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

[Jann Horn]

On Tue, May 25, 2021 at 1:40 AM Vlastimil Babka <vbabka@xxxxxxx> 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...)