linux-kernel - Re: [PATCH v2] module: Replace kmap() with kmap_local

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Date:   Wed, 20 Jul 2022 14:29:41 -0700
From:   Luis Chamberlain <mcgrof@...nel.org>
To:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Ira Weiny <ira.weiny@...el.com>, linux-modules@...r.kernel.org,
        linux-kernel@...r.kernel.org, Song Liu <song@...nel.org>,
        Takashi Iwai <tiwai@...e.de>,
        Adam Manzanares <a.manzanares@...sung.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Matthew Wilcox <willy@...ff-1.hugedomains.com>
Subject: Re: [PATCH v2] module: Replace kmap() with kmap_local_page()

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@...radead.com>
> Suggested-by: Ira Weiny <ira.weiny@...el.com>
> Signed-off-by: Fabio M. De Francesco <fmdefrancesco@...il.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