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.