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

[Andrew Morton]

On Tue,  5 Jul 2022 20:35:32 +0800 Muchun Song <songmuchun@xxxxxxxxxxxxx> 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?