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

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Message-ID: <x49h6zzvn1a.fsf@segfault.boston.devel.redhat.com>
Date:   Wed, 19 Oct 2022 11:41:21 -0400
From:   Jeff Moyer <jmoyer@...hat.com>
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()

"Fabio M. De Francesco" <fmdefrancesco@...il.com> writes:

> 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.

That sentence is very ambiguous.  I don't know what "its" refers to, and
I'm not sure what "safe" means in this context.

The patch looks okay to me.

Reviewed-by: Jeff Moyer <jmoyer@...hat.com>