On 27.03.24 16:23, peterx@xxxxxxxxxx wrote:
From: Peter Xu <peterx@xxxxxxxxxx>
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@xxxxxxxxxxxxx>
Cc: Lorenzo Stoakes <lstoakes@xxxxxxxxx>
Cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
Cc: linuxppc-dev@xxxxxxxxxxxxxxxx
Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
---
@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(outdated) comment.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
