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

[Steven Sistare <steven.sistare@oracle.com>]

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.

>>> +        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.

- Steve