linux-kernel - [PATCH 0/3] hfs: Replace kmap() with kmap_local

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Message-Id: <20220809152004.9223-1-fmdefrancesco@gmail.com>
Date:   Tue,  9 Aug 2022 17:20:01 +0200
From:   "Fabio M. De Francesco" <fmdefrancesco@...il.com>
To:     "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        "Fabio M. De Francesco" <fmdefrancesco@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Damien Le Moal <damien.lemoal@...nsource.wdc.com>,
        Jeff Layton <jlayton@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Muchun Song <songmuchun@...edance.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH 0/3] hfs: Replace kmap() with kmap_local_page()

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

There are two main problems with kmap(): (1) It comes with an overhead as
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.

Since its use in fs/hfs is safe everywhere, it should be preferred.

Therefore, replace kmap() with kmap_local_page() in fs/hfs. Where
possible, use the suited standard helpers (memzero_page(), memcpy_page())
instead of open coding kmap_local_page() plus memset() or memcpy().

Fix a bug due to a page being not unmapped if the code jumps to the
"fail_page" label (1/3).

Tested in a QEMU/KVM x86_32 VM, 6GB RAM, booting a kernel with
HIGHMEM64GB enabled.

Fabio M. De Francesco (3):
  hfs: Unmap the page in the "fail_page" label
  hfs: Replace kmap() with kmap_local_page() in bnode.c
  hfs: Replace kmap() with kmap_local_page() in btree.c

 fs/hfs/bnode.c | 32 ++++++++++++--------------------
 fs/hfs/btree.c | 29 ++++++++++++++++-------------
 2 files changed, 28 insertions(+), 33 deletions(-)

-- 
2.37.1