linux-kernel - [PATCH v1] mm/page_alloc: drop pr_info_ratelimited() in alloc_contig

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Message-Id: <20210301150945.77012-1-david@redhat.com>
Date:   Mon,  1 Mar 2021 16:09:45 +0100
From:   David Hildenbrand <david@...hat.com>
To:     linux-kernel@...r.kernel.org
Cc:     linux-mm@...ck.org, David Hildenbrand <david@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Minchan Kim <minchan@...nel.org>,
        Oscar Salvador <osalvador@...e.de>,
        Michal Hocko <mhocko@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>
Subject: [PATCH v1] mm/page_alloc: drop pr_info_ratelimited() in alloc_contig_range()

The information that some PFNs are busy is:
a) not helpful for ordinary users: we don't even know *who* called
   alloc_contig_range(). This is certainly not worth a pr_info.*().
b) not really helpful for debugging: we don't have any details *why*
   these PFNs are busy, and that is what we usually care about.
c) not complete: there are other cases where we fail alloc_contig_range()
   using different paths that are not getting recorded.

For example, we reach this path once we succeeded in isolating pageblocks,
but failed to migrate some pages - which can happen easily on
ZONE_NORMAL (i.e., has_unmovable_pages() is racy) but also on ZONE_MOVABLE
i.e., we would have to retry longer to migrate).

For example via virtio-mem when unplugging memory, we can create quite
some noise (especially with ZONE_NORMAL) that is not of interest to
users - it's expected that some allocations may fail as memory is busy.

Let's just drop that pr_info_ratelimit() and rather implement a dynamic
debugging mechanism in the future that can give us a better reason why
alloc_contig_range() failed on specific pages.

Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Minchan Kim <minchan@...nel.org>
Cc: Oscar Salvador <osalvador@...e.de>
Cc: Michal Hocko <mhocko@...nel.org>
Cc: Vlastimil Babka <vbabka@...e.cz>
Signed-off-by: David Hildenbrand <david@...hat.com>
---
 mm/page_alloc.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 519a60d5b6f7..efb924fb13e8 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -8647,8 +8647,6 @@ int alloc_contig_range(unsigned long start, unsigned long end,

 	/* Make sure the range is really isolated. */
 	if (test_pages_isolated(outer_start, end, 0)) {
-		pr_info_ratelimited("%s: [%lx, %lx) PFNs busy\n",
-			__func__, outer_start, end);
 		ret = -EBUSY;
 		goto done;
 	}
-- 
2.29.2