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

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

On Tue, Nov 11, 2014 at 05:56:03PM -0800, Andrew Morton wrote:
> On Wed, 12 Nov 2014 03:47:03 +0200 "Kirill A. Shutemov" <kirill@shutemov.name> 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.
> > 
> > Looks like it's reasonable to sanitize flags in shrink_slab() by dropping
> > flags incompatible with slab expectation. Like this:
> > 
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index dcb47074ae03..eb165d29c5e5 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -369,6 +369,8 @@ unsigned long shrink_slab(struct shrink_control *shrinkctl,
> >         if (nr_pages_scanned == 0)
> >                 nr_pages_scanned = SWAP_CLUSTER_MAX;
> >  
> > +       shrinkctl->gfp_mask &= ~(__GFP_DMA32 | __GFP_HIGHMEM);
> > +
> >         if (!down_read_trylock(&shrinker_rwsem)) {
> >                 /*
> >                  * If we would return 0, our callers would understand that we
> 
> Well no, because nobody is supposed to be passing this gfp_mask back
> into a new allocation attempt anyway.  It would be better to do
> 
> 	shrinkctl->gfp_mask |= __GFP_IMMEDIATELY_GO_BUG;
> 
> ?

>From my POV, the problem is that we combine what-need-to-be-freed gfp_mask
with if-have-to-allocate gfp_mask: we want to respect __GFP_IO/FS on
alloc, but not nessesary both if there's no restriction from the context.

For shrink_slab(), __GFP_DMA32 and __GFP_HIGHMEM don't make sense in both
cases.

__GFP_IMMEDIATELY_GO_BUG would work too, but we also need to provide
macros to construct alloc-suitable mask from the given one for
yes-i-really-have-to-allocate case.

-- 
 Kirill A. Shutemov

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