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

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Date:   Mon, 27 Mar 2023 14:22:46 +0100
From:   Matthew Wilcox <willy@...radead.org>
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>,
        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 Mon, Mar 27, 2023 at 12:08:20PM +0200, Fabio M. De Francesco wrote:
> 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().
> > 
> > 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.

Or should we just stop allocating aio rings from HIGHMEM and remove
the calls to kmap()?  How much memory are we talking about here?