On Mon, Nov 02, 2020 at 02:49:27PM +0000, Matthew Wilcox wrote: > On Mon, Nov 02, 2020 at 09:41:10AM -0500, Johannes Weiner wrote: > > On Thu, Oct 29, 2020 at 06:44:53PM +0800, Alex Shi wrote: > > > From: Hugh Dickins <hughd@xxxxxxxxxx> > > > > > > It is necessary for page_idle_get_page() to recheck PageLRU() after > > > get_page_unless_zero(), but holding lru_lock around that serves no > > > useful purpose, and adds to lru_lock contention: delete it. > > > > > > See https://lore.kernel.org/lkml/20150504031722.GA2768@blaptop for the > > > discussion that led to lru_lock there; but __page_set_anon_rmap() now > > > uses WRITE_ONCE(), > > > > That doesn't seem to be the case in Linus's or Andrew's tree. Am I > > missing a dependent patch series? > > > > > and I see no other risk in page_idle_clear_pte_refs() using > > > rmap_walk() (beyond the risk of racing PageAnon->PageKsm, mostly but > > > not entirely prevented by page_count() check in ksm.c's > > > write_protect_page(): that risk being shared with page_referenced() > > > and not helped by lru_lock). > > > > Isn't it possible, as per Minchan's description, for page->mapping to > > point to a struct anon_vma without PAGE_MAPPING_ANON set, and rmap > > thinking it's looking at a struct address_space? > > I don't think it can point to an anon_vma without the ANON bit set. > Minchan's concern in that email was that it might still be NULL. Hm, no, the thread is a lengthy discussion about whether the store could be split such that page->mapping is actually pointing to something invalid (anon_vma without the PageAnon bit). >From his email: CPU 0 CPU 1 do_anonymous_page __page_set_anon_rmap /* out of order happened so SetPageLRU is done ahead */ SetPageLRU(page) /* Compilr changed store operation like below */ page->mapping = (struct address_space *) anon_vma; /* Big stall happens */ /* idletacking judged it as LRU page so pass the page in page_reference */ page_refernced page_rmapping return true because page->mapping has some vaule but not complete so it calls rmap_walk_file. it's okay to pass non-completed anon page in rmap_walk_file?
