I'm still not understanding. The terms you're using are a bit ambiguous.
What does "find some dirty unallocated blocks" mean? Find a page which is
dirty and which does not have a disk mapping?
Normally the above operation would be implemented via
ext4_writeback_writepage(), and it runs under lock_page().
going to commit
find inode I dirty
do NOT find these blocks because they're
allocated only, but pages/bhs aren't mapped
to them
start commit
I think you're assuming here that commit would be using ->t_sync_datalist
to locate dirty buffer_heads.
But under this proposal, t_sync_datalist just gets removed: the new
ordered-data mode _only_ need to do the sb->inode->page walk. So if I'm
understanding you, the way in which we'd handle any such race is to make
kjournald's writeback of the dirty pages block in lock_page(). Once it
gets the page lock it can look to see if some other thread has mapped the
page to disk.
It may turn out that kjournald needs a private way of getting at the
I_DIRTY_PAGES inodes to do this properly, but I don't _think_ so. If we
had the radix-tree-of-dirty-inodes thing then that's easy enough to do
anyway, with a tagged search. But I expect that a single pass through the
superblock's dirty inodes would suffice for ordered-data. Files which
have chattr +j would screw things up, as usual.
I assume (hope) that your delayed allocation code implements
->writepages()? Doing the allocation one-page-at-a-time sounds painful...