Re: [PATCH] btrfs: Convert zlib_compress_pages() to use kmap_local_page()

[Qu Wenruo]

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@xxxxxxxx>
> > Suggested-by: Ira Weiny <ira.weiny@xxxxxxxxx>
> > Signed-off-by: Fabio M. De Francesco <fmdefrancesco@xxxxxxxxx>
> > ---
> >
> > 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@xxxxxxxx/
> >
>
> 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