linux-kernel - Re: [PATCH] btrfs: Convert zlib_compress_pages() to use kmap_local

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Message-ID: <4412617.LvFx2qVVIh@opensuse>
Date:   Mon, 27 Jun 2022 18:41:49 +0200
From:   "Fabio M. De Francesco" <fmdefrancesco@...il.com>
To:     Qu Wenruo <wqu@...e.com>
Cc:     Chris Mason <clm@...com>, Josef Bacik <josef@...icpanda.com>,
        David Sterba <dsterba@...e.com>,
        Nick Terrell <terrelln@...com>,
        Chris Down <chris@...isdown.name>,
        Filipe Manana <fdmanana@...e.com>,
        Nikolay Borisov <nborisov@...e.com>,
        Gabriel Niebler <gniebler@...e.com>,
        linux-btrfs@...r.kernel.org, linux-kernel@...r.kernel.org,
        Ira Weiny <ira.weiny@...el.com>
Subject: Re: [PATCH] btrfs: Convert zlib_compress_pages() to use kmap_local_page()

On domenica 26 giugno 2022 01:03:55 CEST Qu Wenruo wrote:
> 
> On 2022/6/25 22:41, Fabio M. De Francesco wrote:
> > On sabato 18 giugno 2022 11:27:52 CEST Fabio M. De Francesco wrote:
> >> The use of kmap() is being deprecated in favor of kmap_local_page(). 
With
> >> kmap_local_page(), the mapping is per thread, CPU local and not 
globally
> >> visible.
> >>
> >> Therefore, use kmap_local_page() / kunmap_local() in
> > zlib_compress_pages()
> >> because in this function the mappings are per thread and are not 
visible
> >> in other contexts.
> >>
> >> Tested with xfstests on QEMU + KVM 32-bit VM with 4GB of RAM and
> >> HIGHMEM64G enabled. This patch passes 26/26 tests of group "compress".
> >>
> >> Cc: Qu Wenruo <wqu@...e.com>
> >> Suggested-by: Ira Weiny <ira.weiny@...el.com>
> >> Signed-off-by: Fabio M. De Francesco <fmdefrancesco@...il.com>
> >> ---
> >>
> >> This patch builds only on top of
> >> "[PATCH] btrfs: zlib: refactor how we prepare the input buffer" by Qu
> > Wenruo".
> >> https://lore.kernel.org/linux-btrfs/
> > d0bfc791b5509df7b9ad44e41ada197d1b3149b3.1655519730.git.wqu@...e.com/
> >>
> > 
> > I've seen that Qu sent a v2 of the above patch. However David thinks 
that
> > it is better not to map pages allocated in zlib.c for output (out_page)
> > since they cannot come from highmem because of "alloc_page(GFP_NOFS);".
> > 
> > @David:
> > 
> > I suppose that, since it builds _only_ on top of the refactor submitted 
by
> > Qu, I'll have to wait and see what you decide.
> > 
> > If you don't want kmap_local_page() and prefer using page_address() on
> > "out_page", please drop this patch and let me know, so that I can send 
a
> > new patch which will be in accordance to your preference.
> 
> And that would also make the convert much easier for kmap_local_page() 
> of input pages.
> 
> I'll hold the refactor patch after all the kmap code is converted.
> 
> Thanks,
> Qu

Thanks for holding the refactor patch for the time I needed.

Now zlib.c is converted to kmap_local_page() and tested:
https://lore.kernel.org/lkml/20220627163305.24116-1-fmdefrancesco@gmail.com/

Again thanks,

Fabio