Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Wed, 20 Jul 2022 14:29:41 -0700
From:   Luis Chamberlain <mcgrof@kernel.org>
To:     "Fabio M. De Francesco" <fmdefrancesco@gmail.com>
Cc:     Matthew Wilcox <willy@infradead.org>,
        Ira Weiny <ira.weiny@intel.com>, linux-modules@vger.kernel.org,
        linux-kernel@vger.kernel.org, Song Liu <song@kernel.org>,
        Takashi Iwai <tiwai@suse.de>,
        Adam Manzanares <a.manzanares@samsung.com>,
        Davidlohr Bueso <dave@stgolabs.net>,
        Matthew Wilcox <willy@traff-1.hugedomains.com>
Subject: Re: [PATCH v2] module: Replace kmap() with kmap_local_page()
Message-ID: <YthzxUOgyPKgiyPY@bombadil.infradead.org>
References: <20220720161932.9567-1-fmdefrancesco@gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
In-Reply-To: <20220720161932.9567-1-fmdefrancesco@gmail.com>
Sender: Luis Chamberlain <mcgrof@infradead.org>
Precedence: bulk

On Wed, Jul 20, 2022 at 06:19:32PM +0200, Fabio M. De Francesco wrote:
> kmap() is being deprecated in favor of kmap_local_page().
> 
> 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).
> Tasks can be preempted and, when scheduled to run again, the kernel
> virtual addresses are restored and still valid.
> 
> kmap_local_page() is faster than kmap() in kernels with HIGHMEM enabled.
> 
> Since the use of kmap_local_page() in module_gzip_decompress() and in
> module_xz_decompress() is safe (i.e., it does not break the strict rules
> of use), it should be preferred over kmap().
> 
> Therefore, replace kmap() with kmap_local_page().
> 
> Tested on a QEMU/KVM x86_32 VM with 4GB RAM, booting kernels with
> HIGHMEM64GB enabled. Modules compressed with XZ or GZIP decompress
> properly.
> 
> Cc: Matthew Wilcox <willy@infradead.com>
> Suggested-by: Ira Weiny <ira.weiny@intel.com>
> Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>

Thanks for the enhanced commit log! Queued onto modules-testing, if that
doesn't blow something up I'll move to modules-next afterwards.

  Luis