linux-ext4 - Re: [PATCH RFC PKS/PMEM 33/58] fs/cramfs: Utilize new kmap

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Date:   Tue, 13 Oct 2020 20:36:43 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     "Weiny, Ira" <ira.weiny@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
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Subject: Re: [PATCH RFC PKS/PMEM 33/58] fs/cramfs: Utilize new kmap_thread()

On Tue, Oct 13, 2020 at 11:44:29AM -0700, Dan Williams wrote:
> On Fri, Oct 9, 2020 at 12:52 PM <ira.weiny@...el.com> wrote:
> >
> > From: Ira Weiny <ira.weiny@...el.com>
> >
> > The kmap() calls in this FS are localized to a single thread.  To avoid
> > the over head of global PKRS updates use the new kmap_thread() call.
> >
> > Cc: Nicolas Pitre <nico@...xnic.net>
> > Signed-off-by: Ira Weiny <ira.weiny@...el.com>
> > ---
> >  fs/cramfs/inode.c | 10 +++++-----
> >  1 file changed, 5 insertions(+), 5 deletions(-)
> >
> > diff --git a/fs/cramfs/inode.c b/fs/cramfs/inode.c
> > index 912308600d39..003c014a42ed 100644
> > --- a/fs/cramfs/inode.c
> > +++ b/fs/cramfs/inode.c
> > @@ -247,8 +247,8 @@ static void *cramfs_blkdev_read(struct super_block *sb, unsigned int offset,
> >                 struct page *page = pages[i];
> >
> >                 if (page) {
> > -                       memcpy(data, kmap(page), PAGE_SIZE);
> > -                       kunmap(page);
> > +                       memcpy(data, kmap_thread(page), PAGE_SIZE);
> > +                       kunmap_thread(page);
> 
> Why does this need a sleepable kmap? This looks like a textbook
> kmap_atomic() use case.

There's a lot of code of this form.  Could we perhaps have:

static inline void copy_to_highpage(struct page *to, void *vfrom, unsigned int size)
{
	char *vto = kmap_atomic(to);

	memcpy(vto, vfrom, size);
	kunmap_atomic(vto);
}

in linux/highmem.h ?