linux-kernel - Re: [RESEND PATCH] fs/aio: Replace kmap{,_atomic}() with kmap_local

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Message-ID: <Y8kQM9kzC7cHh/Wh@moria.home.lan>
Date:   Thu, 19 Jan 2023 04:41:07 -0500
From:   Kent Overstreet <kent.overstreet@...ux.dev>
To:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Benjamin LaHaise <bcrl@...ck.org>,
        linux-fsdevel@...r.kernel.org, linux-aio@...ck.org,
        linux-kernel@...r.kernel.org,
        "Venkataramanan, Anirudh" <anirudh.venkataramanan@...el.com>,
        Ira Weiny <ira.weiny@...el.com>
Subject: Re: [RESEND PATCH] fs/aio: Replace kmap{,_atomic}() with
 kmap_local_page()

On Sun, Oct 16, 2022 at 05:06:56PM +0200, Fabio M. De Francesco wrote:
> The use of kmap() and kmap_atomic() are 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.
> 
> Since its use in fs/aio.c is safe everywhere, it should be preferred.
> 
> Therefore, replace kmap() and kmap_atomic() with kmap_local_page() in
> fs/aio.c.
> 
> Tested with xfstests on a QEMU/KVM x86_32 VM, 6GB RAM, booting a kernel
> with HIGHMEM64GB enabled.

I was just looking over this code and made the same kmap -> kmap_local
change, but you've done a more complete version - nice.

For context, I was the one who added the kmap() call, because
copy_to_user() can sleep - anything else at the time would've been more
awkward, and highmem machines were already on the way out. But
kmap_local is perfect here :)

Reviewed-by: Kent Overstreet <kent.overstreet@...ux.dev>