linux-kernel - Re: [PATCH v2 1/4] mm: migrate: Move the page count validation to the proper place

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Date:   Sun, 15 Aug 2021 11:34:18 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc:     akpm@...ux-foundation.org, apopple@...dia.com, shy828301@...il.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/4] mm: migrate: Move the page count validation to
 the proper place

On Sun, Aug 15, 2021 at 02:23:03PM +0800, Baolin Wang wrote:
> We've got the expected count for anonymous page or file page by
> expected_page_refs() at the beginning of migrate_page_move_mapping(),
> thus we should move the page count validation a little forward to
> reduce duplicated code.
> 
> Moreover the i_pages lock is not used to guarantee the page refcount
> safety in migrate_page_move_mapping(), so we can move the getting page
> count out of the i_pages lock. Since now the migration page has
> established a migration pte under the page lock now, with the page
> refcount freezing validation, to ensure that the page references
> meet the migration requirement.

I remain unconvinced by this.  

Looking at folio_migrate_mapping() a little more deeply, I don't
understand why we first check folio_ref_count() and then attempt
to free the refcount.  Why not just try to freeze it directly?

ie instead of your patch, this:

+++ b/mm/migrate.c
@@ -403,13 +403,8 @@ int folio_migrate_mapping(struct address_space *mapping,
        newzone = folio_zone(newfolio);

        xas_lock_irq(&xas);
-       if (folio_ref_count(folio) != expected_count ||
-           xas_load(&xas) != folio) {
-               xas_unlock_irq(&xas);
-               return -EAGAIN;
-       }
-
-       if (!folio_ref_freeze(folio, expected_count)) {
+       if (xas_load(&xas) != folio ||
+           !folio_ref_freeze(folio, expected_count)) {
                xas_unlock_irq(&xas);
                return -EAGAIN;
        }

And since we've got the lock on the page, how can somebody else be
removing it from the page cache?  I think that xas_load() can be
removed too.  So even more simply,

+++ b/mm/migrate.c
@@ -403,12 +403,6 @@ int folio_migrate_mapping(struct address_space *mapping,
        newzone = folio_zone(newfolio);

        xas_lock_irq(&xas);
-       if (folio_ref_count(folio) != expected_count ||
-           xas_load(&xas) != folio) {
-               xas_unlock_irq(&xas);
-               return -EAGAIN;
-       }
-
        if (!folio_ref_freeze(folio, expected_count)) {
                xas_unlock_irq(&xas);
                return -EAGAIN;

but I'm not really set up to test page migration.  Does your test suite
test migrating file-backed pages?