Re: [Bug 87891] New: kernel BUG at mm/slab.c:2625!

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Wed, Nov 12, 2014 at 11:17:16AM +0900, Joonsoo Kim wrote:
> On Wed, Nov 12, 2014 at 03:22:41AM +0200, Kirill A. Shutemov wrote:
> > On Tue, Nov 11, 2014 at 04:49:13PM -0800, Andrew Morton wrote:
> > > On Tue, 11 Nov 2014 18:36:28 -0600 (CST) Christoph Lameter <cl@linux.com> wrote:
> > > 
> > > > On Tue, 11 Nov 2014, Andrew Morton wrote:
> > > > 
> > > > > There's no point in doing
> > > > >
> > > > > 	#define GFP_SLAB_BUG_MASK (__GFP_DMA32|__GFP_HIGHMEM|~__GFP_BITS_MASK)
> > > > >
> > > > > because __GFP_DMA32|__GFP_HIGHMEM are already part of ~__GFP_BITS_MASK.
> > > > 
> > > > ?? ~__GFP_BITS_MASK means bits 25 to 31 are set.
> > > > 
> > > > __GFP_DMA32 is bit 2 and __GFP_HIGHMEM is bit 1.
> > > 
> > > Ah, yes, OK.
> > > 
> > > I suppose it's possible that __GFP_HIGHMEM was set.
> > > 
> > > do_huge_pmd_anonymous_page
> > > ->pte_alloc_one
> > >   ->alloc_pages(__userpte_alloc_gfp==__GFP_HIGHMEM)
> > 
> > do_huge_pmd_anonymous_page
> >  alloc_hugepage_vma
> >   alloc_pages_vma(GFP_TRANSHUGE)
> > 
> > GFP_TRANSHUGE contains GFP_HIGHUSER_MOVABLE, which has __GFP_HIGHMEM.
> 
> Hello, Kirill.
> 
> BTW, why does GFP_TRANSHUGE have MOVABLE flag despite it isn't
> movable? After breaking hugepage, it could be movable, but, it may
> prevent CMA from working correctly until break.

Again, the same alloc vs. free gfp_mask: we want page allocator to move
pages around to find space from THP, but resulting page is no really
movable.

I've tried to look into making THP movable: it requires quite a bit of
infrastructure changes around rmap: try_to_unmap*(), remove_migration_pmd(),
migration entries for PMDs, etc. I gets ugly pretty fast :-/
I probably need to give it second try. No promises.

-- 
 Kirill A. Shutemov

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