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

Shakeel Butt
2020-09-25T17:35:20Z

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.

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().

Shakeel