Re: [PATCH v2 4/8] mm: migrate: use a folio in migrate_misplaced_page()

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On 2023/8/29 10:10, Matthew Wilcox wrote:
On Tue, Aug 29, 2023 at 08:49:31AM +0800, Huang, Ying wrote:
Zi Yan <ziy@xxxxxxxxxx> writes:

On 21 Aug 2023, at 7:56, Kefeng Wang wrote:

Use a folio in migrate_misplaced_page() to save compound_head() calls.

Signed-off-by: Kefeng Wang <wangkefeng.wang@xxxxxxxxxx>
---
  mm/migrate.c | 23 ++++++++++++-----------
  1 file changed, 12 insertions(+), 11 deletions(-)

LGTM. And a comment below. Reveiwed-by: Zi Yan <ziy@xxxxxxxxxx>


diff --git a/mm/migrate.c b/mm/migrate.c
index 281eafdf8e63..fc728f9a383f 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -2521,17 +2521,18 @@ int migrate_misplaced_page(struct page *page, struct vm_area_struct *vma,
  			   int node)
  {
  	pg_data_t *pgdat = NODE_DATA(node);
+	struct folio *folio = page_folio(page);
  	int isolated;
  	int nr_remaining;
  	unsigned int nr_succeeded;
  	LIST_HEAD(migratepages);
-	int nr_pages = thp_nr_pages(page);
+	int nr_pages = folio_nr_pages(folio);

  	/*
  	 * Don't migrate file pages that are mapped in multiple processes
  	 * with execute permissions as they are probably shared libraries.
  	 */
-	if (page_mapcount(page) != 1 && page_is_file_lru(page) &&
+	if (page_mapcount(page) != 1 && folio_is_file_lru(folio) &&

page_mapcount() is not converted, since folio_mapcount() is not equivalent
to page_mapcount(). It can be converted and this function can be converted
to migrate_misplaced_folio() once we have something like folio_num_sharers().

It seems that we can use folio_estimated_sharers() here.

So, funny thing, page_mapcount() was always wrong here.  We have two
callers, do_huge_pmd_numa_page() and do_numa_page().  do_numa_page()
has a check for PageCompound() (and /* TODO: handle PTE-mapped THP */).
do_huge_pmd_numa_page() returns pfn_to_page(), after it got the pfn
fromm pmd_pfn(pmd).

for PMD-mapped page,do_huge_pmd_numa_page() return head page

   unsigned long haddr = vmf->address & HPAGE_PMD_MASK;
   page = vm_normal_page_pmd(vma, haddr, pmd);

do_numa_page() only handle base page with PageCompound() check, so we
could use folio_estimated_sharers(), I am not clear why page_mapcount
is wrong here, could you explain more, thank.


That makes folio_estimated_sharers() an improvement, possibly even
a bugfix.  Also, we should look at removing the PageCompound() check
in do_numa_page().

Yes, for pte-mapped thp and the large folio numa migrate support, we
need remove the check, we could do it in another patchset.







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