NR_UNSTABLE_FS vs. NR_FILE_DIRTY: double counting pages?

NR_UNSTABLE_FS vs. NR_FILE_DIRTY: double counting pages?

[Ethan Solomita]

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

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Re: NR_UNSTABLE_FS vs. NR_FILE_DIRTY: double counting pages?

[Trond Myklebust]

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.

Trond

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Re: NR_UNSTABLE_FS vs. NR_FILE_DIRTY: double counting pages?

[Ethan Solomita]

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

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Re: NR_UNSTABLE_FS vs. NR_FILE_DIRTY: double counting pages?

[Ethan Solomita]

Ethan Solomita wrote:
> Trond Myklebust wrote:
>>
>> 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?

	Sorry about my quick reply, I'd misunderstood what you were saying. 
I'll take a look at what you say.

	Thanks,
	-- Ethan

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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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