linux-kernel - Re: [PATCH] mm: be more verbose for alloc_contig

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Message-ID: <YEYdR8azcawau9Rl@dhcp22.suse.cz>
Date:   Mon, 8 Mar 2021 13:49:11 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Minchan Kim <minchan@...nel.org>
Cc:     David Hildenbrand <david@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>, joaodias@...gle.com
Subject: Re: [PATCH] mm: be more verbose for alloc_contig_range faliures

On Thu 04-03-21 10:22:51, Minchan Kim wrote:
[...]
> How about this?
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 238d0fc232aa..489e557b9390 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -8481,7 +8481,8 @@ static inline void dump_migrate_failure_pages(struct list_head *page_list)
> 
>  /* [start, end) must belong to a single zone. */
>  static int __alloc_contig_migrate_range(struct compact_control *cc,
> -                                       unsigned long start, unsigned long end)
> +                                       unsigned long start, unsigned long end,
> +                                       bool nofail)

This sounds like a very bad idea to me. Your nofail definition might
differ from what we actually define as __GFP_NOFAIL but I do not think
this interface should ever promise anything that strong.
Sure movable, cma regions should effectively never fail but there will
never be any _guarantee_ for that.

Earlier in the discussion I have suggested dynamic debugging facility.
Documentation/admin-guide/dynamic-debug-howto.rst. Have you tried to
look into that direction?

>  {
>         /* This function is based on compact_zone() from compaction.c. */
>         unsigned int nr_reclaimed;
> @@ -8522,7 +8523,8 @@ static int __alloc_contig_migrate_range(struct compact_control *cc,
>                                 NULL, (unsigned long)&mtc, cc->mode, MR_CONTIG_RANGE);
>         }
>         if (ret < 0) {
> -               dump_migrate_failure_pages(&cc->migratepages);
> +               if (ret == -EBUSY && nofail)
> +                       dump_migrate_failure_pages(&cc->migratepages);
>                 putback_movable_pages(&cc->migratepages);
>                 return ret;
>         }
> @@ -8610,7 +8612,9 @@ int alloc_contig_range(unsigned long start, unsigned long end,
>          * allocated.  So, if we fall through be sure to clear ret so that
>          * -EBUSY is not accidentally used or returned to caller.
>          */
> -       ret = __alloc_contig_migrate_range(&cc, start, end);
> +       ret = __alloc_contig_migrate_range(&cc, start, end,
> +                                       migratetype == CMA ||
> +                                       zone_idx(cc.zone) == ZONE_MOVABLE);
>         if (ret && ret != -EBUSY)
>                 goto done;
>         ret =0;

-- 
Michal Hocko
SUSE Labs