linux-kernel - Re: [RFC v1 2/2] mm/gup/writeback: add callbacks for inaccessible pages

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Message-ID: <cc2e1664-cff1-de1e-2087-475ec7c6aa56@nvidia.com>
Date:   Sat, 29 Feb 2020 12:07:01 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     Claudio Imbrenda <imbrenda@...ux.ibm.com>
CC:     Christian Borntraeger <borntraeger@...ibm.com>,
        <linux-next@...r.kernel.org>, <akpm@...ux-foundation.org>,
        <david@...hat.com>, <aarcange@...hat.com>, <linux-mm@...ck.org>,
        <frankja@...ux.ibm.com>, <sfr@...b.auug.org.au>,
        <linux-kernel@...r.kernel.org>, <linux-s390@...r.kernel.org>,
        Will Deacon <will@...nel.org>
Subject: Re: [RFC v1 2/2] mm/gup/writeback: add callbacks for inaccessible
 pages

On 2/29/20 2:49 AM, Claudio Imbrenda wrote:
>> ...
>>>> @@ -458,7 +457,6 @@ static struct page *follow_page_pte(struct
>>>> vm_area_struct *vma, }
>>>>   
>>>>   	if (flags & FOLL_SPLIT && PageTransCompound(page)) {
>>>> -		int ret;
>>>>   		get_page(page);
>>>>   		pte_unmap_unlock(ptep, ptl);
>>>>   		lock_page(page);
>>>> @@ -475,6 +473,14 @@ static struct page *follow_page_pte(struct
>>>> vm_area_struct *vma, page = ERR_PTR(-ENOMEM);
>>>>   		goto out;
>>>>   	}
>>>> +	if (flags & FOLL_PIN) {
>>
>>
>> What about FOLL_GET? Unless your calling code has some sort of
>> BUG_ON(flags & FOLL_GET), I'm not sure it's a good idea to leave that
>> case unhandled.
> 
> if I understood the semantics of FOLL_PIN correctly, then we don't need
> to make the page accessible for FOLL_GET. FOLL_PIN indicates intent to
> access the content of the page, whereas FOLL_GET is only for the struct
> page.
> 
> if we are not touching the content of the page, there is no need to
> make it accessible
> 


OK, I hope I'm not overlooking anything, but that sounds correct to me.


thanks,
-- 
John Hubbard
NVIDIA