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SubjectRe: [PATCH] mm: page_alloc: document kmemleak's non-blockable __GFP_NOFAIL case
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On 7/13/19 12:39 PM, 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:
>
> But bio_alloc_bioset() plays with gfp_mask itself: are we sure that it
> isn't the one clearing __GFP_DIRECT_RECLAIM itself before falling back to
> saved_gfp?
>
> In other words do we also want this?
>
> diff --git a/block/bio.c b/block/bio.c
> --- a/block/bio.c
> +++ b/block/bio.c
> @@ -462,16 +462,16 @@ struct bio *bio_alloc_bioset(gfp_t gfp_mask, unsigned int nr_iovecs,
> * We solve this, and guarantee forward progress, with a rescuer
> * workqueue per bio_set. If we go to allocate and there are
> * bios on current->bio_list, we first try the allocation
> - * without __GFP_DIRECT_RECLAIM; if that fails, we punt those
> - * bios we would be blocking to the rescuer workqueue before
> - * we retry with the original gfp_flags.
> + * without __GFP_DIRECT_RECLAIM or __GFP_NOFAIL; if that fails,
> + * we punt those bios we would be blocking to the rescuer
> + * workqueue before we retry with the original gfp_flags.
> */
> -
> if (current->bio_list &&
> (!bio_list_empty(&current->bio_list[0]) ||
> !bio_list_empty(&current->bio_list[1])) &&
> bs->rescue_workqueue)
> - gfp_mask &= ~__GFP_DIRECT_RECLAIM;
> + gfp_mask &= ~(__GFP_DIRECT_RECLAIM |
> + __GFP_NOFAIL);
>
> p = mempool_alloc(&bs->bio_pool, gfp_mask);
> if (!p && gfp_mask != saved_gfp) {

I don't think it will make any difference by removing __GFP_NOFAIL
outside kmemleak. The problem is the commit
d9570ee3bd1d4f20ce63485f5ef05663866fe6c0 ("kmemleak: allow to coexist
with fault injection") makes __GFP_NOFAIL is set for kmemleak always in
order to turn off fault-injection for kmemleak.

As long as kmemleak is called in ~__GFP_DIRECT_RECLAIM path, the warning
might be hit.

And since kmemleak is just a debugging tool, so IMHO I don't think this
is worth fixing, so I came up with the patch to document it.


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