Re: [PATCH 1/1] mm: thp: relocate flush_cache_range() in migrate_misplaced_transhuge_page()

[Andrew Morton <akpm@linux-foundation.org>]

On Mon, 15 Oct 2018 16:23:11 -0400 Andrea Arcangeli <aarcange@redhat.com> wrote:

> There should be no cache left by the time we overwrite the old
> transhuge pmd with the new one. It's already too late to flush through
> the virtual address because we already copied the page data to the new
> physical address.
> 
> So flush the cache before the data copy.
> 
> Also delete the "end" variable to shutoff a "unused variable" warning
> on x86 where flush_cache_range() is a noop.

migrate_misplaced_transhuge_page() has changed a bit.  This is how I
figure the patch should be.  Please check:

--- a/mm/migrate.c~mm-thp-relocate-flush_cache_range-in-migrate_misplaced_transhuge_page
+++ a/mm/migrate.c
@@ -1999,6 +1999,8 @@ int migrate_misplaced_transhuge_page(str
 	/* anon mapping, we can simply copy page->mapping to the new page: */
 	new_page->mapping = page->mapping;
 	new_page->index = page->index;
+	/* flush the cache before copying using the kernel virtual address */
+	flush_cache_range(vma, mmun_start, mmun_end);
 	migrate_page_copy(new_page, page);
 	WARN_ON(PageLRU(new_page));
 
@@ -2037,7 +2039,6 @@ int migrate_misplaced_transhuge_page(str
 	 * The SetPageUptodate on the new page and page_add_new_anon_rmap
 	 * guarantee the copy is visible before the pagetable update.
 	 */
-	flush_cache_range(vma, mmun_start, mmun_end);
 	page_add_anon_rmap(new_page, vma, mmun_start, true);
 	pmdp_huge_clear_flush_notify(vma, mmun_start, pmd);
 	set_pmd_at(mm, mmun_start, pmd, entry);
_