Re: [PATCH STABLE 4.4 5/8] mm: prevent get_user_pages() from overflowing page refcount

[Vlastimil Babka]

On 12/3/19 1:25 PM, Ajay Kaher wrote:
> 
> 
> ïOn 08/11/19, 3:08 PM, "Vlastimil Babka" <vbabka@xxxxxxx> wrote:
> 
>> From: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
>>
>> commit 8fde12ca79aff9b5ba951fce1a2641901b8d8e64 upstream.
>>    
>> [ 4.4 backport: there's get_page_foll(), so add try_get_page()-like checks
>>                 in there, enabled by a new parameter, which is false where
>>                 upstream patch doesn't replace get_page() with try_get_page()
>>                 (the THP and hugetlb callers).
> 
> Could we have try_get_page_foll(), as in:
> https://lore.kernel.org/stable/1570581863-12090-3-git-send-email-akaher@xxxxxxxxxx/
> 
> + Code will be in sync as we have try_get_page()
> + No need to add extra argument to try_get_page()
> + No need to modify the callers of try_get_page()
> 
>> 		In gup_pte_range(), we don't expect tail pages, so just check
>>                 page ref count instead of try_get_compound_head()
> 
> Technically it's fine. If you want to keep the code of stable versions in sync
> with latest versions then this could be done in following ways (without any
> modification in upstream patch for gup_pte_range()):
> 
> Apply 7aef4172c7957d7e65fc172be4c99becaef855d4 before applying
> 8fde12ca79aff9b5ba951fce1a2641901b8d8e64, as done here:
> https://lore.kernel.org/stable/1570581863-12090-4-git-send-email-akaher@xxxxxxxxxx/

Yup, I have considered that, and deliberately didn't add that commit
7aef4172c795 ("mm: handle PTE-mapped tail pages in gerneric fast gup
implementaiton") as it's part of a large THP refcount rework. In 4.4 we
don't expect to GUP tail pages so I wanted to keep it that way -
minimally, the compound_head() operation is a unnecessary added cost,
although it would also work.