NR_UNSTABLE_FS vs. NR_FILE_DIRTY: double counting pages?

There are several places where we add together NR_UNSTABLE_FS and NF_FILE_DIRTY:

sync_inodes_sb()
balance_dirty_pages()
wakeup_pdflush()
wb_kupdate()
prefetch_suitable()

I can trace a standard codepath where it seems both of these are set
on the same page:

nfs_file_aops.commit_write ->
nfs_commit_write
nfs_updatepages
nfs_writepage_setup
nfs_wb_page
nfs_wb_page_priority
nfs_writepage_locked
nfs_flush_mapping
nfs_flush_list
nfs_flush_multi
nfs_write_partial_ops.rpc_call_done
nfs_writeback_done_partial
nfs_writepage_release
nfs_reschedule_unstable_write
nfs_mark_request_commit
incr NR_UNSTABLE_NFS

nfs_file_aops.commit_write ->
nfs_commit_write
nfs_updatepage
__set_page_dirty_nobuffers
incr NF_FILE_DIRTY

This is the standard code path that derives from sys_write(). Can
someone either show how this code sequence can't happen, or confirm for me that there's a bug?
-- Ethan

Quoted 36 lines · Ethan Solomita

There are several places where we add together NR_UNSTABLE_FS and NF_FILE_DIRTY:

sync_inodes_sb()
balance_dirty_pages()
wakeup_pdflush()
wb_kupdate()
prefetch_suitable()

I can trace a standard codepath where it seems both of these are set
on the same page:

nfs_file_aops.commit_write ->
nfs_commit_write
nfs_updatepages
nfs_writepage_setup
nfs_wb_page
nfs_wb_page_priority
nfs_writepage_locked
nfs_flush_mapping
nfs_flush_list
nfs_flush_multi
nfs_write_partial_ops.rpc_call_done
nfs_writeback_done_partial
nfs_writepage_release
nfs_reschedule_unstable_write
nfs_mark_request_commit
incr NR_UNSTABLE_NFS

nfs_file_aops.commit_write ->
nfs_commit_write
nfs_updatepage
__set_page_dirty_nobuffers
incr NF_FILE_DIRTY

Quoted 4 lines

This is the standard code path that derives from sys_write(). Can
someone either show how this code sequence can't happen, or confirm for me that there's a bug?
-- Ethan

It should not happen. If the page is on the unstable list, then it will be committed before nfs_updatepage is allowed to redirty it. See the recent fixes in 2.6.21-rc7.

Quoted 4 lines · Trond Myklebust

It should not happen. If the page is on the unstable list, then it will be committed before nfs_updatepage is allowed to redirty it. See the recent fixes in 2.6.21-rc7.

Above I present a codepath called straight from sys_write() which
seems to do what I say. I could be wrong, but can you address the code paths I show above which seem to set both?

Quoted 42 lines · Trond Myklebust

Quoted 40 lines · Ethan Solomita

There are several places where we add together NR_UNSTABLE_FS and NF_FILE_DIRTY:

sync_inodes_sb()
balance_dirty_pages()
wakeup_pdflush()
wb_kupdate()
prefetch_suitable()

I can trace a standard codepath where it seems both of these are set
on the same page:

nfs_file_aops.commit_write ->
nfs_commit_write
nfs_updatepages
nfs_writepage_setup
nfs_wb_page
nfs_wb_page_priority
nfs_writepage_locked
nfs_flush_mapping
nfs_flush_list
nfs_flush_multi
nfs_write_partial_ops.rpc_call_done
nfs_writeback_done_partial
nfs_writepage_release
nfs_reschedule_unstable_write
nfs_mark_request_commit
incr NR_UNSTABLE_NFS

nfs_file_aops.commit_write ->
nfs_commit_write
nfs_updatepage
__set_page_dirty_nobuffers
incr NF_FILE_DIRTY

This is the standard code path that derives from sys_write(). Can
someone either show how this code sequence can't happen, or confirm for me that there's a bug?
-- Ethan

Quoted 3 lines

It should not happen. If the page is on the unstable list, then it will be committed before nfs_updatepage is allowed to redirty it. See the recent fixes in 2.6.21-rc7.

Above I present a codepath called straight from sys_write() which seems
to do what I say. I could be wrong, but can you address the code paths I show above which seem to set both?
-- Ethan