Re: [PATCH] ksm: Revert "use GET_KSM_PAGE_NOLOCK to get ksm page in remove_rmap_item_from_tree()"

[Miaohe Lin]

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@xxxxxxxxxx>
>>> ---
>>>
>>>  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))
> .
>