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

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Date:   Tue, 5 Jul 2022 20:00:42 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Muchun Song <songmuchun@...edance.com>
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 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.