On Tue, Oct 26, 2021 at 02:30:25PM -0400, Pasha Tatashin wrote: > On Tue, Oct 26, 2021 at 2:24 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > > I think this is overkill. Won't we get exactly the same protection > > by simply testing that page->_refcount == 0 in set_page_count()? > > Anything which triggers that BUG_ON would already be buggy because > > it can race with speculative gets. > > We can't because set_page_count(v) is used for > 1. changing _refcount form a current value to unconstrained v > 2. initialize _refcount from undefined state to v. > > In this work we forbid the first case, and reduce the second case to > initialize only to 1. Anything that is calling set_page_refcount() on something which is not 0 is buggy today. There are several ways to increment the page refcount speculatively if it is not 0. eg lockless GUP and page cache reads. So we could have: CPU 0: alloc_page() (refcount now 1) CPU 1: get_page_unless_zero() (refcount now 2) CPU 0: set_page_refcount(5) (refcount now 5) CPU 1: put_page() (refcount now 4) Now the refcount is wrong. So it is *only* safe to call set_page_refcount() if the refcount is 0. If you can find somewhere that's calling set_page_refcount() on a non-0 refcount, that's a bug that needs to be fixed.
