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

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Message-Id: <20220705141819.804eb972d43be3434dc70192@linux-foundation.org>
Date:   Tue, 5 Jul 2022 14:18:19 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     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,  5 Jul 2022 20:35:32 +0800 Muchun Song <songmuchun@...edance.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?