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

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Date:   Mon, 03 Apr 2023 07:27:40 +0200
From:   "Fabio M. De Francesco" <fmdefrancesco@...il.com>
To:     Matthew Wilcox <willy@...radead.org>
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 lunedì 27 marzo 2023 15:23:41 CEST Matthew Wilcox wrote:
> 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.

Matthew,

It's been a few days since you wrote but I still haven't done anything or 
replied. For now, I just want you to know that it's not my intention to ignore 
your comment.

The only reasons I hesitate to follow your directions are that I built this 
series based on specific suggestions from Al. 

While I understand and, for sure, heed what you said, I also think I should 
wait for Al to chime in with what he thinks about all this before taking a 
different route for removing kmap() from fs/ufs.

Thanks for looking at this series.

Fabio