mm/swap: stop using page->private on tail pages for THP_SWAP

Patch series "mm/swap: stop using page->private on tail pages for THP_SWAP
+ cleanups".

This series stops using page->private on tail pages for THP_SWAP, replaces
folio->private by folio->swap for swapcache folios, and starts using
"new_folio" for tail pages that we are splitting to remove the usage of
page->private for swapcache handling completely.


This patch (of 4):

Let's stop using page->private on tail pages, making it possible to just
unconditionally reuse that field in the tail pages of large folios.

The remaining usage of the private field for THP_SWAP is in the THP
splitting code (mm/huge_memory.c), that we'll handle separately later.

Update the THP_SWAP documentation and sanity checks in mm_types.h and
__split_huge_page_tail().

[david@redhat.com: stop using page->private on tail pages for THP_SWAP]
  Link: https://lkml.kernel.org/r/6f0a82a3-6948-20d9-580b-be1dbf415701@redhat.com
Link: https://lkml.kernel.org/r/20230821160849.531668-1-david@redhat.com
Link: https://lkml.kernel.org/r/20230821160849.531668-2-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>	[arm64]
Reviewed-by: Yosry Ahmed <yosryahmed@google.com>
Cc: Dan Streetman <ddstreet@ieee.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Peter Xu <peterx@redhat.com>
Cc: Seth Jennings <sjenning@redhat.com>
Cc: Vitaly Wool <vitaly.wool@konsulko.com>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 6 insertions, 9 deletions

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index cb4432792b88..a28e9fe16585 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2446,18 +2446,15 @@ static void __split_huge_page_tail(struct page *head, int tail,
 	page_tail->index = head->index + tail;
 
 	/*
-	 * page->private should not be set in tail pages with the exception
-	 * of swap cache pages that store the swp_entry_t in tail pages.
-	 * Fix up and warn once if private is unexpectedly set.
-	 *
-	 * What of 32-bit systems, on which folio->_pincount overlays
-	 * head[1].private?  No problem: THP_SWAP is not enabled on 32-bit, and
-	 * pincount must be 0 for folio_ref_freeze() to have succeeded.
+	 * page->private should not be set in tail pages. Fix up and warn once
+	 * if private is unexpectedly set.
 	 */
-	if (!folio_test_swapcache(page_folio(head))) {
-		VM_WARN_ON_ONCE_PAGE(page_tail->private != 0, page_tail);
+	if (unlikely(page_tail->private)) {
+		VM_WARN_ON_ONCE_PAGE(true, page_tail);
 		page_tail->private = 0;
 	}
+	if (PageSwapCache(head))
+		set_page_private(page_tail, (unsigned long)head->private + tail);
 
 	/* Page flags must be visible before we make the page non-compound. */
 	smp_wmb();