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SubjectRe: [PATCH STABLE 4.4 5/8] mm: prevent get_user_pages() from overflowing page refcount
On 12/3/19 1:25 PM, Ajay Kaher wrote:
> On 08/11/19, 3:08 PM, "Vlastimil Babka" <> wrote:
>> From: Linus Torvalds <>
>> 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:
> + 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:

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.

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