Re: NR_UNSTABLE_FS vs. NR_FILE_DIRTY: double counting pages?

[Trond Myklebust]

On Sun, 2007-04-29 at 17:26 -0700, Ethan Solomita wrote:
> Trond Myklebust wrote:
> > On Fri, 2007-04-27 at 18:21 -0700, Ethan Solomita wrote:
> >> 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
> > 
> > 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

Look carefully at nfs_update_request(): if !nfs_dirty_request(), then it
returns -EBUSY, and so nfs_writepage_setup() will loop on nfs_wb_page().
IOW: if PG_NEED_COMMIT is set (which it should be if on the commit list)
then nfs_writepage_setup() will loop...

Trond

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