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Date: Tue, 5 Jul 2022 14:18:19 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Muchun Song <songmuchun@bytedance.com>
Cc: willy@infradead.org, jgg@ziepe.ca, jhubbard@nvidia.com,
 william.kucharski@oracle.com, dan.j.williams@intel.com, jack@suse.cz,
 linux-mm@kvack.org, linux-kernel@vger.kernel.org,
 linux-fsdevel@vger.kernel.org, nvdimm@lists.linux.dev,
 stable@vger.kernel.org
Subject: Re: [PATCH v2] mm: fix missing wake-up event for FSDAX pages
Message-Id: <20220705141819.804eb972d43be3434dc70192@linux-foundation.org>
In-Reply-To: <20220705123532.283-1-songmuchun@bytedance.com>
References: <20220705123532.283-1-songmuchun@bytedance.com>
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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?