Re: [PATCH v21 06/19] mm/rmap: stop store reordering issue on page->mapping

[Johannes Weiner]

On Fri, Nov 06, 2020 at 09:20:04AM +0800, Alex Shi wrote:
> From 2fd278b1ca6c3e260ad249808b62f671d8db5a7b Mon Sep 17 00:00:00 2001
> From: Alex Shi <alex.shi@xxxxxxxxxxxxxxxxx>
> Date: Thu, 5 Nov 2020 11:38:24 +0800
> Subject: [PATCH v21 06/19] 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

Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>

Thanks Alex!