linux-kernel - Re: [PATCH 4/7] mm/hugetlb: Prepare hugetlb_follow_page_mask() for FOLL

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Date:   Wed, 14 Jun 2023 17:17:13 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Matthew Wilcox <willy@...radead.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        John Hubbard <jhubbard@...dia.com>,
        Mike Rapoport <rppt@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        James Houghton <jthoughton@...gle.com>,
        Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH 4/7] mm/hugetlb: Prepare hugetlb_follow_page_mask() for
 FOLL_PIN

On 14.06.23 17:11, Peter Xu wrote:
> On Wed, Jun 14, 2023 at 04:57:37PM +0200, David Hildenbrand wrote:
>> On 13.06.23 23:53, Peter Xu wrote:
>>> It's coming, not yet, but soon.  Loose the restriction.
>>>
>>> Signed-off-by: Peter Xu <peterx@...hat.com>
>>> ---
>>>    mm/hugetlb.c | 7 -------
>>>    1 file changed, 7 deletions(-)
>>>
>>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>>> index f037eaf9d819..31d8f18bc2e4 100644
>>> --- a/mm/hugetlb.c
>>> +++ b/mm/hugetlb.c
>>> @@ -6467,13 +6467,6 @@ struct page *hugetlb_follow_page_mask(struct vm_area_struct *vma,
>>>    	spinlock_t *ptl;
>>>    	pte_t *pte, entry;
>>> -	/*
>>> -	 * FOLL_PIN is not supported for follow_page(). Ordinary GUP goes via
>>> -	 * follow_hugetlb_page().
>>> -	 */
>>> -	if (WARN_ON_ONCE(flags & FOLL_PIN))
>>> -		return NULL;
>>> -
>>>    	hugetlb_vma_lock_read(vma);
>>>    	pte = hugetlb_walk(vma, haddr, huge_page_size(h));
>>>    	if (!pte)
>> Did you fix why the warning was placed there in the first place? (IIRC, at
>> least unsharing support needs to be added, maybe more)
> 
> Feel free to have a look at patch 2 - it should be done there, hopefully in
> the right way.  And IIUC it could be a bug to not do that before (besides
> CoR there was also the pgtable permission checks that was missing).  More
> details in patch 2's commit message.  Thanks,

Oh, that slipped my eyes (unsharing is not really a permission check) -- 
and the patch description could have been more explicit about why we can 
now lift the restrictions.

For the records: we don't use CoR terminology upstream. As suggested by 
John, we use "GUP-triggered unsharing".

As unsharing only applies to FOLL_PIN, it doesn't quite fit into patch 
#2. Either move that to this patch or squash both.

-- 
Cheers,

David / dhildenb