linux-kernel - [PATCH v2 0/4] fs/sysv: Replace kmap() with kmap_local

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Message-Id: <20230109170639.19757-1-fmdefrancesco@gmail.com>
Date:   Mon,  9 Jan 2023 18:06:35 +0100
From:   "Fabio M. De Francesco" <fmdefrancesco@...il.com>
To:     Christoph Hellwig <hch@...radead.org>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Al Viro <viro@...iv.linux.org.uk>,
        Ira Weiny <ira.weiny@...el.com>
Cc:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>
Subject: [PATCH v2 0/4] fs/sysv: Replace kmap() with kmap_local_page() 

kmap() is 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.

kmap_local_page() in fs/sysv does not violate any of the strict rules of
its use, therefore it should be preferred.

Therefore, replace kmap() with kmap_local_page() in fs/sysv. kunmap_local()
requires the mapping address, so return that address from dir_get_page()
to be used in dir_put_page().

I had submitted a patch with the same purpose but it has been replaced
by this series.[1] This is based on a long series of very appreciated
comments and suggestions kindly provided by Al Viro (again thanks!).[2][3][4]

Changes from v1:[5]
	1/4 - No changes.
	2/4 - Delete an unnecessary assignment (thanks to Dan Carpenter).
	3/4 - No changes.
	4/4 - No changes.

[1] https://lore.kernel.org/lkml/20221016164636.8696-1-fmdefrancesco@gmail.com/
[2] https://lore.kernel.org/lkml/Y4E++JERgUMoqfjG@ZenIV/#t
[3] https://lore.kernel.org/lkml/Y4FG0O7VWTTng5yh@ZenIV/#t
[4] https://lore.kernel.org/lkml/Y4ONIFJatIGsVNpf@ZenIV/#t
[5] https://lore.kernel.org/lkml/20221231075717.10258-1-fmdefrancesco@gmail.com/

Cc: Al Viro <viro@...iv.linux.org.uk>
Cc: Ira Weiny <ira.weiny@...el.com>
Signed-off-by: Fabio M. De Francesco <fmdefrancesco@...il.com>

Fabio M. De Francesco (4):
  fs/sysv: Use the offset_in_page() helper
  fs/sysv: Change the signature of dir_get_page()
  fs/sysv: Use dir_put_page() in sysv_rename()
  fs/sysv: Replace kmap() with kmap_local_page()

 fs/sysv/dir.c   | 118 +++++++++++++++++++++++++++---------------------
 fs/sysv/namei.c |   9 ++--
 fs/sysv/sysv.h  |   1 +
 3 files changed, 71 insertions(+), 57 deletions(-)

-- 
2.39.0