Re: [PATCH v2] mm: fix missing wake-up event for FSDAX pages

[Muchun Song <songmuchun@bytedance.com>]

On Tue, Jul 05, 2022 at 04:47:10PM -0700, Andrew Morton wrote:
> On Wed, 6 Jul 2022 00:38:41 +0100 Matthew Wilcox <willy@infradead.org> wrote:
> 
> > On Tue, Jul 05, 2022 at 02:18:19PM -0700, Andrew Morton wrote:
> > > On Tue,  5 Jul 2022 20:35:32 +0800 Muchun Song <songmuchun@bytedance.com> wrote:
> > > 
> > > > FSDAX page refcounts are 1-based, rather than 0-based: if refcount is
> > > > 1, then the page is freed.  The FSDAX pages can be pinned through GUP,
> > > > then they will be unpinned via unpin_user_page() using a folio variant
> > > > to put the page, however, folio variants did not consider this special
> > > > case, the result will be to miss a wakeup event (like the user of
> > > > __fuse_dax_break_layouts()).  Since FSDAX pages are only possible get
> > > > by GUP users, so fix GUP instead of folio_put() to lower overhead.
> > > > 
> > > 
> > > What are the user visible runtime effects of this bug?
> > 
> > "missing wake up event" seems pretty obvious to me?  Something goes to
> > sleep waiting for a page to become unused, and is never woken.
> 
> No, missed wakeups are often obscured by another wakeup coming in
> shortly afterwards.
> 

I need to clarify the task will never be woken up.

> If this wakeup is not one of these, then are there reports from the
> softlockup detector?
> 
> Do we have reports of processes permanently stuck in D state?
>

No. The task is in an TASK_INTERRUPTIBLE state (see __fuse_dax_break_layouts). 
The hung task reporter only reports D task (TASK_UNINTERRUPTIBLE).

Thanks.
>