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

From: Ajay Kaher
Date: Mon Dec 09 2019 - 03:55:01 EST

ïOn 06/12/19, 8:02 PM, "Vlastimil Babka" <vbabka@xxxxxxx> wrote:

> On 12/6/19 5:15 AM, Ajay Kaher wrote:
>> On 03/12/19, 6:28 PM, "Vlastimil Babka" <vbabka@xxxxxxx> wrote:
>>>>> [ 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()
>> Any reason for not using try_get_page_foll().
> Ah, sorry, I missed that previously. It's certainly possible to do it
> that way, I just didn't care so strongly to rewrite the existing SLES
> patch. It's a stable backport for a rather old LTS, not a codebase for
> further development.

Thanks for your response.

I would appreciate if you would like to include try_get_page_foll(),
and resend this patch series again.

Greg may require Acked-by from my side also, so if it's fine with you,
you can add or I will add once you will post this patch series again.

Let me know if anything else I can do here.

>>>>> 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.

Thanks for above explanation.