On Fri, 6 Nov 2020, Alex Shi wrote: > > updated for comments change from Johannes > > > From 2fd278b1ca6c3e260ad249808b62f671d8db5a7b Mon Sep 17 00:00:00 2001 > From: Alex Shi <[email protected]> > Date: Thu, 5 Nov 2020 11:38:24 +0800 > Subject: [PATCH v21 06/19] mm/rmap: stop store reordering issue on > page->mapping > > Hugh Dickins and Minchan Kim observed a long time issue which > discussed here, but actully the mentioned fix missed. > https://lore.kernel.org/lkml/20150504031722.GA2768@blaptop/ > The store reordering may cause problem in the scenario: > > CPU 0 CPU1 > do_anonymous_page > page_add_new_anon_rmap() > page->mapping = anon_vma + PAGE_MAPPING_ANON > lru_cache_add_inactive_or_unevictable() > spin_lock(lruvec->lock) > SetPageLRU() > spin_unlock(lruvec->lock) > /* idletacking judged it as LRU > * page so pass the page in > * page_idle_clear_pte_refs > */ > page_idle_clear_pte_refs > rmap_walk > if PageAnon(page) > > Johannes give detailed examples how the store reordering could cause > a trouble: > "The concern is the SetPageLRU may get reorder before 'page->mapping' > setting, That would make CPU 1 will observe at page->mapping after > observing PageLRU set on the page. > > 1. anon_vma + PAGE_MAPPING_ANON > > That's the in-order scenario and is fine. > > 2. NULL > > That's possible if the page->mapping store gets reordered to occur > after SetPageLRU. That's fine too because we check for it. > > 3. anon_vma without the PAGE_MAPPING_ANON bit > > That would be a problem and could lead to all kinds of undesirable > behavior including crashes and data corruption. > > Is it possible? AFAICT the compiler is allowed to tear the store to > page->mapping and I don't see anything that would prevent it. > > That said, I also don't see how the reader testing PageLRU under the > lru_lock would prevent that in the first place. AFAICT we need that > WRITE_ONCE() around the page->mapping assignment." > > Signed-off-by: Alex Shi <[email protected]> > Cc: Johannes Weiner <[email protected]> > Cc: Andrew Morton <[email protected]> > Cc: Hugh Dickins <[email protected]>
Acked-by: Hugh Dickins <[email protected]> Many thanks to Johannes for spotting my falsehood in the next patch, and to Alex for making it true with this patch. As I just remarked against the v20, I do have some more of these WRITE_ONCEs, but consider them merely theoretical: so please don't let me hold this series up. Andrew, I am hoping that Alex's v21 will appear in the next mmotm? Thanks, Hugh > Cc: Matthew Wilcox <[email protected]> > Cc: Minchan Kim <[email protected]> > Cc: Vladimir Davydov <[email protected]> > Cc: [email protected] > Cc: [email protected] > --- > mm/rmap.c | 8 +++++++- > 1 file changed, 7 insertions(+), 1 deletion(-) > > diff --git a/mm/rmap.c b/mm/rmap.c > index 1b84945d655c..380c6b9956c2 100644 > --- a/mm/rmap.c > +++ b/mm/rmap.c > @@ -1054,8 +1054,14 @@ static void __page_set_anon_rmap(struct page *page, > if (!exclusive) > anon_vma = anon_vma->root; > > + /* > + * page_idle does a lockless/optimistic rmap scan on page->mapping. > + * Make sure the compiler doesn't split the stores of anon_vma and > + * the PAGE_MAPPING_ANON type identifier, otherwise the rmap code > + * could mistake the mapping for a struct address_space and crash. > + */ > anon_vma = (void *) anon_vma + PAGE_MAPPING_ANON; > - page->mapping = (struct address_space *) anon_vma; > + WRITE_ONCE(page->mapping, (struct address_space *) anon_vma); > page->index = linear_page_index(vma, address); > } > > -- > 1.8.3.1
