Re: [PATCH] mm: fix a race scenario in folio_isolate_lru

[Zhaoyang Huang]

On Fri, Mar 15, 2024 at 8:46 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
> On Thu, Mar 14, 2024 at 04:39:21PM +0800, zhaoyang.huang wrote:
> > From: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxx>
> >
> > Panic[1] reported which is caused by lruvec->list break. Fix the race
> > between folio_isolate_lru and release_pages.
> >
> > race condition:
> > release_pages could meet a non-refered folio which escaped from being
> > deleted from LRU but add to another list_head
>
> I don't think the bug is in folio_isolate_lru() but rather in its
> caller.
>
>  * Context:
>  *
>  * (1) Must be called with an elevated refcount on the folio. This is a
>  *     fundamental difference from isolate_lru_folios() (which is called
>  *     without a stable reference).
>
> So when release_pages() runs, it must not see a refcount decremented to
> zero, because the caller of folio_isolate_lru() is supposed to hold one.
>
> Your stack trace is for the thread which is calling release_pages(), not
> the one calling folio_isolate_lru(), so I can't help you debug further.
Thanks for the comments.  According to my understanding,
folio_put_testzero does the decrement before test which makes it
possible to have release_pages see refcnt equal zero and proceed
further(folio_get in folio_isolate_lru has not run yet).

   #0 folio_isolate_lru          #1 release_pages
BUG_ON(!folio_refcnt)
                                         if (folio_put_testzero())
   folio_get(folio)
   if (folio_test_clear_lru())