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

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Date:   Wed, 6 Jul 2022 11:11:13 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Matthew Wilcox <willy@...radead.org>, jgg@...pe.ca,
        jhubbard@...dia.com, william.kucharski@...cle.com,
        dan.j.williams@...el.com, jack@...e.cz, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        nvdimm@...ts.linux.dev, stable@...r.kernel.org
Subject: Re: [PATCH v2] mm: fix missing wake-up event for FSDAX pages

On Tue, Jul 05, 2022 at 08:00:42PM -0700, Andrew Morton wrote:
> On Wed, 6 Jul 2022 10:47:32 +0800 Muchun Song <songmuchun@...edance.com> wrote:
> 
> > > 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, I updated the changelog a bit.
> 
> : 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()).  This results in a task being permanently
> : stuck in TASK_INTERRUPTIBLE state.
> : 
> : Since FSDAX pages are only possibly obtained by GUP users, so fix GUP
> : instead of folio_put() to lower overhead.
> 
> I believe these details are helpful for -stable maintainers who are
> wondering why they were sent stuff.  Also for maintainers of
> downstreeam older kernels who are scratching heads over some user bug
> report, trying to find a patch which might fix it - for this they want
> to see a description of the user-visible effects, for matching with
> that bug report.
>

Thanks Andrew, it's really helpful.