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

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Message-ID: <2250236.ElGaqSPkdT@opensuse>
Date:   Wed, 06 Jul 2022 12:38:29 +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 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.

> The rest looks good,

Thanks,

Fabio

> Ira
> 
> [snip]
>