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Date:   Tue, 20 Jun 2023 12:06:15 -0400
From:   Peter Xu <peterx@redhat.com>
To:     David Hildenbrand <david@redhat.com>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Andrea Arcangeli <aarcange@redhat.com>,
        Mike Rapoport <rppt@kernel.org>,
        Matthew Wilcox <willy@infradead.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        John Hubbard <jhubbard@nvidia.com>,
        "Kirill A . Shutemov" <kirill@shutemov.name>,
        James Houghton <jthoughton@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Lorenzo Stoakes <lstoakes@gmail.com>,
        Hugh Dickins <hughd@google.com>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Jason Gunthorpe <jgg@nvidia.com>
Subject: Re: [PATCH v2 2/8] mm/hugetlb: Prepare hugetlb_follow_page_mask()
 for FOLL_PIN
Message-ID: <ZJHOdzuSEO14j7r0@x1n>
References: <20230619231044.112894-1-peterx@redhat.com>
 <20230619231044.112894-3-peterx@redhat.com>
 <7cd1db3f-9bdf-dc0b-692c-380a2fa14286@redhat.com>
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On Tue, Jun 20, 2023 at 05:28:28PM +0200, David Hildenbrand wrote:
> On 20.06.23 01:10, Peter Xu wrote:
> > follow_page() doesn't use FOLL_PIN, meanwhile hugetlb seems to not be the
> > target of FOLL_WRITE either.  However add the checks.
> > 
> > Namely, either the need to CoW due to missing write bit, or proper CoR on
> > !AnonExclusive pages over R/O pins to reject the follow page.  That brings
> > this function closer to follow_hugetlb_page().
> > 
> > So we don't care before, and also for now.  But we'll care if we switch
> > over slow-gup to use hugetlb_follow_page_mask().  We'll also care when to
> > return -EMLINK properly, as that's the gup internal api to mean "we should
> > do CoR".  Not really needed for follow page path, though.
> > 
> > When at it, switching the try_grab_page() to use WARN_ON_ONCE(), to be
> > clear that it just should never fail.
> 
> Oh, and does this comment really belong into this patch or am I confused?

Ah yeh, thanks for spotting.  I used to have it in v1 but I kept the old
failure path here to partly address Lorenzo's worry; but then I forgot to
add the WARN_ON_ONCE back to guard.  I'll remember to add that in v3.

-- 
Peter Xu