Re: [PATCH -next 1/2] mm: compaction: convert to use a folio in isolate_migratepages_block()

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On 2023/6/13 9:53, Kefeng Wang wrote:


On 2023/6/12 22:41, Matthew Wilcox wrote:
On Mon, Jun 12, 2023 at 10:34:13PM +0800, Kefeng Wang wrote:
@@ -959,7 +960,8 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
                   * Hugepage was successfully isolated and placed
                   * on the cc->migratepages list.
                   */
-                low_pfn += compound_nr(page) - 1;
+                folio = page_folio(page);
+                low_pfn += folio_nr_pages(folio) - 1;
                  goto isolate_success_no_list;

Why is this safe?  That is, how do we know that the folio can't be
dissolved under us at this point, then reallocated and hit the
VM_BUG_ON_PGFLAGS(PageTail(page), page) in folio_flags() when we
test folio_test_large()?

This is successfully isolated path, after isolate_hugetlb(),  the folio
reference is incremented, so I think the folio can't be dissolved here,
correct me if I am wrong.

Hi Matthew,is it enough for the above conversion from page to folio with reference?




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