Re: [PATCH] hugetlb: do not demote poisoned hugetlb pages

[Miaohe Lin]

On 2022/3/17 6:31, Mike Kravetz wrote:
> On 3/8/22 05:43, Miaohe Lin wrote:
>> On 2022/3/8 5:57, Mike Kravetz wrote:
>>> It is possible for poisoned hugetlb pages to reside on the free lists.
>>> The huge page allocation routines which dequeue entries from the free
>>> lists make a point of avoiding poisoned pages.  There is no such check
>>> and avoidance in the demote code path.
>>>
>>> If a hugetlb page on the is on a free list, poison will only be set in
>>> the head page rather then the page with the actual error.  If such a
>>> page is demoted, then the poison flag may follow the wrong page.  A page
>>> without error could have poison set, and a page with poison could not
>>> have the flag set.
>>>
>>> Check for poison before attempting to demote a hugetlb page.  Also,
>>> return -EBUSY to the caller if only poisoned pages are on the free list.
>>>
>>> Fixes: 8531fc6f52f5 ("hugetlb: add hugetlb demote page support")
>>> Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
>>> Cc: <stable@xxxxxxxxxxxxxxx>
>>> ---
>>>  mm/hugetlb.c | 17 ++++++++++-------
>>>  1 file changed, 10 insertions(+), 7 deletions(-)
>>>
>>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>>> index b34f50156f7e..f8ca7cca3c1a 100644
>>> --- a/mm/hugetlb.c
>>> +++ b/mm/hugetlb.c
>>> @@ -3475,7 +3475,6 @@ static int demote_pool_huge_page(struct hstate *h, nodemask_t *nodes_allowed)
>>>  {
>>>  	int nr_nodes, node;
>>>  	struct page *page;
>>> -	int rc = 0;
>>>  
>>>  	lockdep_assert_held(&hugetlb_lock);
>>>  
>>> @@ -3486,15 +3485,19 @@ static int demote_pool_huge_page(struct hstate *h, nodemask_t *nodes_allowed)
>>>  	}
>>>  
>>>  	for_each_node_mask_to_free(h, nr_nodes, node, nodes_allowed) {
>>> -		if (!list_empty(&h->hugepage_freelists[node])) {
>>> -			page = list_entry(h->hugepage_freelists[node].next,
>>> -					struct page, lru);
>>> -			rc = demote_free_huge_page(h, page);
>>> -			break;
>>> +		list_for_each_entry(page, &h->hugepage_freelists[node], lru) {
>>> +			if (PageHWPoison(page))
>>> +				continue;
>>> +
>>> +			return demote_free_huge_page(h, page);
>>
>> It seems this patch is not ideal. Memory failure can hit the hugetlb page anytime without
>> holding the hugetlb_lock. So the page might become HWPoison just after the check. But this
>> patch should have handled the common case. Many thanks for your work. :)
>>
> 
> Correct, this patch handles the common case of not demoting a hugetlb
> page if HWPoison is set.  This is similar to code in the dequeue path
> used when allocating a huge page for allocation use.
> 
> As you point out, work still needs to be done to better coordinate
> memory failure with demote as well as huge page freeing.  As you know
> Naoya is working on this now.  It is unclear if that work will be limited
> to memory error handling code, or if greater coordination with hugetlb
> code will be required.
> 
> Unless you have objections, I believe this patch should move forward and
> be backported to stable trees.  If we determine that more coordination
> between memory error and hugetlb code is needed, that can be added later. 

I think this patch is good enough to move forward and be backported to stable trees.
Many thanks. :)

>