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

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Date:   Sun, 26 Jun 2022 07:03:55 +0800
From:   Qu Wenruo <wqu@...e.com>
To:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>,
        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
Cc:     Ira Weiny <ira.weiny@...el.com>
Subject: Re: [PATCH] btrfs: Convert zlib_compress_pages() to use
 kmap_local_page()



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,
> 
> Fabio
> 
>