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Date: Wed, 13 May 2015 12:00:57 +0900 From: Minchan Kim <minchan@...nel.org> To: Rik van Riel <riel@...hat.com> Cc: Vladimir Davydov <vdavydov@...allels.com>, Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org, linux-kernel@...r.kernel.org, "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>, "Kirill A. Shutemov" <kirill@...temov.name>, Hugh Dickins <hughd@...gle.com> Subject: Re: [PATCH v2] rmap: fix theoretical race between do_wp_page and shrink_active_list Separate from Vladimir's thread. I don't want to make a noise in there. On Tue, May 12, 2015 at 10:04:12PM -0400, Rik van Riel wrote: > On 05/12/2015 09:43 PM, Minchan Kim wrote: > > Hi, Rik > > > > I'd like to bring up the issue in this thread although I already gave > > my Acked-by. > > > > Below issue causes by no PG_locked page in page_referenced while > > page_move_anon_rmap depends on PG_locked to prevent race with rmap code. > > > > So, although this patch fixes below one example, we still have a problem > > in rmap. > > > > If page_referenced holds PG_locked for all of pages unconditionally, > > we don't need this patch and might remove READ_ONCE introduced by > > 80e148 and more than. > > > > What do you think about? > > Maybe the reclaim code and page_referenced are fine. > > However, I have seen one real world bug report of a page->mapping > pointing to an anon_vma without the PAGE_MAPPING_ANON bit being > set. > > This is a pretty hard to hit race, so I have only ever heard of > it happening once, and I do not remember the details of exactly > what code blew up trying to follow the page->mapping pointer in > the wrong way. > > I wish I remember what needs this patch, but I have a rather > strong suspicion there is something that needs it... > > Acked-by: Rik van Riel <riel@...hat.com> It seems you misunderstood my point. My bad. My point is you wrote down below comment above page_move_anon_rmap. "Protected against the rmap code by the page lock" but rmap code doesn't hold a page lock sometime so anon_vma would be stale in rmap traverse. But when I reviewed the code, worst case is rmap will look up all of parent, siblings but it wouldn't affect integrity. One thing I suspect is load-tearing when we get anon_vma from the page->mapping but we used READ_ONCE for that so I couldn't find any serious bug. So is it okay to remove above wrong comment? diff --git a/mm/memory.c b/mm/memory.c index 22e037e..e35a782 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -2329,7 +2329,6 @@ static int do_wp_page(struct mm_struct *mm, struct vm_area_struct *vma, /* * The page is all ours. Move it to our anon_vma so * the rmap code will not search our parent or siblings. - * Protected against the rmap code by the page lock. */ page_move_anon_rmap(old_page, vma, address); unlock_page(old_page); > > > On Tue, May 12, 2015 at 01:18:39PM +0300, Vladimir Davydov wrote: > >> As noted by Paul the compiler is free to store a temporary result in a > >> variable on stack, heap or global unless it is explicitly marked as > >> volatile, see: > >> > >> http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4455.html#sample-optimizations > >> > >> This can result in a race between do_wp_page() and shrink_active_list() > >> as follows. > >> > >> In do_wp_page() we can call page_move_anon_rmap(), which sets > >> page->mapping as follows: > >> > >> anon_vma = (void *) anon_vma + PAGE_MAPPING_ANON; > >> page->mapping = (struct address_space *) anon_vma; > >> > >> The page in question may be on an LRU list, because nowhere in > >> do_wp_page() we remove it from the list, neither do we take any LRU > >> related locks. Although the page is locked, shrink_active_list() can > >> still call page_referenced() on it concurrently, because the latter does > >> not require an anonymous page to be locked: > >> > >> CPU0 CPU1 > >> ---- ---- > >> do_wp_page shrink_active_list > >> lock_page page_referenced > >> PageAnon->yes, so skip trylock_page > >> page_move_anon_rmap > >> page->mapping = anon_vma > >> rmap_walk > >> PageAnon->no > >> rmap_walk_file > >> BUG > >> page->mapping += PAGE_MAPPING_ANON > >> > >> This patch fixes this race by explicitly forbidding the compiler to > >> split page->mapping store in page_move_anon_rmap() with the aid of > >> WRITE_ONCE. > >> > >> Signed-off-by: Vladimir Davydov <vdavydov@...allels.com> > >> Cc: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> > >> Cc: "Kirill A. Shutemov" <kirill@...temov.name> > >> Cc: Rik van Riel <riel@...hat.com> > >> Cc: Hugh Dickins <hughd@...gle.com> > >> --- > >> Changes in v2: > >> - do not add READ_ONCE to PageAnon and WRITE_ONCE to > >> __page_set_anon_rmap and __hugepage_set_anon_rmap (Kirill) > >> > >> mm/rmap.c | 2 +- > >> 1 file changed, 1 insertion(+), 1 deletion(-) > >> > >> diff --git a/mm/rmap.c b/mm/rmap.c > >> index 24dd3f9fee27..8b18fd4227d1 100644 > >> --- a/mm/rmap.c > >> +++ b/mm/rmap.c > >> @@ -950,7 +950,7 @@ void page_move_anon_rmap(struct page *page, > >> VM_BUG_ON_PAGE(page->index != linear_page_index(vma, address), 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); > >> } > >> > >> /** > >> -- > >> 1.7.10.4 > >> > >> -- > >> To unsubscribe, send a message with 'unsubscribe linux-mm' in > >> the body to majordomo@...ck.org. For more info on Linux MM, > >> see: http://www.linux-mm.org/ . > >> Don't email: <a href=mailto:"dont@...ck.org"> email@...ck.org </a> > > > > > -- > All rights reversed -- Kind regards, Minchan Kim -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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