linux-kernel - [PATCH v3 5/6] gfs2: Replace kmap() by kmap_local_page() in gfs2_read

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Message-ID: <7ad72f9d1a97d673483c206d8ac9a88f2e32d3b9.1688073459.git.drv@mailo.com>
Date:   Fri, 30 Jun 2023 03:21:53 +0530
From:   Deepak R Varma <drv@...lo.com>
To:     Bob Peterson <rpeterso@...hat.com>,
        Andreas Gruenbacher <agruenba@...hat.com>,
        cluster-devel@...hat.com, linux-kernel@...r.kernel.org
Cc:     Ira Weiny <ira.weiny@...el.com>,
        "Fabio M. De Francesco" <fmdefrancesco@...il.com>,
        Sumitra Sharma <sumitraartsy@...il.com>,
        Deepak R Varma <drv@...lo.com>
Subject: [PATCH v3 5/6] gfs2: Replace kmap() by kmap_local_page() in
 gfs2_read_super

The use of kmap() is being deprecated in favor of kmap_local_page().

There are two main problems with kmap(): (1) It comes with an overhead as
the mapping space is restricted and protected by a global lock for
synchronization and (2) it also requires global TLB invalidation when the
kmap’s pool wraps and it might block when the mapping space is fully
utilized until a slot becomes available.

With kmap_local_page() the mappings are per thread, CPU local, can take
page faults, and can be called from any context (including interrupts).
It is faster than kmap() in kernels with HIGHMEM enabled. Furthermore,
the tasks can be preempted and, when they are scheduled to run again, the
kernel virtual addresses are restored and still valid.

Therefore, replace kmap() with kmap_local_page() in gfs2_read_super().

Suggested-by: Fabio M. De Francesco <fmdefrancesco@...il.com>
Signed-off-by: Deepak R Varma <drv@...lo.com>
---
Changes in v3:
   - Patch included in patch set

Changes in v2:
   - None

 fs/gfs2/ops_fstype.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/gfs2/ops_fstype.c b/fs/gfs2/ops_fstype.c
index 8a27957dbfee..80fe61662412 100644
--- a/fs/gfs2/ops_fstype.c
+++ b/fs/gfs2/ops_fstype.c
@@ -264,9 +264,9 @@ static int gfs2_read_super(struct gfs2_sbd *sdp, sector_t sector, int silent)
 		__free_page(page);
 		return -EIO;
 	}
-	p = kmap(page);
+	p = kmap_local_page(page);
 	gfs2_sb_in(sdp, p);
-	kunmap(page);
+	kunmap_local(p);
 	__free_page(page);
 	return gfs2_check_sb(sdp, silent);
 }
-- 
2.34.1