[linux-yocto] [PATCH 4.19-rt] Revert "mm: handle lru_add_drain_all for UP properly"

[Bruce Ashfield]

On Mon, Apr 15, 2019 at 12:02 PM Paul Gortmaker
<paul.gortmaker at windriver.com> wrote:
>
> This reverts commit e6e9d6e290028b0a6b83b563fad9fafa7f1d515e.
>
> It was a 4.19.31 backport of commit 6ea183d60c46 ("mm: handle
> lru_add_drain_all for UP properly").  In summary, what that did
> was to fix a possible harmless WARN_ON on non-SMP, introduced at
> commit 4d43d395fed1 ("workqueue: Try to catch flush_work() without
> INIT_WORK().") by adding non-SMP variants of lru functions.
>
> The combination of that, with the -rt commit 473f14a9f234 ("mm:
> perform lru_add_drain_all() remotely") at the merge of the two
> results in the following build failure:
>
>   mm/swap.c:736:2: error: #endif without #if
>
> since the -rt change wants RT specific lru and the stable backport
> wants non-SMP specific lru, and a chunk of the backport with
> an #ifdef CONFIG_SMP is missing.
>
> However, before we add a four way cluster of ifdeffery to handle all
> cases, we note 4d43d395fed1 was added to the v5.1 release, and it
> was not (currently) backported to any 4.19.x stable release - so it is
> unclear to me why this commit was ever backported to 4.19.31 at all.
>
> Further, we note this change was to mm/swap.c -- and by definition,
> any preempt-rt deployment that uses swap for anything other than a
> failure contingency mitigation is broken by design.
>
> Given all that, I decided that the best path forward was to revert
> the two of the three chunks of the backport that remain in the -rt
> branch, and return us to the pre-4.19.31 merge behaviour for -rt.
>

Paul, this is now merged. There's been a lot of churn in these
functions, so thanks for the help fixing it up!

Bruce

> Signed-off-by: Paul Gortmaker <paul.gortmaker at windriver.com>
>
> diff --git a/mm/swap.c b/mm/swap.c
> index 7e0bcaf450a5..9217027671c8 100644
> --- a/mm/swap.c
> +++ b/mm/swap.c
> @@ -325,6 +325,11 @@ static inline void activate_page_drain(int cpu)
>  {
>  }
>
> +static bool need_activate_page_drain(int cpu)
> +{
> +       return false;
> +}
> +
>  void activate_page(struct page *page)
>  {
>         struct zone *zone = page_zone(page);
> @@ -728,12 +733,6 @@ void lru_add_drain_all(void)
>
>         mutex_unlock(&lock);
>  }
> -#else
> -void lru_add_drain_all(void)
> -{
> -       lru_add_drain();
> -}
> -#endif
>
>  /**
>   * release_pages - batched put_page()
> --
> 2.7.4
>


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