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Subject: Re: [RFC 25/26] mm, slub: use migrate_disable() in put_cpu_partial()
To:     Jann Horn <jannh@google.com>
Cc:     Linux-MM <linux-mm@kvack.org>,
        kernel list <linux-kernel@vger.kernel.org>,
        Christoph Lameter <cl@linux.com>,
        David Rientjes <rientjes@google.com>,
        Pekka Enberg <penberg@kernel.org>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Thomas Gleixner <tglx@linutronix.de>,
        Mel Gorman <mgorman@techsingularity.net>,
        Jesper Dangaard Brouer <brouer@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>
References: <20210524233946.20352-1-vbabka@suse.cz>
 <20210524233946.20352-26-vbabka@suse.cz>
 <CAG48ez1mvUuXwg0YPH5ANzhQLpbphqk-ZS+jbRz+H66fvm4FcA@mail.gmail.com>
From:   Vlastimil Babka <vbabka@suse.cz>
Message-ID: <2a5cddf1-100d-9d24-fb98-8a5bd439a804@suse.cz>
Date:   Wed, 9 Jun 2021 10:41:43 +0200
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On 5/25/21 5:33 PM, Jann Horn wrote:
> On Tue, May 25, 2021 at 1:40 AM Vlastimil Babka <vbabka@suse.cz> wrote:
>> In put_cpu_partial, we need a stable cpu, but being preempted is not an issue.
>> So, disable migration instead of preemption.
> 
> I wouldn't say "not an issue", more like "you're not making it worse".
> 
> From what I can tell, the following race can already theoretically happen:
> 
> task A: put_cpu_partial() calls preempt_disable()
> task A: oldpage = this_cpu_read(s->cpu_slab->partial)
> interrupt: kfree() reaches unfreeze_partials() and discards the page
> task B (on another CPU): reallocates page as page cache
> task A: reads page->pages and page->pobjects, which are actually
> halves of the pointer page->lru.prev
> task B (on another CPU): frees page
> interrupt: allocates page as SLUB page and places it on the percpu partial list
> task A: this_cpu_cmpxchg() succeeds

Oops, nice find. Thanks.

> which would cause page->pages and page->pobjects to end up containing
> halves of pointers that would then influence when put_cpu_partial()
> happens and show up in root-only sysfs files. Maybe that's acceptable,
> I don't know. But there should probably at least be a comment for now
> to point out that we're reading union fields of a page that might be
> in a completely different state.
> 
> (Someone should probably fix that code sometime and get rid of
> page->pobjects entirely, given how inaccurate it is...)

I'll try to address it separately later. Probably just target a number of pages,
instead of objects, on the list and store the number as part of struct
kmem_cache_cpu, not struct page. The inaccuracy leading to potentially long
lists is a good reason enough, the race scenario above is another one...