Re: [PATCH V1 2/5] mm/hugetlb: fix memfd_pin_folios free_huge_pages leak

[Steven Sistare <steven.sistare@oracle.com>]

On 9/3/2024 8:45 PM, Kasireddy, Vivek wrote:
> Hi Steve,
> 
>> Subject: [PATCH V1 2/5] mm/hugetlb: fix memfd_pin_folios free_huge_pages
>> leak
>>
>> memfd_pin_folios followed by unpin_folios fails to restore
>> free_huge_pages if the pages were not already faulted in, because the
>> folio refcount for pages created by memfd_alloc_folio never goes to 0.
>> memfd_pin_folios needs another folio_put to undo the folio_try_get
>> below:
>>
>> memfd_alloc_folio()
>>    alloc_hugetlb_folio_nodemask()
>>      dequeue_hugetlb_folio_nodemask()
>>        dequeue_hugetlb_folio_node_exact()
>>          folio_ref_unfreeze(folio, 1);    ; adds 1 refcount
>>    folio_try_get()                        ; adds 1 refcount
> I wonder if it is more optimal to skip the folio_try_get() above.
> I think it (folio_try_get) was probably added because I didn't realize that
> alloc_hugetlb_folio_nodemask() already adds a reference.

Agreed, that is a simpler fix.  I tried it and my tests pass.

>>    hugetlb_add_to_page_cache()            ; adds 512 refcount (on x86)
>>
>> With the fix, after memfd_pin_folios + unpin_folios, the refcount for
>> the (unfaulted) page is 512, which is correct, as the refcount for a
>> faulted unpinned page is 513.
>>
>> Fixes: 89c1905d9c14 ("mm/gup: introduce memfd_pin_folios() for pinning
>> memfd folios")
>>
>> Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
>> ---
>>   mm/gup.c | 4 +++-
>>   1 file changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/mm/gup.c b/mm/gup.c
>> index 54d0dc3..5b92f1d 100644
>> --- a/mm/gup.c
>> +++ b/mm/gup.c
>> @@ -3618,7 +3618,7 @@ long memfd_pin_folios(struct file *memfd, loff_t
>> start, loff_t end,
>>   	pgoff_t start_idx, end_idx, next_idx;
>>   	struct folio *folio = NULL;
>>   	struct folio_batch fbatch;
>> -	struct hstate *h;
>> +	struct hstate *h = NULL;
>>   	long ret = -EINVAL;
>>
>>   	if (start < 0 || start > end || !max_folios)
>> @@ -3662,6 +3662,8 @@ long memfd_pin_folios(struct file *memfd, loff_t
>> start, loff_t end,
>>   							     &fbatch);
>>   			if (folio) {
>>   				folio_put(folio);
>> +				if (h)
>> +					folio_put(folio);
> If we stick with this change, I guess there needs to be an additional
> folio_put() in memfd_alloc_folio() as well if adding the folio to the
> page cache fails.

Indeed, another reason why just deleting the folio_try_get is better.  Thanks!

I will submit a V2 of this patch.

- Steve