__fsync_super and inode dirty pages

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

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 ?


-aneesh