linux-kernel - Re: [RFC PATCH 1/7] mm/gup: introduce pin_user

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Date:   Mon, 28 Feb 2022 13:14:54 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     David Hildenbrand <david@...hat.com>, Jens Axboe <axboe@...nel.dk>,
        Jan Kara <jack@...e.cz>, Christoph Hellwig <hch@...radead.org>,
        Dave Chinner <dchinner@...hat.com>,
        "Darrick J . Wong" <djwong@...nel.org>,
        Theodore Ts'o <tytso@....edu>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Miklos Szeredi <miklos@...redi.hu>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Chaitanya Kulkarni <kch@...dia.com>
Cc:     linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-xfs@...r.kernel.org, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 1/7] mm/gup: introduce pin_user_page()

On 2/28/22 05:27, David Hildenbrand wrote:
...
>> diff --git a/mm/gup.c b/mm/gup.c
>> index 5c3f6ede17eb..44446241c3a9 100644
>> --- a/mm/gup.c
>> +++ b/mm/gup.c
>> @@ -3034,6 +3034,40 @@ long pin_user_pages(unsigned long start, unsigned long nr_pages,
>>   }
>>   EXPORT_SYMBOL(pin_user_pages);
>>   
>> +/**
>> + * pin_user_page() - apply a FOLL_PIN reference to a page ()
>> + *
>> + * @page: the page to be pinned.
>> + *
>> + * Similar to get_user_pages(), in that the page's refcount is elevated using
>> + * FOLL_PIN rules.
>> + *
>> + * IMPORTANT: That means that the caller must release the page via
>> + * unpin_user_page().
>> + *
>> + */
>> +void pin_user_page(struct page *page)
>> +{
>> +	struct folio *folio = page_folio(page);
>> +
>> +	WARN_ON_ONCE(folio_ref_count(folio) <= 0);
>> +
>> +	/*
>> +	 * Similar to try_grab_page(): be sure to *also*
>> +	 * increment the normal page refcount field at least once,
>> +	 * so that the page really is pinned.
>> +	 */
>> +	if (folio_test_large(folio)) {
>> +		folio_ref_add(folio, 1);
>> +		atomic_add(1, folio_pincount_ptr(folio));
>> +	} else {
>> +		folio_ref_add(folio, GUP_PIN_COUNTING_BIAS);
>> +	}
>> +
>> +	node_stat_mod_folio(folio, NR_FOLL_PIN_ACQUIRED, 1);
>> +}
>> +EXPORT_SYMBOL(pin_user_page);
>> +
>>   /*
>>    * pin_user_pages_unlocked() is the FOLL_PIN variant of
>>    * get_user_pages_unlocked(). Behavior is the same, except that this one sets
> 
> I assume that function will only get called on a page that has been
> obtained by a previous pin_user_pages_fast(), correct?
> 

Well, no. This is meant to be used in place of get_page(), for code that
knows that the pages will be released via unpin_user_page(). So there is
no special prerequisite there.


thanks,
-- 
John Hubbard
NVIDIA