Re: [PATCH 4/4] mm/hugeltb: handle the error case in hugetlb_fix_reserve_counts()

[Miaohe Lin]

On 2021/4/9 7:25, Mike Kravetz wrote:
> On 4/2/21 2:32 AM, Miaohe Lin wrote:
>> A rare out of memory error would prevent removal of the reserve map region
>> for a page. hugetlb_fix_reserve_counts() handles this rare case to avoid
>> dangling with incorrect counts. Unfortunately, hugepage_subpool_get_pages
>> and hugetlb_acct_memory could possibly fail too. We should correctly handle
>> these cases.
> 
> Yes, this is a potential issue.
> 
> The 'good news' is that hugetlb_fix_reserve_counts() is unlikely to ever
> be called.  To do so would imply we could not allocate a region entry
> which is only 6 words in size.  We also keep a 'cache' of entries so we
> may not even need to allocate.
> 
> But, as mentioned it is a potential issue.

Yes, a potential *theoretical* issue.

> 
>> Fixes: b5cec28d36f5 ("hugetlbfs: truncate_hugepages() takes a range of pages")
> 
> This is likely going to make this get picked by by stable releases.
> That is unfortunate as mentioned above this is mostly theoretical.
> 

I will drop this. This does not worth backport.

>> Signed-off-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
>> ---
>>  mm/hugetlb.c | 11 +++++++++--
>>  1 file changed, 9 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>> index bdff8d23803f..ca5464ed04b7 100644
>> --- a/mm/hugetlb.c
>> +++ b/mm/hugetlb.c
>> @@ -745,13 +745,20 @@ void hugetlb_fix_reserve_counts(struct inode *inode)
>>  {
>>  	struct hugepage_subpool *spool = subpool_inode(inode);
>>  	long rsv_adjust;
>> +	bool reserved = false;
>>  
>>  	rsv_adjust = hugepage_subpool_get_pages(spool, 1);
>> -	if (rsv_adjust) {
>> +	if (rsv_adjust > 0) {
>>  		struct hstate *h = hstate_inode(inode);
>>  
>> -		hugetlb_acct_memory(h, 1);
>> +		if (!hugetlb_acct_memory(h, 1))
>> +			reserved = true;
>> +	} else if (!rsv_adjust) {
>> +		reserved = true;
>>  	}
>> +
>> +	if (!reserved)
>> +		pr_warn("hugetlb: fix reserve count failed\n");
> 
> We should expand this warning message a bit to indicate what this may
> mean to the user.  Add something like"
> 	"Huge Page Reserved count may go negative".
> 

Will add it in v2. Many thanks for review and nice suggestion ! :)