linux-kernel - Re: [PATCH v5 0/4] fs/ufs: Replace kmap() with kmap_local

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Message-ID: <ZCGY3c5avRefahms@casper.infradead.org>
Date:   Mon, 27 Mar 2023 14:23:41 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>
Cc:     Evgeniy Dushistov <dushistov@...l.ru>,
        Al Viro <viro@...iv.linux.org.uk>,
        Ira Weiny <ira.weiny@...el.com>, linux-kernel@...r.kernel.org,
        bpf@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v5 0/4] fs/ufs: Replace kmap() with kmap_local_page

On Mon, Mar 27, 2023 at 12:13:08PM +0200, Fabio M. De Francesco wrote:
> On giovedì 29 dicembre 2022 23:50:56 CEST Fabio M. De Francesco wrote:
> > kmap() is 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/ufs is safe everywhere, it should be preferred.
> > 
> > Therefore, replace kmap() with kmap_local_page() in fs/ufs. kunmap_local()
> > requires the mapping address, so return that address from ufs_get_page()
> > to be used in ufs_put_page().
> 
> Hi Al,
> 
> I see that this series is here since Dec 29, 2022.
> Is there anything that prevents its merging? 
> Can you please its four patches in your tree?

I'm pretty sure UFS directories should simply be allocated from lowmem.
There's really no reason to put them in highmem these days.