Re: [PATCH V1 1/9] mm/gup: repin_folio_unhugely

[David Hildenbrand <david@redhat.com>]

On 18.09.24 16:51, Steven Sistare wrote:
> On 9/17/2024 8:25 AM, David Hildenbrand wrote:
>> On 14.09.24 15:19, Steven Sistare wrote:
>>> cc'ing linux-mm for review of this one patch of the series.
>>>
>>> This proposes a new KAPI function repin_folio_unhugely(), for use in this
>>> patch of the iommu_ioas_map_file series:
>>>
>>>      iommufd: IOMMU_IOAS_MAP_FILE implementation
>>>      https://lore.kernel.org/linux-iommu/1726319158-283074-7-git-send-email-steven.sistare@oracle.com
>>>
>>> - Steve
>>>
>>> On 9/14/2024 9:05 AM, Steve Sistare wrote:
>>>> Export a function that repins a huge-page folio at small-page granularity.
>>>> This allows any range of small pages within the folio to be unpinned later.
>>>> For example, pages pinned via memfd_pin_folios and modified by
>>>> repin_folio_unhugely could be unpinned via unpin_user_page(s).
>>>>
>>>> Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
>>>> Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
>>>> ---
>>>>     include/linux/mm.h |  1 +
>>>>     mm/gup.c           | 18 ++++++++++++++++++
>>>>     2 files changed, 19 insertions(+)
>>>>
>>>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>>>> index 1470736..ba8344f 100644
>>>> --- a/include/linux/mm.h
>>>> +++ b/include/linux/mm.h
>>>> @@ -2514,6 +2514,7 @@ long pin_user_pages_unlocked(unsigned long start, unsigned long nr_pages,
>>>>     long memfd_pin_folios(struct file *memfd, loff_t start, loff_t end,
>>>>                   struct folio **folios, unsigned int max_folios,
>>>>                   pgoff_t *offset);
>>>> +void repin_folio_unhugely(struct folio *folio, unsigned long npin);
>>>>     int get_user_pages_fast(unsigned long start, int nr_pages,
>>>>                 unsigned int gup_flags, struct page **pages);
>>>> diff --git a/mm/gup.c b/mm/gup.c
>>>> index 947881ff..f8f3f2a 100644
>>>> --- a/mm/gup.c
>>>> +++ b/mm/gup.c
>>>> @@ -3720,3 +3720,21 @@ long memfd_pin_folios(struct file *memfd, loff_t start, loff_t end,
>>>>         return ret;
>>>>     }
>>>>     EXPORT_SYMBOL_GPL(memfd_pin_folios);
>>>> +
>>>> +/**
>>>> + * repin_folio_unhugely() - repin a folio at small page granularity
>>>> + * @folio: the folio to repin
>>>> + * @npin:  the number of pages pinned in the folio
>>>> + *
>>>> + * Given a huge page folio that is already pinned, and the number of small
>>
>> s/huge page folio/large folio/
>>
>>>> + * pages that are pinned in it, adjust the pincount to reflect small-page
>>>> + * granularity.  Each small page can later be unpinned individually.
>>>> + */
>>>> +void repin_folio_unhugely(struct folio *folio, unsigned long npin)
>>>> +{
>>>> +    if (!folio_test_large(folio) || is_huge_zero_folio(folio) || npin == 1)
>>
>> Why not the huge zero folio? That looks very odd here.
> 
> The zero page is treated specially here and elsewhere, it can never be deleted so
> reference fiddling is skipped.

Please point me in mm/gup.c at that handling.

IIRC is_zero_folio() does *not* include the huge zero page.

Yes, we should likely be special-casing the huge zeropage in mm/gup.c, 
but it's not that easy because PINs can outlive MMs ... so *not* 
grabbing a reference could currently be harmful.

But that has do be changed consistently, not with doing things here 
different compared to other gup.c functions.

> 
>>>> +        return;
>>>> +    atomic_add(npin - 1, &folio->_refcount);
>>>> +    atomic_add(npin - 1, &folio->_pincount);
>>>> +}
>>>> +EXPORT_SYMBOL_GPL(repin_folio_unhugely);
>>
>> Can we ... find a better name? For example, it's "large" folio not "huge"...
>>
>> And repin is really misleading. We are simply adding more pins to an already pinned one ...
> 
> Jason suggests a better name in the other thread.

I would prefer something that simply adds more pins to an already pinned 
folio. Much easier to get.

-- 
Cheers,

David / dhildenb