Re: [PATCH STABLE 4.4 5/8] mm: prevent get_user_pages() from overflowing page refcount
From: Ajay Kaher
Date: Thu Dec 05 2019 - 23:16:11 EST
ïOn 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:
>> https://nam04.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.kernel.org%2Fstable%2F1570581863-12090-3-git-send-email-akaher%40vmware.com%2F&data=02%7C01%7Cakaher%40vmware.com%7Cb6592f0fbec040aa045f08d777f06a9f%7Cb39138ca3cee4b4aa4d6cd83d9dd62f0%7C0%7C0%7C637109746821395444&sdata=cYBj3SvEikPbiHsVZj3zCys8t9ISLiHKzAlsSqiZRW8%3D&reserved=0
>>
>> + 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().
>>> 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://nam04.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.kernel.org%2Fstable%2F1570581863-12090-4-git-send-email-akaher%40vmware.com%2F&data=02%7C01%7Cakaher%40vmware.com%7Cb6592f0fbec040aa045f08d777f06a9f%7Cb39138ca3cee4b4aa4d6cd83d9dd62f0%7C0%7C0%7C637109746821395444&sdata=gTJMJ3Yx6G0ng46TQsBzCS2DowwP7YtIjluKJuqvN6o%3D&reserved=0
> 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.