linux-kernel - Re: [PATCH 3/4] mm/gup: add a range variant of unpin_user_pages_dirty

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Date:   Wed, 3 Feb 2021 15:37:26 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     Joao Martins <joao.m.martins@...cle.com>, <linux-mm@...ck.org>
CC:     <linux-kernel@...r.kernel.org>, <linux-rdma@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Doug Ledford <dledford@...hat.com>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH 3/4] mm/gup: add a range variant of
 unpin_user_pages_dirty_lock()

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...

> 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@...dia.com>
> Signed-off-by: Joao Martins <joao.m.martins@...cle.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);


thanks,
-- 
John Hubbard
NVIDIA

> +
>   	for (ntails = 1; ntails < npages; ntails++) {
>   		if (compound_head(pages[ntails]) != head)
>   			break;
> @@ -229,20 +234,32 @@ static inline unsigned int count_ntails(struct page **pages, unsigned long npage
>   }
>   
>   static inline void compound_next(unsigned long i, unsigned long npages,
> -				 struct page **list, struct page **head,
> -				 unsigned int *ntails)
> +				 struct page **list, bool range,
> +				 struct page **head, unsigned int *ntails)
>   {
> +	struct page *p, **next = &p;
> +
>   	if (i >= npages)
>   		return;
>   
> -	*ntails = count_ntails(list + i, npages - i);
> -	*head = compound_head(list[i]);
> +	if (range)
> +		*next = *list + i;
> +	else
> +		next = list + i;
> +
> +	*ntails = count_ntails(next, npages - i, range);
> +	*head = compound_head(*next);
>   }
>   
> +#define for_each_compound_range(i, list, npages, head, ntails) \
> +	for (i = 0, compound_next(i, npages, list, true, &head, &ntails); \
> +	     i < npages; i += ntails, \
> +	     compound_next(i, npages, list, true,  &head, &ntails))
> +
>   #define for_each_compound_head(i, list, npages, head, ntails) \
> -	for (i = 0, compound_next(i, npages, list, &head, &ntails); \
> +	for (i = 0, compound_next(i, npages, list, false, &head, &ntails); \
>   	     i < npages; i += ntails, \
> -	     compound_next(i, npages, list, &head, &ntails))
> +	     compound_next(i, npages, list, false,  &head, &ntails))
>   
>   /**
>    * unpin_user_pages_dirty_lock() - release and optionally dirty gup-pinned pages
> @@ -306,6 +323,21 @@ void unpin_user_pages_dirty_lock(struct page **pages, unsigned long npages,
>   }
>   EXPORT_SYMBOL(unpin_user_pages_dirty_lock);
>   
> +void unpin_user_page_range_dirty_lock(struct page *page, unsigned long npages,
> +				      bool make_dirty)
> +{
> +	unsigned long index;
> +	struct page *head;
> +	unsigned int ntails;
> +
> +	for_each_compound_range(index, &page, npages, head, ntails) {
> +		if (make_dirty && !PageDirty(head))
> +			set_page_dirty_lock(head);
> +		put_compound_head(head, ntails, FOLL_PIN);
> +	}
> +}
> +EXPORT_SYMBOL(unpin_user_page_range_dirty_lock);
> +
>   /**
>    * unpin_user_pages() - release an array of gup-pinned pages.
>    * @pages:  array of pages to be marked dirty and released.
>