Re: [RFC 25/26] mm, slub: use migrate_disable() in put_cpu_partial()

[Vlastimil Babka <vbabka@suse.cz>]

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...