linux-kernel - Re: [PATCH] affs: add kunmap in error path

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Date:	Tue, 2 Feb 2016 02:28:05 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Insu Yun <wuninsu@...il.com>
Cc:	akpm@...ux-foundation.org, fabf@...net.be, osandov@...ndov.com,
	geert@...ux-m68k.org, tsgatesv@...il.com,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	taesoo@...ech.edu, yeongjin.jang@...ech.edu, insu@...ech.edu,
	changwoo@...ech.edu
Subject: Re: [PATCH] affs: add kunmap in error path

On Tue, Feb 02, 2016 at 02:24:36AM +0000, Al Viro wrote:
> On Mon, Feb 01, 2016 at 08:55:17PM -0500, Insu Yun wrote:
> > When error occurs, it needs to unmap page.
> 
> Point, but...  I'm not sure there's any point in keeping it mapped all
> along.  After all, what we are doing is a bunch of memcpy() into the
> parts of page, so kmap_atomic()/kunmap_atomic() just around memcpy
> would do just fine.

IOW, something like this (completely untested):

diff --git a/fs/affs/file.c b/fs/affs/file.c
index 0548c53..22fc7c8 100644
--- a/fs/affs/file.c
+++ b/fs/affs/file.c
@@ -511,8 +511,6 @@ affs_do_readpage_ofs(struct page *page, unsigned to)
 	pr_debug("%s(%lu, %ld, 0, %d)\n", __func__, inode->i_ino,
 		 page->index, to);
 	BUG_ON(to > PAGE_CACHE_SIZE);
-	kmap(page);
-	data = page_address(page);
 	bsize = AFFS_SB(sb)->s_data_blksize;
 	tmp = page->index << PAGE_CACHE_SHIFT;
 	bidx = tmp / bsize;
@@ -524,14 +522,15 @@ affs_do_readpage_ofs(struct page *page, unsigned to)
 			return PTR_ERR(bh);
 		tmp = min(bsize - boff, to - pos);
 		BUG_ON(pos + tmp > to || tmp > bsize);
+		data = kmap_atomic(page);
 		memcpy(data + pos, AFFS_DATA(bh) + boff, tmp);
+		kunmap_atomic(data);
 		affs_brelse(bh);
 		bidx++;
 		pos += tmp;
 		boff = 0;
 	}
 	flush_dcache_page(page);
-	kunmap(page);
 	return 0;
 }