Re: [PATCH v4 06/13] mm/gup: Drop folio_fast_pin_allowed() in hugepd processing

[David Hildenbrand <david@redhat.com>]

On 27.03.24 16:23, peterx@redhat.com wrote:
> From: Peter Xu <peterx@redhat.com>
> 
> Hugepd format for GUP is only used in PowerPC with hugetlbfs.  There are
> some kernel usage of hugepd (can refer to hugepd_populate_kernel() for
> PPC_8XX), however those pages are not candidates for GUP.
> 
> Commit a6e79df92e4a ("mm/gup: disallow FOLL_LONGTERM GUP-fast writing to
> file-backed mappings") added a check to fail gup-fast if there's potential
> risk of violating GUP over writeback file systems.  That should never apply
> to hugepd.  Considering that hugepd is an old format (and even
> software-only), there's no plan to extend hugepd into other file typed
> memories that is prone to the same issue.
> 
> Drop that check, not only because it'll never be true for hugepd per any
> known plan, but also it paves way for reusing the function outside
> fast-gup.
> 
> To make sure we'll still remember this issue just in case hugepd will be
> extended to support non-hugetlbfs memories, add a rich comment above
> gup_huge_pd(), explaining the issue with proper references.
> 
> Cc: Christoph Hellwig <hch@infradead.org>
> Cc: Lorenzo Stoakes <lstoakes@gmail.com>
> Cc: Michael Ellerman <mpe@ellerman.id.au>
> Cc: linuxppc-dev@lists.ozlabs.org
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---

@Andrew, you properly adjusted the code to remove the 
gup_fast_folio_allowed() call instead of the folio_fast_pin_allowed() 
call, but

(1) the commit subject
(2) comment for gup_huge_pd()

Still mention folio_fast_pin_allowed().

The patch "mm/gup: handle hugepd for follow_page()" then moves that 
(outdated) comment.

-- 
Cheers,

David / dhildenb