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SubjectRe: [PATCH 3/4] mm/gup: add a range variant of unpin_user_pages_dirty_lock()
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On 2/3/21 11:37 PM, John Hubbard wrote:
> On 2/3/21 2:00 PM, Joao Martins wrote:
>> Add a unpin_user_page_range() API which takes a starting page
>> and how many consecutive pages we want to dirty.
>>
>> Given that we won't be iterating on a list of changes, change
>> compound_next() to receive a bool, whether to calculate from the starting
>> page, or walk the page array. Finally add a separate iterator,
>
> A bool arg is sometimes, but not always, a hint that you really just want
> a separate set of routines. Below...
>
Yes.

I was definitely wrestling back and forth a lot about having separate routines for two
different iterators helpers i.e. compound_next_head()or having it all merged into one
compound_next() / count_ntails().

>> for_each_compound_range() that just operate in page ranges as opposed
>> to page array.
>>
>> For users (like RDMA mr_dereg) where each sg represents a
>> contiguous set of pages, we're able to more efficiently unpin
>> pages without having to supply an array of pages much of what
>> happens today with unpin_user_pages().
>>
>> Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
>> Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
>> ---
>> include/linux/mm.h | 2 ++
>> mm/gup.c | 48 ++++++++++++++++++++++++++++++++++++++--------
>> 2 files changed, 42 insertions(+), 8 deletions(-)
>>
>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>> index a608feb0d42e..b76063f7f18a 100644
>> --- a/include/linux/mm.h
>> +++ b/include/linux/mm.h
>> @@ -1265,6 +1265,8 @@ static inline void put_page(struct page *page)
>> void unpin_user_page(struct page *page);
>> void unpin_user_pages_dirty_lock(struct page **pages, unsigned long npages,
>> bool make_dirty);
>> +void unpin_user_page_range_dirty_lock(struct page *page, unsigned long npages,
>> + bool make_dirty);
>> void unpin_user_pages(struct page **pages, unsigned long npages);
>>
>> /**
>> diff --git a/mm/gup.c b/mm/gup.c
>> index 971a24b4b73f..1b57355d5033 100644
>> --- a/mm/gup.c
>> +++ b/mm/gup.c
>> @@ -215,11 +215,16 @@ void unpin_user_page(struct page *page)
>> }
>> EXPORT_SYMBOL(unpin_user_page);
>>
>> -static inline unsigned int count_ntails(struct page **pages, unsigned long npages)
>> +static inline unsigned int count_ntails(struct page **pages,
>> + unsigned long npages, bool range)
>> {
>> - struct page *head = compound_head(pages[0]);
>> + struct page *page = pages[0], *head = compound_head(page);
>> unsigned int ntails;
>>
>> + if (range)
>> + return (!PageCompound(head) || compound_order(head) <= 1) ? 1 :
>> + min_t(unsigned int, (head + compound_nr(head) - page), npages);
>
> Here, you clearly should use a separate set of _range routines. Because you're basically
> creating two different routines here! Keep it simple.
>
> Once you're in a separate routine, you might feel more comfortable expanding that to
> a more readable form, too:
>
> if (!PageCompound(head) || compound_order(head) <= 1)
> return 1;
>
> return min_t(unsigned int, (head + compound_nr(head) - page), npages);
>
Yes.

Let me also try instead to put move everything into two sole iterator helper routines,
compound_next() and compound_next_range(), and thus get rid of this count_ntails(). It
should also help in removing a compound_head() call which should save cycles.

Joao

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