linux-kernel - Re: [PATCH v2 3/4] mm/gup: add a range variant of unpin_user_pages_dirty

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Date:   Thu, 4 Feb 2021 20:49:36 -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 v2 3/4] mm/gup: add a range variant of
 unpin_user_pages_dirty_lock()

On 2/4/21 12:24 PM, Joao Martins wrote:
> Add a unpin_user_page_range_dirty_lock() API which takes a starting page
> and how many consecutive pages we want to unpin and optionally 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

Thankfully, that claim is stale and can now be removed from this commit
description.

> page, or walk the page array. Finally add a separate iterator,
> 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           | 64 ++++++++++++++++++++++++++++++++++++++++++++++
>   2 files changed, 66 insertions(+)
> 
> 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 5a3dd235017a..3426736a01b2 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -215,6 +215,34 @@ void unpin_user_page(struct page *page)
>   }
>   EXPORT_SYMBOL(unpin_user_page);
>   
> +static inline void range_next(unsigned long i, unsigned long npages,
> +			      struct page **list, struct page **head,
> +			      unsigned int *ntails)

Would compound_range_next() be a better name?

> +{
> +	struct page *next, *page;
> +	unsigned int nr = 1;
> +
> +	if (i >= npages)
> +		return;
> +
> +	npages -= i;
> +	next = *list + i;
> +
> +	page = compound_head(next);
> +	if (PageCompound(page) && compound_order(page) > 1)
> +		nr = min_t(unsigned int,
> +			   page + compound_nr(page) - next, npages);

This pointer arithmetic will involve division. Which may be unnecessarily
expensive, if there is a way to calculate this with indices instead of
pointer arithmetic. I'm not sure if there is, off hand, but thought it
worth mentioning because the point is sometimes overlooked.

> +
> +	*head = page;
> +	*ntails = nr;
> +}
> +
> +#define for_each_compound_range(__i, __list, __npages, __head, __ntails) \
> +	for (__i = 0, \
> +	     range_next(__i, __npages, __list, &(__head), &(__ntails)); \
> +	     __i < __npages; __i += __ntails, \
> +	     range_next(__i, __npages, __list, &(__head), &(__ntails)))
> +
>   static inline void compound_next(unsigned long i, unsigned long npages,
>   				 struct page **list, struct page **head,
>   				 unsigned int *ntails)
> @@ -306,6 +334,42 @@ void unpin_user_pages_dirty_lock(struct page **pages, unsigned long npages,
>   }
>   EXPORT_SYMBOL(unpin_user_pages_dirty_lock);
>   
> +/**
> + * unpin_user_page_range_dirty_lock() - release and optionally dirty
> + * gup-pinned page range
> + *
> + * @page:  the starting page of a range maybe marked dirty, and definitely released.
> + * @npages: number of consecutive pages to release.
> + * @make_dirty: whether to mark the pages dirty
> + *
> + * "gup-pinned page range" refers to a range of pages that has had one of the
> + * get_user_pages() variants called on that page.
> + *
> + * For the page ranges defined by [page .. page+npages], make that range (or
> + * its head pages, if a compound page) dirty, if @make_dirty is true, and if the
> + * page range was previously listed as clean.
> + *
> + * set_page_dirty_lock() is used internally. If instead, set_page_dirty() is
> + * required, then the caller should a) verify that this is really correct,
> + * because _lock() is usually required, and b) hand code it:
> + * set_page_dirty_lock(), unpin_user_page().
> + *
> + */
> +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.
> 

Didn't spot any actual problems with how this works.

thanks,
-- 
John Hubbard
NVIDIA