that it 
> _uses_ the allocation in a way that means that it is movable.
> 
> So it shouldn't be a property of the mapping itself, it should always be a 
> property of the code that actually does the allocation.
> 
> Hmm?

Not anything I feel strongly about, but I don't see it that way.

mapping_gfp_mask() seems to me nothing more than a pragmatic way
of getting the appropriate gfp_mask down to page_cache_alloc().

alloc_inode() initializes it to whatever suits most filesystems
(currently GFP_HIGHUSER), and those who differ adjust it (e.g.
block_dev has good reason to avoid highmem so sets it to GFP_USER
instead).  It used to be the case that several filesystems lacked
kmap() where needed, and those too would set GFP_USER: what you call
a constraint seems to me equally a property of the surrounding code.

If __GFP_MOVABLE is coming in, and most fs's are indeed allocating
movable pages, then I don't see why MOVABLE shouldn't be in the
mapping_gfp_mask.  Specifying MOVABLE constrains both the caller's
use of the pages, and the way they are allocated; as does HIGHMEM.

And we shouldn't be guided by the way tmpfs (ab?)uses that gfp_mask
for its metadata allocations as well as its page_cache_alloc()s:
that's just a special case.  Though the ramfs case is more telling
(its pagecache pages being not at present movable).

Hugh

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