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

[Shakeel Butt]

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.