Re: __fsync_super and inode dirty pages

["Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>]

On Tue, Aug 05, 2008 at 11:24:39PM +0530, Aneesh Kumar K.V wrote:
> Hi,
> 
> I am trying to understand the expectation on filesystem
> writepages by __fsync_super -> generic_sync_sb_inodes call path.
> __fsync_super make two calls to generic_sync_sb_inodes and the 
> second call is made with WB_SYNC_ALL. The problem is, i am finding 
> multiple ways we may 'break' from the generic_sync_sb_inodes
> while (!list_empty(&sb->s_io)) loop.  For ex:
> 
> a) When wbc->nr_to_write <=0 
>    This should normally indicate that we wrote more than we requested
>    That means we have more dirty pages . So why break from loop ?
>    This may be ok for nr_to_write limited writeback. But is it ok
>    for __fsync_super ?
> 
> b) pages_skipped != wbc->pages_skipped.
>    That means writepages was not able to write some of the pages
>    and it redirtied the same. We add such inode to s_dirty list. But
>    we don't loop through the s_dirty list later trying to write the
>    dirty pages. We actually call generic_sync_sb_inodes again. That
>    can actually cause s_dirty list content to move to s_io once
>    via queue_io. But then we can have pages skipped again in the second
>    call to generic_sync_sb_inodes ?
> 
> 
> Shouldn't we have a variant that actually loop trough s_dirty and s_more_io
> list MULTIPLE times and make sure we force all the dirty pages to disk
> before __fsync_super returns ? Or is there an expectation from file
> system writepages that would make sure calling generic_sync_sb_inodes
> TWICE would guarantee that ?

The problem I am lookin at is listed here.

http://thread.gmane.org/gmane.comp.file-systems.ext4/8277

basically a mount -o remount,ro followed by a sync is indicating that
there are dirty pages left after remount,ro

-aneesh