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Date: Wed, 6 Jul 2022 00:38:41 +0100
From: Matthew Wilcox <willy@infradead.org>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Muchun Song <songmuchun@bytedance.com>, 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
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References: <20220705123532.283-1-songmuchun@bytedance.com>
 <20220705141819.804eb972d43be3434dc70192@linux-foundation.org>
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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.