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

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Message-ID: <9e70bd93-783f-3d0d-c53f-ec32480477d6@oracle.com>
Date:   Thu, 4 Feb 2021 11:47:51 +0000
From:   Joao Martins <joao.m.martins@...cle.com>
To:     John Hubbard <jhubbard@...dia.com>
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>, linux-mm@...ck.org
Subject: Re: [PATCH 3/4] mm/gup: add a range variant of
 unpin_user_pages_dirty_lock()



On 2/4/21 12:11 AM, John Hubbard wrote:
> On 2/3/21 2:00 PM, Joao Martins wrote:
> ...
>> +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);
>> +
> 
> Also, looking at this again while trying to verify the sg diffs in patch #4, I noticed
> that the name is tricky. Usually a "range" would not have a single struct page* as the
> argument. So in this case, perhaps a comment above the function would help, something
> approximately like this:
> 
> /*
>   * The "range" in the function name refers to the fact that the page may be a
>   * compound page. If so, then that compound page will be treated as one or more
>   * ranges of contiguous tail pages.
>   */
> 
> ...I guess. Or maybe the name is just wrong (a comment block explaining a name is
> always a bad sign). 

Naming aside, a comment is probably worth nonetheless to explain what the function does.

> Perhaps we should rename it to something like:
> 
> 	unpin_user_compound_page_dirty_lock()
> 
> ?

The thing though, is that it doesn't *only* unpin a compound page. It unpins a contiguous
range of pages (hence page_range) *and* if these are compound pages it further accelerates
things.

Albeit, your name suggestion then probably hints the caller that you should be passing a
compound page anyways, so your suggestion does have a ring to it.

	Joao