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:
> > @@ -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
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