linux-kernel - Re: [PATCH] mm/zswap: Replace kmap_atomic() with kmap_local

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Date:   Wed, 29 Nov 2023 11:03:52 -0800
From:   Christopher Li <chrisl@...nel.org>
To:     "Fabio M. De Francesco" <fabio.maria.de.francesco@...ux.intel.com>
Cc:     Seth Jennings <sjenning@...hat.com>,
        Dan Streetman <ddstreet@...e.org>,
        Vitaly Wool <vitaly.wool@...sulko.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Ira Weiny <ira.weiny@...el.com>,
        Nhat Pham <nphamcs@...il.com>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH] mm/zswap: Replace kmap_atomic() with kmap_local_page()

Hi Fabio,

On Wed, Nov 29, 2023 at 3:41 AM Fabio M. De Francesco
<fabio.maria.de.francesco@...ux.intel.com> wrote:
> > > The kernel virtual addresses returned by these two API are
> > > only valid in the context of the callers (i.e., they cannot be handed to
> > > other threads).
> > >
> > > With kmap_local_page() the mappings are per thread and CPU local like
> > > in kmap_atomic(); however, they can handle page-faults and can be called
> > > from any context (including interrupts). The tasks that call
> > > kmap_local_page() can be preempted and, when they are scheduled to run
> > > again, the kernel virtual addresses are restored and are still valid.
> >
> > As far as I can tell, the kmap_atomic() is the same as
> > kmap_local_page() with the following additional code before calling to
> > "__kmap_local_page_prot(page, prot)", which is common between these
> > two functions.
> >
> >         if (IS_ENABLED(CONFIG_PREEMPT_RT))
> >                 migrate_disable();
> >         else
> >                 preempt_disable();
> >
> >         pagefault_disable();
> >
>
> This is what I tried to explain with that sentence. I think you overlooked it
> :)

I did read your description. It is not that I don't trust your
description. I want to see how the code does what you describe at the
source code level. In this case the related code is fairly easy to
isolate.

>
> BTW, please have a look at the Highmem documentation. It has initially been
> written by Peter Z. and I reworked and largely extended it authoring  the
> patches with my gmail address (6 - 7 different patches, if I remember
> correctly).
>
> You will find there everything you may want to know about these API and how to
> do conversions from the older to the newer.

Will do, thanks for the pointer.

Chris