linux-kernel - Re: [RFC PATCH v2 2/3] btrfs: Use kmap_local_page() on "out_page" in zlib_compress

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Message-ID: <3674366.kQq0lBPeGt@opensuse>
Date:   Fri, 17 Jun 2022 19:46:21 +0200
From:   "Fabio M. De Francesco" <fmdefrancesco@...il.com>
To:     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>, Qu Wenruo <wqu@...e.com>,
        Nikolay Borisov <nborisov@...e.com>,
        Gabriel Niebler <gniebler@...e.com>,
        Ira Weiny <ira.weiny@...el.com>, linux-btrfs@...r.kernel.org,
        linux-kernel@...r.kernel.org, Qu Wenruo <quwenruo.btrfs@....com>
Subject: Re: [RFC PATCH v2 2/3] btrfs: Use kmap_local_page() on "out_page" in zlib_compress_pages()

On venerdì 17 giugno 2022 14:54:22 CEST Qu Wenruo wrote:
> 
> On 2022/6/17 20:05, 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() for "out_page" 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 bits VM with 4GB of RAM and
> > HIGHMEM64G enabled. This patch passes 26/26 tests of group "compress".
> >
> > Suggested-by: Ira Weiny <ira.weiny@...el.com>
> > Signed-off-by: Fabio M. De Francesco <fmdefrancesco@...il.com>
> 
> Looks good to me.
> 
> Reviewed-by: Qu Wenruo <wqu@...e.com>
> 

Again, thanks.

> The change is just to use @cpage_out to indicate if it's mapped (NULL =
> not mapped).
> 

Most of the conversions are quite easy, I would say "mechanical".
As you already noted in 3/3, there are cases where it's not that simple :-(

> Just a small nit inlined below.
> 
> > ---
> >   fs/btrfs/zlib.c | 20 +++++++++++---------
> >   1 file changed, 11 insertions(+), 9 deletions(-)
> >
> > diff --git a/fs/btrfs/zlib.c b/fs/btrfs/zlib.c
> > index 770c4c6bbaef..c7c69ce4a1a9 100644
> > --- a/fs/btrfs/zlib.c
> > +++ b/fs/btrfs/zlib.c
> > @@ -97,8 +97,8 @@ int zlib_compress_pages(struct list_head *ws, struct 
address_space *mapping,
> >   {
> >   	struct workspace *workspace = list_entry(ws, struct workspace, 
list);
> >   	int ret;
> > -	char *data_in;
> > -	char *cpage_out;
> > +	char *data_in = NULL;
> 
> I didn't see any diff touching @data_in, any idea why it's initialized
> to NULL?
> 

I suppose it's a relic of RFC v1 that I overlooked when I split it into 
three patches. I will remember to avoid initialization in the "real" patch.

Fabio

>
> Thanks,
> Qu
> 
> > +	char *cpage_out = NULL;
> >   	int nr_pages = 0;
> >   	struct page *in_page = NULL;
> >   	struct page *out_page = NULL;
> > @@ -126,7 +126,7 @@ int zlib_compress_pages(struct list_head *ws, 
struct address_space *mapping,
> >   		ret = -ENOMEM;
> >   		goto out;
> >   	}
> > -	cpage_out = kmap(out_page);
> > +	cpage_out = kmap_local_page(out_page);
> >   	pages[0] = out_page;
> >   	nr_pages = 1;
> >
> > @@ -196,7 +196,8 @@ int zlib_compress_pages(struct list_head *ws, 
struct address_space *mapping,
> >   		 * the stream end if required
> >   		 */
> >   		if (workspace->strm.avail_out == 0) {
> > -			kunmap(out_page);
> > +			kunmap_local(cpage_out);
> > +			cpage_out = NULL;
> >   			if (nr_pages == nr_dest_pages) {
> >   				out_page = NULL;
> >   				ret = -E2BIG;
> > @@ -207,7 +208,7 @@ int zlib_compress_pages(struct list_head *ws, 
struct address_space *mapping,
> >   				ret = -ENOMEM;
> >   				goto out;
> >   			}
> > -			cpage_out = kmap(out_page);
> > +			cpage_out = kmap_local_page(out_page);
> >   			pages[nr_pages] = out_page;
> >   			nr_pages++;
> >   			workspace->strm.avail_out = PAGE_SIZE;
> > @@ -234,7 +235,8 @@ int zlib_compress_pages(struct list_head *ws, 
struct address_space *mapping,
> >   			goto out;
> >   		} else if (workspace->strm.avail_out == 0) {
> >   			/* get another page for the stream end */
> > -			kunmap(out_page);
> > +			kunmap_local(cpage_out);
> > +			cpage_out = NULL;
> >   			if (nr_pages == nr_dest_pages) {
> >   				out_page = NULL;
> >   				ret = -E2BIG;
> > @@ -245,7 +247,7 @@ int zlib_compress_pages(struct list_head *ws, 
struct address_space *mapping,
> >   				ret = -ENOMEM;
> >   				goto out;
> >   			}
> > -			cpage_out = kmap(out_page);
> > +			cpage_out = kmap_local_page(out_page);
> >   			pages[nr_pages] = out_page;
> >   			nr_pages++;
> >   			workspace->strm.avail_out = PAGE_SIZE;
> > @@ -264,8 +266,8 @@ int zlib_compress_pages(struct list_head *ws, 
struct address_space *mapping,
> >   	*total_in = workspace->strm.total_in;
> >   out:
> >   	*out_pages = nr_pages;
> > -	if (out_page)
> > -		kunmap(out_page);
> > +	if (cpage_out)
> > +		kunmap_local(cpage_out);
> >
> >   	if (in_page) {
> >   		kunmap(in_page);
>