linux-kernel - Re: [PATCH v3] fs/aio: Replace kmap{,_atomic}() with kmap_local

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Message-ID: <23757331.ouqheUzb2q@suse>
Date:   Fri, 09 Jun 2023 17:04:56 +0200
From:   "Fabio M. De Francesco" <fmdefrancesco@...il.com>
To:     Alexander Viro <viro@...iv.linux.org.uk>,
        Benjamin LaHaise <bcrl@...ck.org>,
        Ira Weiny <ira.weiny@...el.com>,
        Matthew Wilcox <matthew.wilcox@...cle.com>
Cc:     linux-fsdevel@...r.kernel.org, linux-aio@...ck.org,
        linux-kernel@...r.kernel.org, Jeff Moyer <jmoyer@...hat.com>,
        Kent Overstreet <kent.overstreet@...ux.dev>
Subject: Re: [PATCH v3] fs/aio: Replace kmap{,_atomic}() with kmap_local_page()

On giovedì 19 gennaio 2023 17:20:55 CEST Fabio M. De Francesco wrote:
> The use of kmap() and kmap_atomic() are being deprecated in favor of
> kmap_local_page().

According to a suggestion by Matthew, I just sent another patch which stops 
allocating aio rings from ZONE_HIGHMEM.[1]

Therefore, please drop this patch.

Since the purpose of the new patch is entirely different from this, I changed 
the subject and reset the version number to v1.

Thanks,

Fabio

[1] https://lore.kernel.org/lkml/20230609145937.17610-1-fmdefrancesco@gmail.com/
 
> 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.
> 
> The use of kmap_local_page() in fs/aio.c is "safe" in the sense that the
> code don't hands the returned kernel virtual addresses to other threads
> and there are no nesting which should be handled with the stack based
> (LIFO) mappings/un-mappings order. Furthermore, the code between the old
> kmap_atomic()/kunmap_atomic() did not depend on disabling page-faults
> and/or preemption, so that there is no need to call pagefault_disable()
> and/or preempt_disable() before the mappings.
> 
> 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.
> 
> Cc: "Venkataramanan, Anirudh" <anirudh.venkataramanan@...el.com>
> Suggested-by: Ira Weiny <ira.weiny@...el.com>
> Reviewed-by: Ira Weiny <ira.weiny@...el.com>
> Reviewed-by: Jeff Moyer <jmoyer@...hat.com>
> Reviewed-by: Kent Overstreet <kent.overstreet@...ux.dev>
> Signed-off-by: Fabio M. De Francesco <fmdefrancesco@...il.com>
> ---