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

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Date:   Wed, 19 May 2021 12:49:47 +0800
From:   "Huang, Ying" <ying.huang@...el.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Matthew Wilcox <willy@...radead.org>,
        Peter Xu <peterx@...hat.com>, Hugh Dickins <hughd@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...riel.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Michal Hocko <mhocko@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Tim Chen <tim.c.chen@...el.com>
Subject: Re: [PATCH] mm: move idle swap cache pages to the tail of LRU after
 COW

Linus Torvalds <torvalds@...ux-foundation.org> writes:

> On Tue, May 18, 2021 at 5:17 PM Linus Torvalds
> <torvalds@...ux-foundation.org> 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