* [PATCH] ksm: Revert "use GET_KSM_PAGE_NOLOCK to get ksm page in remove_rmap_item_from_tree()"
@ 2021-05-10 5:59 ` Hugh Dickins
0 siblings, 0 replies; 6+ messages in thread
From: Hugh Dickins @ 2021-05-10 5:59 UTC (permalink / raw)
To: Andrew Morton; +Cc: Hugh Dickins, Miaohe Lin, linux-kernel, linux-mm
This reverts commit 3e96b6a2e9ad929a3230a22f4d64a74671a0720b.
General Protection Fault in rmap_walk_ksm() under memory pressure:
remove_rmap_item_from_tree() needs to take page lock, of course.
Signed-off-by: Hugh Dickins <hughd@google.com>
---
mm/ksm.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
--- 5.13-rc1/mm/ksm.c 2021-05-09 17:03:44.010422188 -0700
+++ linux/mm/ksm.c 2021-05-09 22:12:39.403008350 -0700
@@ -776,11 +776,12 @@ static void remove_rmap_item_from_tree(s
struct page *page;
stable_node = rmap_item->head;
- page = get_ksm_page(stable_node, GET_KSM_PAGE_NOLOCK);
+ page = get_ksm_page(stable_node, GET_KSM_PAGE_LOCK);
if (!page)
goto out;
hlist_del(&rmap_item->hlist);
+ unlock_page(page);
put_page(page);
if (!hlist_empty(&stable_node->hlist))
^ permalink raw reply [flat|nested] 6+ messages in thread
* [PATCH] ksm: Revert "use GET_KSM_PAGE_NOLOCK to get ksm page in remove_rmap_item_from_tree()"
@ 2021-05-10 5:59 ` Hugh Dickins
0 siblings, 0 replies; 6+ messages in thread
From: Hugh Dickins @ 2021-05-10 5:59 UTC (permalink / raw)
To: Andrew Morton; +Cc: Hugh Dickins, Miaohe Lin, linux-kernel, linux-mm
This reverts commit 3e96b6a2e9ad929a3230a22f4d64a74671a0720b.
General Protection Fault in rmap_walk_ksm() under memory pressure:
remove_rmap_item_from_tree() needs to take page lock, of course.
Signed-off-by: Hugh Dickins <hughd@google.com>
---
mm/ksm.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
--- 5.13-rc1/mm/ksm.c 2021-05-09 17:03:44.010422188 -0700
+++ linux/mm/ksm.c 2021-05-09 22:12:39.403008350 -0700
@@ -776,11 +776,12 @@ static void remove_rmap_item_from_tree(s
struct page *page;
stable_node = rmap_item->head;
- page = get_ksm_page(stable_node, GET_KSM_PAGE_NOLOCK);
+ page = get_ksm_page(stable_node, GET_KSM_PAGE_LOCK);
if (!page)
goto out;
hlist_del(&rmap_item->hlist);
+ unlock_page(page);
put_page(page);
if (!hlist_empty(&stable_node->hlist))
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH] ksm: Revert "use GET_KSM_PAGE_NOLOCK to get ksm page in remove_rmap_item_from_tree()"
2021-05-10 5:59 ` Hugh Dickins
(?)
@ 2021-05-10 6:57 ` Miaohe Lin
2021-05-10 23:42 ` Hugh Dickins
-1 siblings, 1 reply; 6+ messages in thread
From: Miaohe Lin @ 2021-05-10 6:57 UTC (permalink / raw)
To: Hugh Dickins, Andrew Morton; +Cc: linux-kernel, linux-mm
On 2021/5/10 13:59, Hugh Dickins wrote:
> This reverts commit 3e96b6a2e9ad929a3230a22f4d64a74671a0720b.
> General Protection Fault in rmap_walk_ksm() under memory pressure:
> remove_rmap_item_from_tree() needs to take page lock, of course.
>
I'am really sorry about it! And many thanks for this bugfix!
It seems rmap_walk_ksm() relies on the page lock to protect against
concurrent modifications to that page's node of the stable tree.
Could you please add a comment in remove_rmap_item_from_tree() to
clarify this in case similar trouble again? Many thanks!
> Signed-off-by: Hugh Dickins <hughd@google.com>
> ---
>
> mm/ksm.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> --- 5.13-rc1/mm/ksm.c 2021-05-09 17:03:44.010422188 -0700
> +++ linux/mm/ksm.c 2021-05-09 22:12:39.403008350 -0700
> @@ -776,11 +776,12 @@ static void remove_rmap_item_from_tree(s
> struct page *page;
>
> stable_node = rmap_item->head;
> - page = get_ksm_page(stable_node, GET_KSM_PAGE_NOLOCK);
> + page = get_ksm_page(stable_node, GET_KSM_PAGE_LOCK);
> if (!page)
> goto out;
>
> hlist_del(&rmap_item->hlist);
> + unlock_page(page);
> put_page(page);
>
> if (!hlist_empty(&stable_node->hlist))
> .
>
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH] ksm: Revert "use GET_KSM_PAGE_NOLOCK to get ksm page in remove_rmap_item_from_tree()"
2021-05-10 6:57 ` Miaohe Lin
@ 2021-05-10 23:42 ` Hugh Dickins
0 siblings, 0 replies; 6+ messages in thread
From: Hugh Dickins @ 2021-05-10 23:42 UTC (permalink / raw)
To: Miaohe Lin; +Cc: Hugh Dickins, Andrew Morton, linux-kernel, linux-mm
On Mon, 10 May 2021, Miaohe Lin wrote:
> On 2021/5/10 13:59, Hugh Dickins wrote:
> > This reverts commit 3e96b6a2e9ad929a3230a22f4d64a74671a0720b.
> > General Protection Fault in rmap_walk_ksm() under memory pressure:
> > remove_rmap_item_from_tree() needs to take page lock, of course.
> >
>
> I'am really sorry about it! And many thanks for this bugfix!
> It seems rmap_walk_ksm() relies on the page lock to protect against
> concurrent modifications to that page's node of the stable tree.
> Could you please add a comment in remove_rmap_item_from_tree() to
> clarify this in case similar trouble again? Many thanks!
Sorry, no. Page lock is held by callers of stable_tree_append() when
adding an rmap_item to the tree, and held by callers of rmap_walk_ksm()
(see VM_BUG_ON_PAGE there) when walking the tree: you would surely
expect some kind of locking when removing an rmap_item from the tree,
and the appropriate page lock is what GET_KSM_PAGE_LOCK provided.
I do not want us to go through the kernel source adding a comment
/* We really mean to take this lock: it protects against concurrency */
every time we take a lock in the kernel: you should generally assume
that if a lock is taken, then the writer intended it to be taken.
There are sure to be some exceptions, where a lock is taken pointlessly:
but please look deeper before assuming that is the case.
Hugh
>
> > Signed-off-by: Hugh Dickins <hughd@google.com>
> > ---
> >
> > mm/ksm.c | 3 ++-
> > 1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > --- 5.13-rc1/mm/ksm.c 2021-05-09 17:03:44.010422188 -0700
> > +++ linux/mm/ksm.c 2021-05-09 22:12:39.403008350 -0700
> > @@ -776,11 +776,12 @@ static void remove_rmap_item_from_tree(s
> > struct page *page;
> >
> > stable_node = rmap_item->head;
> > - page = get_ksm_page(stable_node, GET_KSM_PAGE_NOLOCK);
> > + page = get_ksm_page(stable_node, GET_KSM_PAGE_LOCK);
> > if (!page)
> > goto out;
> >
> > hlist_del(&rmap_item->hlist);
> > + unlock_page(page);
> > put_page(page);
> >
> > if (!hlist_empty(&stable_node->hlist))
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH] ksm: Revert "use GET_KSM_PAGE_NOLOCK to get ksm page in remove_rmap_item_from_tree()"
@ 2021-05-10 23:42 ` Hugh Dickins
0 siblings, 0 replies; 6+ messages in thread
From: Hugh Dickins @ 2021-05-10 23:42 UTC (permalink / raw)
To: Miaohe Lin; +Cc: Hugh Dickins, Andrew Morton, linux-kernel, linux-mm
On Mon, 10 May 2021, Miaohe Lin wrote:
> On 2021/5/10 13:59, Hugh Dickins wrote:
> > This reverts commit 3e96b6a2e9ad929a3230a22f4d64a74671a0720b.
> > General Protection Fault in rmap_walk_ksm() under memory pressure:
> > remove_rmap_item_from_tree() needs to take page lock, of course.
> >
>
> I'am really sorry about it! And many thanks for this bugfix!
> It seems rmap_walk_ksm() relies on the page lock to protect against
> concurrent modifications to that page's node of the stable tree.
> Could you please add a comment in remove_rmap_item_from_tree() to
> clarify this in case similar trouble again? Many thanks!
Sorry, no. Page lock is held by callers of stable_tree_append() when
adding an rmap_item to the tree, and held by callers of rmap_walk_ksm()
(see VM_BUG_ON_PAGE there) when walking the tree: you would surely
expect some kind of locking when removing an rmap_item from the tree,
and the appropriate page lock is what GET_KSM_PAGE_LOCK provided.
I do not want us to go through the kernel source adding a comment
/* We really mean to take this lock: it protects against concurrency */
every time we take a lock in the kernel: you should generally assume
that if a lock is taken, then the writer intended it to be taken.
There are sure to be some exceptions, where a lock is taken pointlessly:
but please look deeper before assuming that is the case.
Hugh
>
> > Signed-off-by: Hugh Dickins <hughd@google.com>
> > ---
> >
> > mm/ksm.c | 3 ++-
> > 1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > --- 5.13-rc1/mm/ksm.c 2021-05-09 17:03:44.010422188 -0700
> > +++ linux/mm/ksm.c 2021-05-09 22:12:39.403008350 -0700
> > @@ -776,11 +776,12 @@ static void remove_rmap_item_from_tree(s
> > struct page *page;
> >
> > stable_node = rmap_item->head;
> > - page = get_ksm_page(stable_node, GET_KSM_PAGE_NOLOCK);
> > + page = get_ksm_page(stable_node, GET_KSM_PAGE_LOCK);
> > if (!page)
> > goto out;
> >
> > hlist_del(&rmap_item->hlist);
> > + unlock_page(page);
> > put_page(page);
> >
> > if (!hlist_empty(&stable_node->hlist))
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH] ksm: Revert "use GET_KSM_PAGE_NOLOCK to get ksm page in remove_rmap_item_from_tree()"
2021-05-10 23:42 ` Hugh Dickins
(?)
@ 2021-05-11 1:40 ` Miaohe Lin
-1 siblings, 0 replies; 6+ messages in thread
From: Miaohe Lin @ 2021-05-11 1:40 UTC (permalink / raw)
To: Hugh Dickins; +Cc: Andrew Morton, linux-kernel, linux-mm
On 2021/5/11 7:42, Hugh Dickins wrote:
> On Mon, 10 May 2021, Miaohe Lin wrote:
>> On 2021/5/10 13:59, Hugh Dickins wrote:
>>> This reverts commit 3e96b6a2e9ad929a3230a22f4d64a74671a0720b.
>>> General Protection Fault in rmap_walk_ksm() under memory pressure:
>>> remove_rmap_item_from_tree() needs to take page lock, of course.
>>>
>>
>> I'am really sorry about it! And many thanks for this bugfix!
>> It seems rmap_walk_ksm() relies on the page lock to protect against
>> concurrent modifications to that page's node of the stable tree.
>> Could you please add a comment in remove_rmap_item_from_tree() to
>> clarify this in case similar trouble again? Many thanks!
>
> Sorry, no. Page lock is held by callers of stable_tree_append() when
> adding an rmap_item to the tree, and held by callers of rmap_walk_ksm()
> (see VM_BUG_ON_PAGE there) when walking the tree: you would surely
> expect some kind of locking when removing an rmap_item from the tree,
> and the appropriate page lock is what GET_KSM_PAGE_LOCK provided.
>
> I do not want us to go through the kernel source adding a comment
> /* We really mean to take this lock: it protects against concurrency */
> every time we take a lock in the kernel: you should generally assume
> that if a lock is taken, then the writer intended it to be taken.
>
> There are sure to be some exceptions, where a lock is taken pointlessly:
> but please look deeper before assuming that is the case.
>
I see. I should have been more careful. Many thanks for your detailed explanation
and sorry about trouble!
> Hugh
>
>>
>>> Signed-off-by: Hugh Dickins <hughd@google.com>
>>> ---
>>>
>>> mm/ksm.c | 3 ++-
>>> 1 file changed, 2 insertions(+), 1 deletion(-)
>>>
>>> --- 5.13-rc1/mm/ksm.c 2021-05-09 17:03:44.010422188 -0700
>>> +++ linux/mm/ksm.c 2021-05-09 22:12:39.403008350 -0700
>>> @@ -776,11 +776,12 @@ static void remove_rmap_item_from_tree(s
>>> struct page *page;
>>>
>>> stable_node = rmap_item->head;
>>> - page = get_ksm_page(stable_node, GET_KSM_PAGE_NOLOCK);
>>> + page = get_ksm_page(stable_node, GET_KSM_PAGE_LOCK);
>>> if (!page)
>>> goto out;
>>>
>>> hlist_del(&rmap_item->hlist);
>>> + unlock_page(page);
>>> put_page(page);
>>>
>>> if (!hlist_empty(&stable_node->hlist))
> .
>
^ permalink raw reply [flat|nested] 6+ messages in thread
end of thread, other threads:[~2021-05-11 1:40 UTC | newest]
Thread overview: 6+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2021-05-10 5:59 [PATCH] ksm: Revert "use GET_KSM_PAGE_NOLOCK to get ksm page in remove_rmap_item_from_tree()" Hugh Dickins
2021-05-10 5:59 ` Hugh Dickins
2021-05-10 6:57 ` Miaohe Lin
2021-05-10 23:42 ` Hugh Dickins
2021-05-10 23:42 ` Hugh Dickins
2021-05-11 1:40 ` Miaohe Lin
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