Re: REGRESSION: 37f4a24c2469: blk-mq: centralise related handling into blk_mq_get_driver_tag

Shakeel Butt
2020-09-25T17:59:09Z

On Fri, Sep 25, 2020 at 10:48 AM Roman Gushchin <guro@fb.com> wrote:
>
> On Fri, Sep 25, 2020 at 10:35:03AM -0700, Shakeel Butt wrote:
> > On Fri, Sep 25, 2020 at 10:22 AM Shakeel Butt <shakeelb@google.com> wrote:
> > >
> > > On Fri, Sep 25, 2020 at 10:17 AM Linus Torvalds
> > > <torvalds@linux-foundation.org> wrote:
> > > >
> > > > On Fri, Sep 25, 2020 at 9:19 AM Ming Lei <ming.lei@redhat.com> wrote:
> > > > >
> > > > > git bisect shows the first bad commit:
> > > > >
> > > > > [10befea91b61c4e2c2d1df06a2e978d182fcf792] mm: memcg/slab: use a single set of
> > > > > kmem_caches for all allocations
> > > > >
> > > > > And I have double checked that the above commit is really the first bad
> > > > > commit for the list corruption issue of 'list_del corruption, ffffe1c241b00408->next
> > > > > is LIST_POISON1 (dead000000000100)',
> > > >
> > > > Thet commit doesn't revert cleanly, but I think that's purely because
> > > > we'd also need to revert
> > > >
> > > > 849504809f86 ("mm: memcg/slab: remove unused argument by charge_slab_page()")
> > > > 74d555bed5d0 ("mm: slab: rename (un)charge_slab_page() to
> > > > (un)account_slab_page()")
> > > >
> > > > too.
> > > >
> > > > Can you verify that a
> > > >
> > > > git revert 74d555bed5d0 849504809f86 10befea91b61
> > > >
> > > > on top of current -git makes things work for you again?
> > > >
> > > > I'm going to do an rc8 this release simply because we have another VM
> > > > issue that I hope to get fixed - but there we know what the problem
> > > > and the fix _is_, it just needs some care.
> > > >
> > > > So if Roman (or somebody else) can see what's wrong and we can fix
> > > > this quickly, we don't need to go down the revert path, but ..
> > > >
> > >
> > > I think I have a theory. The issue is happening due to the potential
> > > infinite recursion:
> > >
> > > [ 5060.124412] ___cache_free+0x488/0x6b0
> > > *****Second recursion
> > > [ 5060.128666] kfree+0xc9/0x1d0
> > > [ 5060.131947] kmem_freepages+0xa0/0xf0
> > > [ 5060.135746] slab_destroy+0x19/0x50
> > > [ 5060.139577] slabs_destroy+0x6d/0x90
> > > [ 5060.143379] ___cache_free+0x4a3/0x6b0
> > > *****First recursion
> > > [ 5060.147896] kfree+0xc9/0x1d0
> > > [ 5060.151082] kmem_freepages+0xa0/0xf0
> > > [ 5060.155121] slab_destroy+0x19/0x50
> > > [ 5060.159028] slabs_destroy+0x6d/0x90
> > > [ 5060.162920] ___cache_free+0x4a3/0x6b0
> > > [ 5060.167097] kfree+0xc9/0x1d0
> > >
> > > ___cache_free() is calling cache_flusharray() to flush the local cpu
> > > array_cache if the cache has more elements than the limit (ac->avail
> > > >= ac->limit).
> > >
> > > cache_flusharray() is removing batchcount number of element from local
> > > cpu array_cache and pass it slabs_destroy (if the node shared cache is
> > > also full).
> > >
> > > Note that we have not updated local cpu array_cache size yet and
> > > called slabs_destroy() which can call kfree() through
> > > unaccount_slab_page().
> > >
> > > We are on the same CPU and this recursive kfree again check the
> > > (ac->avail >= ac->limit) and call cache_flusharray() again and recurse
> > > indefinitely.
>
> It's a coll theory! And it explains why we haven't seen it with SLUB.
>
> >
> > I can see two possible fixes. We can either do async kfree of
> > page_obj_cgroups(page) or we can update the local cpu array_cache's
> > size before slabs_destroy().
>
> I wonder if something like this can fix the problem?
> (completely untested).
>
> --
>
> diff --git a/mm/slab.c b/mm/slab.c
> index 684ebe5b0c7a..c94b9ccfb803 100644
> --- a/mm/slab.c
> +++ b/mm/slab.c
> @@ -186,6 +186,7 @@ struct array_cache {
> unsigned int limit;
> unsigned int batchcount;
> unsigned int touched;
> + bool flushing;
> void *entry[]; /*
> * Must have this definition in here for the proper
> * alignment of array_cache. Also simplifies accessing
> @@ -526,6 +527,7 @@ static void init_arraycache(struct array_cache *ac, int limit, int batch)
> ac->limit = limit;
> ac->batchcount = batch;
> ac->touched = 0;
> + ac->flushing = false;
> }
> }
>
> @@ -3368,6 +3370,11 @@ static void cache_flusharray(struct kmem_cache *cachep, struct array_cache *ac)
> int node = numa_mem_id();
> LIST_HEAD(list);
>
> + if (ac->flushing)
> + return;
> +
> + ac->flushing = true;
> +
> batchcount = ac->batchcount;
>
> check_irq_off();
> @@ -3404,6 +3411,7 @@ static void cache_flusharray(struct kmem_cache *cachep, struct array_cache *ac)
> spin_unlock(&n->list_lock);
> slabs_destroy(cachep, &list);
> ac->avail -= batchcount;
> + ac->flushing = false;
> memmove(ac->entry, &(ac->entry[batchcount]), sizeof(void *)*ac->avail);
> }
>

I don't think you can ignore the flushing. The __free_once() in
___cache_free() assumes there is a space available.

BTW do_drain() also have the same issue.

Why not move slabs_destroy() after we update ac->avail and memmove()?