Locking issue on try_to_swap_out()

Locking issue on try_to_swap_out()

[Marcelo Tosatti]

Hi Linus,

In theory, there is nothing which guarantees that nobody will mess with
the page between "UnlockPage" and "deactivate_page" (that is pretty hard
to happen, I suppose, but anyway)

--- mm/vmscan.c.orig       Sun Jan 14 13:23:55 2001
+++ mm/vmscan.c    Sun Jan 14 13:24:16 2001
@@ -72,10 +72,10 @@
                swap_duplicate(entry);
                set_pte(page_table, swp_entry_to_pte(entry));
 drop_pte:
-               UnlockPage(page);
                mm->rss--;
                if (!page->age)
                        deactivate_page(page);
+               UnlockPage(page);
                page_cache_release(page);
                return;
        }


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Re: Locking issue on try_to_swap_out()

[Rik van Riel]

On Sun, 14 Jan 2001, Marcelo Tosatti wrote:

> In theory, there is nothing which guarantees that nobody will
> mess with the page between "UnlockPage" and "deactivate_page"
> (that is pretty hard to happen, I suppose, but anyway)
>
> --- mm/vmscan.c.orig       Sun Jan 14 13:23:55 2001
> +++ mm/vmscan.c    Sun Jan 14 13:24:16 2001
> @@ -72,10 +72,10 @@
>                 swap_duplicate(entry);
>                 set_pte(page_table, swp_entry_to_pte(entry));
>  drop_pte:
> -               UnlockPage(page);
>                 mm->rss--;
>                 if (!page->age)
>                         deactivate_page(page);
> +               UnlockPage(page);
>                 page_cache_release(page);
>                 return;
>         }

Why do you suppose the page_cache_release(page) is BELOW
the deactivate_page(page) call ?

We are still holding a reference on the page when we call
deactivate_page(page), this is what keeps the page from
going away from under us.

regards,

Rik
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