mm/rmap: split migration into its own function

Migration is currently implemented as a mode of operation for
try_to_unmap_one() generally specified by passing the TTU_MIGRATION flag
or in the case of splitting a huge anonymous page TTU_SPLIT_FREEZE.

However it does not have much in common with the rest of the unmap
functionality of try_to_unmap_one() and thus splitting it into a separate
function reduces the complexity of try_to_unmap_one() making it more
readable.

Several simplifications can also be made in try_to_migrate_one() based on
the following observations:

 - All users of TTU_MIGRATION also set TTU_IGNORE_MLOCK.
 - No users of TTU_MIGRATION ever set TTU_IGNORE_HWPOISON.
 - No users of TTU_MIGRATION ever set TTU_BATCH_FLUSH.

TTU_SPLIT_FREEZE is a special case of migration used when splitting an
anonymous page.  This is most easily dealt with by calling the correct
function from unmap_page() in mm/huge_memory.c - either try_to_migrate()
for PageAnon or try_to_unmap().

Link: https://lkml.kernel.org/r/20210616105937.23201-5-apopple@nvidia.com
Signed-off-by: Alistair Popple <apopple@nvidia.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Ralph Campbell <rcampbell@nvidia.com>
Cc: Ben Skeggs <bskeggs@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Cc: Peter Xu <peterx@redhat.com>
Cc: Shakeel Butt <shakeelb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 10 insertions, 6 deletions

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 1c81aa11d618..8b731d53e9f4 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2309,16 +2309,20 @@ void vma_adjust_trans_huge(struct vm_area_struct *vma,
 
 static void unmap_page(struct page *page)
 {
-	enum ttu_flags ttu_flags = TTU_IGNORE_MLOCK | TTU_SYNC |
-		TTU_RMAP_LOCKED | TTU_SPLIT_HUGE_PMD;
+	enum ttu_flags ttu_flags = TTU_RMAP_LOCKED | TTU_SPLIT_HUGE_PMD |
+		TTU_SYNC;
 
 	VM_BUG_ON_PAGE(!PageHead(page), page);
 
-	/* If TTU_SPLIT_FREEZE is ever extended to file, update remap_page() */
+	/*
+	 * Anon pages need migration entries to preserve them, but file
+	 * pages can simply be left unmapped, then faulted back on demand.
+	 * If that is ever changed (perhaps for mlock), update remap_page().
+	 */
 	if (PageAnon(page))
-		ttu_flags |= TTU_SPLIT_FREEZE;
-
-	try_to_unmap(page, ttu_flags);
+		try_to_migrate(page, ttu_flags);
+	else
+		try_to_unmap(page, ttu_flags | TTU_IGNORE_MLOCK);
 
 	VM_WARN_ON_ONCE_PAGE(page_mapped(page), page);
 }