linux-ext4 - Re: REGRESSION: 37f4a24c2469: blk-mq: centralise related handling into blk_mq_get_driver

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Date:   Fri, 25 Sep 2020 23:42:32 -0700
From:   Roman Gushchin <guro@...com>
To:     Ming Lei <ming.lei@...hat.com>
CC:     Shakeel Butt <shakeelb@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "Theodore Y . Ts'o" <tytso@....edu>, Jens Axboe <axboe@...nel.dk>,
        Ext4 Developers List <linux-ext4@...r.kernel.org>,
        linux-block <linux-block@...r.kernel.org>,
        Vlastimil Babka <vbabka@...e.cz>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: REGRESSION: 37f4a24c2469: blk-mq: centralise related handling
 into blk_mq_get_driver_tag

On Sat, Sep 26, 2020 at 09:43:25AM +0800, Ming Lei wrote:
> On Fri, Sep 25, 2020 at 12:19:02PM -0700, Shakeel Butt wrote:
> > On Fri, Sep 25, 2020 at 10:58 AM Shakeel Butt <shakeelb@...gle.com>
> > wrote:
> > >
> > [snip]
> > >
> > > 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()?
> > 
> > Ming, can you please try the following patch?
> > 
> > 
> > From: Shakeel Butt <shakeelb@...gle.com>
> > 
> > [PATCH] mm: slab: fix potential infinite recursion in ___cache_free
> > 
> > With the commit 10befea91b61 ("mm: memcg/slab: use a single set of
> > kmem_caches for all allocations"), it becomes possible to call kfree()
> > from the slabs_destroy(). However if slabs_destroy() is being called for
> > the array_cache of the local CPU then this opens the potential scenario
> > of infinite recursion because kfree() called from slabs_destroy() can
> > call slabs_destroy() with the same array_cache of the local CPU. Since
> > the array_cache of the local CPU is not updated before calling
> > slabs_destroy(), it will try to free the same pages.
> > 
> > To fix the issue, simply update the cache before calling
> > slabs_destroy().
> > 
> > Signed-off-by: Shakeel Butt <shakeelb@...gle.com>
> > ---
> >  mm/slab.c | 8 ++++++--
> >  1 file changed, 6 insertions(+), 2 deletions(-)
> > 
> > diff --git a/mm/slab.c b/mm/slab.c
> > index 3160dff6fd76..f658e86ec8ce 100644
> > --- a/mm/slab.c
> > +++ b/mm/slab.c
> > @@ -1632,6 +1632,10 @@ static void slab_destroy(struct kmem_cache *cachep, struct page *page)
> >  		kmem_cache_free(cachep->freelist_cache, freelist);
> >  }
> >  
> > +/*
> > + * Update the size of the caches before calling slabs_destroy as it may
> > + * recursively call kfree.
> > + */
> >  static void slabs_destroy(struct kmem_cache *cachep, struct list_head *list)
> >  {
> >  	struct page *page, *n;
> > @@ -2153,8 +2157,8 @@ static void do_drain(void *arg)
> >  	spin_lock(&n->list_lock);
> >  	free_block(cachep, ac->entry, ac->avail, node, &list);
> >  	spin_unlock(&n->list_lock);
> > -	slabs_destroy(cachep, &list);
> >  	ac->avail = 0;
> > +	slabs_destroy(cachep, &list);
> >  }
> >  
> >  static void drain_cpu_caches(struct kmem_cache *cachep)
> > @@ -3402,9 +3406,9 @@ static void cache_flusharray(struct kmem_cache *cachep, struct array_cache *ac)
> >  	}
> >  #endif
> >  	spin_unlock(&n->list_lock);
> > -	slabs_destroy(cachep, &list);
> >  	ac->avail -= batchcount;
> >  	memmove(ac->entry, &(ac->entry[batchcount]), sizeof(void *)*ac->avail);
> > +	slabs_destroy(cachep, &list);
> >  }
> 
> The issue can't be reproduced after applying this patch:
> 
> Tested-by: Ming Lei <ming.lei@...hat.com>

Perfect, thank you very much for the confirmation!

Shakeel, can you, please, resend the patch with the proper fixes tag
and the updated commit log? Please, feel free to add
Reviewed-by: Roman Gushchin <guro@...com> .

Thank you!

Roman