在 2020/11/5 上午1:46, Johannes Weiner 写道: > On Wed, Nov 04, 2020 at 07:27:21PM +0800, Alex Shi wrote: >> 在 2020/11/3 上午4:20, Johannes Weiner 写道: >>> 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) >> >> This SetPageLRU done in __pagevec_lru_add_fn() which under the lru_lock >> protection, so the original memory barrier or lock concern isn't >> correct. that means, the SetPageLRU isn't possible to be here. >> And then no warry on right side 'CPU 1' problem. > > The SetPageLRU is done under lru_lock, but the store to page->mapping > is not, so what ensures ordering between them? And what prevents the > compiler from tearing the store to page->mapping? > Right, I misunderstand the spin_lock in memory barrier. Thanks a lot for point out this. So, is this patch fine to fix the problem? >From 8427121da195a6a386d1ce71abcb41b31211e21f 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 | 21 ++++++++++++++++++++- 1 file changed, 20 insertions(+), 1 deletion(-) diff --git a/mm/rmap.c b/mm/rmap.c index c050dab2ae65..56af18aa57de 100644 --- a/mm/rmap.c +++ b/mm/rmap.c @@ -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); } -- 1.8.3.1 > The writer does this: > > CPU 0 > 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) > > The concern is what 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 that's referenced in > the changelog of this patch but I cannot find in any tree or patch. >
