linux-kernel - Re: [PATCH v5 2/2] btrfs: Replace kmap() with kmap_local

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Date:   Wed, 06 Jul 2022 12:48:46 +0200
From:   "Fabio M. De Francesco" <fmdefrancesco@...il.com>
To:     Ira Weiny <ira.weiny@...el.com>
Cc:     David Sterba <dsterba@...e.com>, Chris Mason <clm@...com>,
        Josef Bacik <josef@...icpanda.com>,
        Nick Terrell <terrelln@...com>, linux-btrfs@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        Kees Cook <keescook@...omium.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        "James E. J. Bottomley" <James.Bottomley@...senpartnership.com>,
        Helge Deller <deller@....de>,
        John David Anglin <dave.anglin@...l.net>,
        linux-parisc@...r.kernel.org, Filipe Manana <fdmanana@...nel.org>
Subject: Re: [PATCH v5 2/2] btrfs: Replace kmap() with kmap_local_page() in zstd.c

On mercoledì 6 luglio 2022 12:38:29 CEST Fabio M. De Francesco wrote:
> On martedì 5 luglio 2022 21:04:04 CEST Ira Weiny wrote:
> > On Mon, Jul 04, 2022 at 05:23:22PM +0200, 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 zstd.c because 
in 
> this
> > > file the mappings are per thread and are not visible in other 
contexts. 
> In
> > > the meanwhile use plain page_address() on pages allocated with the 
> GFP_NOFS
> > > flag instead of calling kmap*() on them (since they are always 
> allocated
> > > from ZONE_NORMAL).
> > > 
> > > Tested with xfstests on QEMU + KVM 32 bits VM with 4GB of RAM and
> > > booting a kernel with HIGHMEM64G enabled.
> > > 
> > > Cc: Filipe Manana <fdmanana@...nel.org>
> > > Suggested-by: Ira Weiny <ira.weiny@...el.com>
> > > Signed-off-by: Fabio M. De Francesco <fmdefrancesco@...il.com>
> > > ---
> > >  fs/btrfs/zstd.c | 34 ++++++++++++++--------------------
> > >  1 file changed, 14 insertions(+), 20 deletions(-)
> > > 
> > > diff --git a/fs/btrfs/zstd.c b/fs/btrfs/zstd.c
> > > index 0fe31a6f6e68..78e0272e770e 100644
> > > --- a/fs/btrfs/zstd.c
> > > +++ b/fs/btrfs/zstd.c
> > > @@ -403,7 +403,7 @@ int zstd_compress_pages(struct list_head *ws, 
> struct address_space *mapping,
> > >  
> > >  	/* map in the first page of input data */
> > >  	in_page = find_get_page(mapping, start >> PAGE_SHIFT);
> > > -	workspace->in_buf.src = kmap(in_page);
> > > +	workspace->in_buf.src = kmap_local_page(in_page);
> > >  	workspace->in_buf.pos = 0;
> > >  	workspace->in_buf.size = min_t(size_t, len, PAGE_SIZE);
> > >  
> > > @@ -415,7 +415,7 @@ int zstd_compress_pages(struct list_head *ws, 
> struct address_space *mapping,
> > >  		goto out;
> > >  	}
> > >  	pages[nr_pages++] = out_page;
> > > -	workspace->out_buf.dst = kmap(out_page);
> > > +	workspace->out_buf.dst = page_address(out_page);
> > >  	workspace->out_buf.pos = 0;
> > >  	workspace->out_buf.size = min_t(size_t, max_out, PAGE_SIZE);
> > >  
> > > @@ -450,9 +450,7 @@ int zstd_compress_pages(struct list_head *ws, 
> struct address_space *mapping,
> > >  		if (workspace->out_buf.pos == workspace->out_buf.size) 
> {
> > >  			tot_out += PAGE_SIZE;
> > >  			max_out -= PAGE_SIZE;
> > > -			kunmap(out_page);
> > >  			if (nr_pages == nr_dest_pages) {
> > > -				out_page = NULL;
> > >  				ret = -E2BIG;
> > >  				goto out;
> > >  			}
> > > @@ -462,7 +460,7 @@ int zstd_compress_pages(struct list_head *ws, 
> struct address_space *mapping,
> > >  				goto out;
> > >  			}
> > >  			pages[nr_pages++] = out_page;
> > > -			workspace->out_buf.dst = kmap(out_page);
> > > +			workspace->out_buf.dst = 
> page_address(out_page);
> > >  			workspace->out_buf.pos = 0;
> > >  			workspace->out_buf.size = min_t(size_t, 
> max_out,
> > >  							
> PAGE_SIZE);
> > > @@ -477,15 +475,15 @@ int zstd_compress_pages(struct list_head *ws, 
> struct address_space *mapping,
> > >  		/* Check if we need more input */
> > >  		if (workspace->in_buf.pos == workspace->in_buf.size) {
> > >  			tot_in += PAGE_SIZE;
> > > -			kunmap(in_page);
> > > +			kunmap_local(workspace->in_buf.src);
> > >  			put_page(in_page);
> > > -
> > >  			start += PAGE_SIZE;
> > >  			len -= PAGE_SIZE;
> > >  			in_page = find_get_page(mapping, start >> 
> PAGE_SHIFT);
> > > -			workspace->in_buf.src = kmap(in_page);
> > > +			workspace->in_buf.src = 
> kmap_local_page(in_page);
> > >  			workspace->in_buf.pos = 0;
> > >  			workspace->in_buf.size = min_t(size_t, len, 
> PAGE_SIZE);
> > > +			workspace->out_buf.dst = 
> page_address(out_page);
> > 
> > Why is this needed?
> 
> Sorry. This initialization is not needed at all.
> Probably made a mistake with copy-pasting snippets of code.
> 
> I'm going to send ASAP the fifth version of this series.

"fifth" -> "sixth".

> 
> > The rest looks good,
> 
> Thanks,
> 
> Fabio
> 
> > Ira
> > 
> > [snip]
> > 
> 
> 
> 
>