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Subject: Re: [PATCH] reiserfs: don't drop PG_dirty when releasing
	sub-page-sized dirty file
From: Peter Zijlstra <peterz@infradead.org>
To: Fengguang Wu <wfg@mail.ustc.edu.cn>
Cc: Maxim Levitsky <maximlevitsky@gmail.com>, linux-kernel@vger.kernel.org,
       Fengguang Wu <fengguang.wu@gmail.com>,
       Andrew Morton <akpm@linux-foundation.org>,
       Jeff Mahoney <jeffm@suse.com>, reiserfs-dev@namesys.com
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Date: Tue, 23 Oct 2007 12:07:07 +0200
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[ adding reiserfs devs to the CC ]

On Tue, 2007-10-23 at 15:55 +0800, Fengguang Wu wrote:
> This is not a new problem in 2.6.23-git17.
> 2.6.22/2.6.23 is buggy in the same way.
> 
> Reiserfs could leave newly created sub-page-size files in dirty state
> for ever.  They cannot be synced to disk by pdflush routines or
> explicit `sync' commands.  Only `umount' can do the trick.
> 
> The direct cause is: the dirty page's PG_dirty is wrongly _cleared_.
> Call trace:
> 	 [<ffffffff8027e920>] cancel_dirty_page+0xd0/0xf0
> 	 [<ffffffff8816d470>] :reiserfs:reiserfs_cut_from_item+0x660/0x710
> 	 [<ffffffff8816d791>] :reiserfs:reiserfs_do_truncate+0x271/0x530
> 	 [<ffffffff8815872d>] :reiserfs:reiserfs_truncate_file+0xfd/0x3b0
> 	 [<ffffffff8815d3d0>] :reiserfs:reiserfs_file_release+0x1e0/0x340
> 	 [<ffffffff802a187c>] __fput+0xcc/0x1b0
> 	 [<ffffffff802a1ba6>] fput+0x16/0x20
> 	 [<ffffffff8029e676>] filp_close+0x56/0x90
> 	 [<ffffffff8029fe0d>] sys_close+0xad/0x110
> 	 [<ffffffff8020c41e>] system_call+0x7e/0x83
> 
> Fix the bug by removing the cancel_dirty_page() call. Tests show that
> it causes no bad behaviors on various write sizes.
> 
> 
> === for the patient ===
> Here are more detailed demonstrations of the problem.
> 
> 1) the page has both PG_dirty(D)/PAGECACHE_TAG_DIRTY(d) after being written to;
>    and then only PAGECACHE_TAG_DIRTY(d) remains after the file is closed.
> 
> ------------------------------ screen 0 ------------------------------
> [T0] root /home/wfg# cat > /test/tiny
> [T1] hi
> [T2] root /home/wfg#
> 
> ------------------------------ screen 1 ------------------------------
> [T1] root /home/wfg# echo /test/tiny > /proc/filecache
> [T1] root /home/wfg# cat /proc/filecache
>      # file /test/tiny
>      # flags R:referenced A:active M:mmap U:uptodate D:dirty W:writeback O:owner B:buffer d:dirty w:writeback
>      # idx   len     state   refcnt
>      0       1       ___UD__Bd_      2
> [T2] root /home/wfg# cat /proc/filecache
>      # file /test/tiny
>      # flags R:referenced A:active M:mmap U:uptodate D:dirty W:writeback O:owner B:buffer d:dirty w:writeback
>      # idx   len     state   refcnt
>      0       1       ___U___Bd_      2
> 
> 2) note the non-zero 'cancelled_write_bytes' after /tmp/hi is copied.
> 
> ------------------------------ screen 0 ------------------------------
> [T0] root /home/wfg# echo hi > /tmp/hi
> [T1] root /home/wfg# cp /tmp/hi /dev/stdin /test
> [T2] hi
> [T3] root /home/wfg#
> 
> ------------------------------ screen 1 ------------------------------
> [T1] root /proc/4397# cd /proc/`pidof cp`
> [T1] root /proc/4713# cat io
>      rchar: 8396
>      wchar: 3
>      syscr: 20
>      syscw: 1
>      read_bytes: 0
>      write_bytes: 20480
>      cancelled_write_bytes: 4096
> [T2] root /proc/4713# cat io
>      rchar: 8399
>      wchar: 6
>      syscr: 21
>      syscw: 2
>      read_bytes: 0
>      write_bytes: 24576
>      cancelled_write_bytes: 4096
> 
> //Question: the 'write_bytes' is a bit more than expected ;-)
> 
> Cc: Maxim Levitsky <maximlevitsky@gmail.com>                                                                           
> Cc: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Fengguang Wu <wfg@mail.ustc.edu.cn>
> ---
>  fs/reiserfs/stree.c |    3 ---
>  1 file changed, 3 deletions(-)
> 
> --- linux-2.6.24-git17.orig/fs/reiserfs/stree.c
> +++ linux-2.6.24-git17/fs/reiserfs/stree.c
> @@ -1458,9 +1458,6 @@ static void unmap_buffers(struct page *p
>  				}
>  				bh = next;
>  			} while (bh != head);
> -			if (PAGE_SIZE == bh->b_size) {
> -				cancel_dirty_page(page, PAGE_CACHE_SIZE);
> -			}
>  		}
>  	}
>  }
> 

-
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