Re: [PATCH v2] fadvise: introduce POSIX_FADV_DONTNEED_FS

On Wed, Apr 27, 2011 at 12:33:08PM -0600, Matthew Wilcox wrote:

On Wed, Apr 27, 2011 at 08:13:47PM +0200, Andrea Righi wrote:

quoted

@@ -127,6 +128,12 @@ SYSCALL_DEFINE(fadvise64_64)(int fd, loff_t offset, loff_t len, int advice)
 			invalidate_mapping_pages(mapping, start_index,
 						end_index);
 		break;
+	case POSIX_FADV_DONTNEED_FS:
+		if (capable(CAP_SYS_ADMIN))
+			drop_pagecache_sb(file->f_dentry->d_sb, NULL);
+		else
+			ret = -EPERM;
+		break;
 	default:
 		ret = -EINVAL;
 	}

Mmm ... what if I open /dev/sdxyz and call fadvise() on it?  I think
you end up flushing /dev's page cache entries, instead of the filesystem
which is on /dev/sdxyz.

If I understand correctly, you want mapping->host->i_sb instead of
file->f_dentry->d_sb.

I did some tests, but I don't get the expected behaviour. In all cases
both if I use mapping->host->i_sb or file->f_dentry->d_sb when I call
fadvise() on any block device all the blockdev pages are dropped
("Buffers" from /proc/meminfo), but page cache pages are not touched:

# df -hT /
Filesystem    Type    Size  Used Avail Use% Mounted on
/dev/sda1     ext4     29G   20G  7.4G  73% /
# grep "^Cached\|Buffers" /proc/meminfo
Buffers:           79772 kB
Cached:            32440 kB
# sudo drop-pagecache /dev/sda1
# grep "^Cached\|Buffers" /proc/meminfo
Buffers:             228 kB
Cached:            32440 kB
# sudo drop-pagecache /
# grep "^Cached\|Buffers" /proc/meminfo
Buffers:             228 kB
Cached:             4884 kB

Thanks,
-Andrea