On Thu, Nov 05, 2020 at 01:03:18PM +0800, Alex Shi wrote: > > > 在 2020/11/5 下午12:57, Matthew Wilcox 写道: > > On Thu, Nov 05, 2020 at 12:52:05PM +0800, Alex Shi wrote: > >> @@ -1054,8 +1054,27 @@ static void __page_set_anon_rmap(struct page *page, > >> if (!exclusive) > >> anon_vma = anon_vma->root; > >> > >> + /* > >> + * w/o the WRITE_ONCE here the following scenario may happens due to > >> + * store reordering. > >> + * > >> + * CPU 0 CPU 1 > >> + * > >> + * do_anonymous_page page_idle_clear_pte_refs > >> + * __page_set_anon_rmap > >> + * page->mapping = anon_vma + PAGE_MAPPING_ANON > >> + * lru_cache_add_inactive_or_unevictable() > >> + * SetPageLRU(page) > >> + * rmap_walk > >> + * if PageAnon(page) > >> + * > >> + * The 'SetPageLRU' may reordered before page->mapping setting, and > >> + * page->mapping may set with anon_vma, w/o anon bit, then rmap_walk > >> + * may goes to rmap_walk_file() for a anon page. > >> + */ > >> + > >> 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); > >> } > > > > I don't like these verbose comments with detailed descriptions in > > the source code. They're fine in changelogs, but they clutter the > > code, and they get outdated really quickly. My preference is for > > something more brief: > > > > /* > > * Prevent page->mapping from pointing to an anon_vma without > > * the PAGE_MAPPING_ANON bit set. This could happen if the > > * compiler stores anon_vma and then adds PAGE_MAPPING_ANON to it. > > */ > > Yeah, I don't think this scenario warrants the full race diagram in the code itself. But the code is highly specific - synchronizing one struct page member for one particular use case. Let's keep at least a reference to what we are synchronizing against. There is a non-zero chance that if the comment goes out of date, so does the code. How about this? /* * 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. */
