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

[Roman Gushchin]

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@xxxxxxxxxx> wrote:
> >
> > On Fri, Sep 25, 2020 at 10:17 AM Linus Torvalds
> > <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> > >
> > > On Fri, Sep 25, 2020 at 9:19 AM Ming Lei <ming.lei@xxxxxxxxxx> 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);
 }