Re: [PATCH] mm: move idle swap cache pages to the tail of LRU after COW

[Huang, Ying]

Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> writes:

> On Tue, May 18, 2021 at 5:17 PM Linus Torvalds
> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>>
>> This looks sensible to me (and numbers talk!), but as Rik says, it
>> would probably be a good idea to move the trylock_page()/unlock_page()
>> into try_to_free_idle_swapcache(), and that would make the calling
>> side a whole lot cleaner and easier to read.
>
> To keep the error handling simple, and keep that "if that didn't work,
> just return" logic in you had, doing it as two functions like:
>
>   static inline void locked_try_to_free_idle_swapcache(struct page *page)
>   { .. your current try_to_free_idle_swapcache() .. }
>
>   void try_to_free_idle_swapcache(struct page *page)
>   {
>         if (trylock_page(page)) {
>                 locked_try_to_free_idle_swapcache(page);
>                 unlock_page(page);
>         }
>   }
>
> would keep that readability and simplicity.
>
> And then the wp_page_copy() code ends up being
>
>         if (page_copied && PageSwapCache(old_page) && !page_mapped(old_page))
>                       try_to_free_idle_swapcache(old_page);
>
> which looks pretty sensible to me: if we copied the page, and the old
> page is a no longer mapped swap cache page, let's try to free it.
>
> That's still a hell of a long conditional, partly because of those
> long names. But at least it's conceptually fairly straightforward and
> easy to understand what's going on.

Thanks!  That looks much better.  I will do that in the next version.

Best Regards,
Huang, Ying