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

[David Hildenbrand <david@redhat.com>]

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.

>> +		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 ...

-- 
Cheers,

David / dhildenb