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 <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: Vladimir Davydov <[email protected]>
Cc: [email protected]
Cc: [email protected]
---
mm/rmap.c | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)
diff --git a/mm/rmap.c b/mm/rmap.c
index 1b84945d655c..078d54da59d4 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
updated for comments change from Johannes
From 2fd278b1ca6c3e260ad249808b62f671d8db5a7b Mon Sep 17 00:00:00 2001
From: Alex Shi <[email protected]>
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 <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: Vladimir Davydov <[email protected]>
Cc: [email protected]
Cc: [email protected]
---
mm/rmap.c | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)
diff --git a/mm/rmap.c b/mm/rmap.c
index 1b84945d655c..380c6b9956c2 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1054,8 +1054,14 @@ static void __page_set_anon_rmap(struct page *page,
if (!exclusive)
anon_vma = anon_vma->root;
+ /*
+ * 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.
+ */
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
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 <[email protected]>
> 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 <[email protected]>
> Cc: Johannes Weiner <[email protected]>
> Cc: Andrew Morton <[email protected]>
> Cc: Hugh Dickins <[email protected]>
> Cc: Matthew Wilcox <[email protected]>
> Cc: Minchan Kim <[email protected]>
> Cc: Vladimir Davydov <[email protected]>
> Cc: [email protected]
> Cc: [email protected]
Acked-by: Johannes Weiner <[email protected]>
Thanks Alex!
On Fri, 6 Nov 2020, Alex Shi wrote:
>
> updated for comments change from Johannes
>
>
> From 2fd278b1ca6c3e260ad249808b62f671d8db5a7b Mon Sep 17 00:00:00 2001
> From: Alex Shi <[email protected]>
> 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 <[email protected]>
> Cc: Johannes Weiner <[email protected]>
> Cc: Andrew Morton <[email protected]>
> Cc: Hugh Dickins <[email protected]>
Acked-by: Hugh Dickins <[email protected]>
Many thanks to Johannes for spotting my falsehood in the next patch,
and to Alex for making it true with this patch. As I just remarked
against the v20, I do have some more of these WRITE_ONCEs, but consider
them merely theoretical: so please don't let me hold this series up.
Andrew, I am hoping that Alex's v21 will appear in the next mmotm?
Thanks,
Hugh
> Cc: Matthew Wilcox <[email protected]>
> Cc: Minchan Kim <[email protected]>
> Cc: Vladimir Davydov <[email protected]>
> Cc: [email protected]
> Cc: [email protected]
> ---
> mm/rmap.c | 8 +++++++-
> 1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 1b84945d655c..380c6b9956c2 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1054,8 +1054,14 @@ static void __page_set_anon_rmap(struct page *page,
> if (!exclusive)
> anon_vma = anon_vma->root;
>
> + /*
> + * 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.
> + */
> 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