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

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?