Re: [PATCH v20 08/20] mm: page_idle_get_page() does not need lru_lock

Johannes Weiner
2020-11-05T15:38:41Z

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 code, and they get outdated really quickly.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comment goes out of date, so does the code.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.
*/