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SubjectRe: [PATCH v2 1/4] mm/gup: add compound page list iterator
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On 2/5/21 4:11 AM, John Hubbard wrote:
> On 2/4/21 12:24 PM, Joao Martins wrote:
>> Add an helper that iterates over head pages in a list of pages. It
>> essentially counts the tails until the next page to process has a
>> different head that the current. This is going to be used by
>> unpin_user_pages() family of functions, to batch the head page refcount
>> updates once for all passed consecutive tail pages.
>>
>> Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
>> Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
>> ---
>> mm/gup.c | 29 +++++++++++++++++++++++++++++
>> 1 file changed, 29 insertions(+)
>>
>> diff --git a/mm/gup.c b/mm/gup.c
>> index d68bcb482b11..d1549c61c2f6 100644
>> --- a/mm/gup.c
>> +++ b/mm/gup.c
>> @@ -215,6 +215,35 @@ void unpin_user_page(struct page *page)
>> }
>> EXPORT_SYMBOL(unpin_user_page);
>>
>> +static inline void compound_next(unsigned long i, unsigned long npages,
>> + struct page **list, struct page **head,
>> + unsigned int *ntails)
>> +{
>> + struct page *page;
>> + unsigned int nr;
>> +
>> + if (i >= npages)
>> + return;
>> +
>> + list += i;
>> + npages -= i;
>
> It is worth noting that this is slightly more complex to read than it needs to be.
> You are changing both endpoints of a loop at once. That's hard to read for a human.
> And you're only doing it in order to gain the small benefit of being able to
> use nr directly at the end of the routine.
>
> If instead you keep npages constant like it naturally wants to be, you could
> just do a "(*ntails)++" in the loop, to take care of *ntails.
>
I didn't do it as such as I would need to deref @ntails per iteration, so
it felt more efficient to do as above. On a second thought, I could alternatively do the
following below, thoughts?

diff --git a/mm/gup.c b/mm/gup.c
index d68bcb482b11..8defe4f670d5 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -215,6 +215,32 @@ void unpin_user_page(struct page *page)
}
EXPORT_SYMBOL(unpin_user_page);

+static inline void compound_next(unsigned long i, unsigned long npages,
+ struct page **list, struct page **head,
+ unsigned int *ntails)
+{
+ struct page *page;
+ unsigned int nr;
+
+ if (i >= npages)
+ return;
+
+ page = compound_head(list[i]);
+ for (nr = i + 1; nr < npages; nr++) {
+ if (compound_head(list[nr]) != page)
+ break;
+ }
+
+ *head = page;
+ *ntails = nr - i;
+}
+

> However, given that the patch is correct and works as-is, the above is really just
> an optional idea, so please feel free to add:
>
> Reviewed-by: John Hubbard <jhubbard@nvidia.com>
>
>
Thanks!

Hopefully I can retain that if the snippet above is preferred?

Joao
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