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

[peterx@redhat.com]

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>
---
 mm/gup.c | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index e7510b6ce765..db35b056fc9a 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -2832,11 +2832,6 @@ static int gup_hugepte(pte_t *ptep, unsigned long sz, unsigned long addr,
 		return 0;
 	}
 
-	if (!folio_fast_pin_allowed(folio, flags)) {
-		gup_put_folio(folio, refs, flags);
-		return 0;
-	}
-
 	if (!pte_write(pte) && gup_must_unshare(NULL, flags, &folio->page)) {
 		gup_put_folio(folio, refs, flags);
 		return 0;
@@ -2847,6 +2842,14 @@ static int gup_hugepte(pte_t *ptep, unsigned long sz, unsigned long addr,
 	return 1;
 }
 
+/*
+ * NOTE: currently GUP for a hugepd is only possible on hugetlbfs file
+ * systems on Power, which does not have issue with folio writeback against
+ * GUP updates.  When hugepd will be extended to support non-hugetlbfs or
+ * even anonymous memory, we need to do extra check as what we do with most
+ * of the other folios. See writable_file_mapping_allowed() and
+ * folio_fast_pin_allowed() for more information.
+ */
 static int gup_huge_pd(hugepd_t hugepd, unsigned long addr,
 		unsigned int pdshift, unsigned long end, unsigned int flags,
 		struct page **pages, int *nr)
-- 
2.44.0