Messages in this thread | | | From | Jann Horn <> | Date | Tue, 25 May 2021 17:33:23 +0200 | Subject | Re: [RFC 25/26] mm, slub: use migrate_disable() in put_cpu_partial() |
| |
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...)
| |