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From:   Jann Horn <jannh@google.com>
Date:   Tue, 25 May 2021 17:33:23 +0200
Message-ID: <CAG48ez1mvUuXwg0YPH5ANzhQLpbphqk-ZS+jbRz+H66fvm4FcA@mail.gmail.com>
Subject: Re: [RFC 25/26] mm, slub: use migrate_disable() in put_cpu_partial()
To:     Vlastimil Babka <vbabka@suse.cz>
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>
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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

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...)