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 code, and they get outdated really quickly.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. > */ > Yes, it's reansonble. So is the following fine? >From f166f0d5df350c5eae1218456b9e6e1bd43434e7 Mon Sep 17 00:00:00 2001 From: Alex Shi <alex.shi@xxxxxxxxxxxxxxxxx> Date: Thu, 5 Nov 2020 11:38:24 +0800 Subject: [PATCH] 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 <alex.shi@xxxxxxxxxxxxxxxxx> Cc: Johannes Weiner <hannes@xxxxxxxxxxx> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> Cc: Hugh Dickins <hughd@xxxxxxxxxx> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx> Cc: Minchan Kim <minchan@xxxxxxxxxx> Cc: Vladimir Davydov <vdavydov.dev@xxxxxxxxx> Cc: linux-kernel@xxxxxxxxxxxxxxx Cc: linux-mm@xxxxxxxxx --- mm/rmap.c | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-) diff --git a/mm/rmap.c b/mm/rmap.c index c050dab2ae65..73788505aa0a 100644 --- a/mm/rmap.c +++ b/mm/rmap.c @@ -1054,8 +1054,13 @@ static void __page_set_anon_rmap(struct page *page, if (!exclusive) anon_vma = anon_vma->root; + /* + * 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. + */ 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
