Re: [PATCH] mm: page_alloc: document kmemleak's non-blockable __GFP_NOFAIL case

[Michal Hocko]

On Sat 13-07-19 12:39:16, David Rientjes wrote:
> On Sat, 13 Jul 2019, Yang Shi wrote:
> 
> > When running ltp's oom test with kmemleak enabled, the below warning was
> > triggerred since kernel detects __GFP_NOFAIL & ~__GFP_DIRECT_RECLAIM is
> > passed in:
> > 
> > WARNING: CPU: 105 PID: 2138 at mm/page_alloc.c:4608 __alloc_pages_nodemask+0x1c31/0x1d50
> > Modules linked in: loop dax_pmem dax_pmem_core
> > ip_tables x_tables xfs virtio_net net_failover virtio_blk failover
> > ata_generic virtio_pci virtio_ring virtio libata
> > CPU: 105 PID: 2138 Comm: oom01 Not tainted 5.2.0-next-20190710+ #7
> > Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.10.2-0-g5f4c7b1-prebuilt.qemu-project.org 04/01/2014
> > RIP: 0010:__alloc_pages_nodemask+0x1c31/0x1d50
> > ...
> >  kmemleak_alloc+0x4e/0xb0
> >  kmem_cache_alloc+0x2a7/0x3e0
> >  ? __kmalloc+0x1d6/0x470
> >  ? ___might_sleep+0x9c/0x170
> >  ? mempool_alloc+0x2b0/0x2b0
> >  mempool_alloc_slab+0x2d/0x40
> >  mempool_alloc+0x118/0x2b0
> >  ? __kasan_check_read+0x11/0x20
> >  ? mempool_resize+0x390/0x390
> >  ? lock_downgrade+0x3c0/0x3c0
> >  bio_alloc_bioset+0x19d/0x350
> >  ? __swap_duplicate+0x161/0x240
> >  ? bvec_alloc+0x1b0/0x1b0
> >  ? do_raw_spin_unlock+0xa8/0x140
> >  ? _raw_spin_unlock+0x27/0x40
> >  get_swap_bio+0x80/0x230
> >  ? __x64_sys_madvise+0x50/0x50
> >  ? end_swap_bio_read+0x310/0x310
> >  ? __kasan_check_read+0x11/0x20
> >  ? check_chain_key+0x24e/0x300
> >  ? bdev_write_page+0x55/0x130
> >  __swap_writepage+0x5ff/0xb20
> > 
> > The mempool_alloc_slab() clears __GFP_DIRECT_RECLAIM, kmemleak has
> > __GFP_NOFAIL set all the time due to commit
> > d9570ee3bd1d4f20ce63485f5ef05663866fe6c0 ("kmemleak: allow to coexist
> > with fault injection").
> > 
> 
> It only clears __GFP_DIRECT_RECLAIM provisionally to see if the allocation 
> would immediately succeed before falling back to the elements in the 
> mempool.  If that fails, and the mempool is empty, mempool_alloc() 
> attempts the allocation with __GFP_DIRECT_RECLAIM.  So for the problem 
> described here, I think what we really want is this:
> 
> diff --git a/mm/mempool.c b/mm/mempool.c
> --- a/mm/mempool.c
> +++ b/mm/mempool.c
> @@ -386,7 +386,7 @@ void *mempool_alloc(mempool_t *pool, gfp_t gfp_mask)
>  	gfp_mask |= __GFP_NORETRY;	/* don't loop in __alloc_pages */
>  	gfp_mask |= __GFP_NOWARN;	/* failures are OK */
>  
> -	gfp_temp = gfp_mask & ~(__GFP_DIRECT_RECLAIM|__GFP_IO);
> +	gfp_temp = gfp_mask & ~(__GFP_DIRECT_RECLAIM|__GFP_IO|__GFP_NOFAIL);
>  
>  repeat_alloc:

No, I do not think we should make mempool allocator more complex for
something that is an implementation problem the kmemleak.
-- 
Michal Hocko
SUSE Labs