Re: [PATCH] Fix mapping->writeback_index to point to the lastwritten page

[Jan Kara]

  Hello,

On Fri 25-02-11 16:55:19, Jun'ichi Nomura wrote:
> For range-cyclic writeback (e.g. kupdate), the writeback code sets
> a continuation point of the next writeback to mapping->writeback_index.
> 
> Current code sets the page next to the last written page.
> I think it's intended for sequential writer.
  Not exactly. The code is meant so that background writeback gets to
writing the end of a file which gets continuously dirtied (if we always
started at the beginning, nr_to_write could always get to 0 before we reach
end of the file).

> However, in many cases, sequential writer is writing in the middle of the page
> and it just redirties the last written page by continuing from that.
> 
> So the next writeback should try to continue from the last written page,
> not the next one.
> (If it's clean because the writer was on the page boundary,
>  pagevec_lookup_tag just skips it. So no problem.)
> 
> Otherwise, the last written page was left dirty until the writeback
> wraps around.
> 
> I.e. if the sequential dirtier has written on pagecache as '*'s below:
> 
>    |*******|*******|****---|-------|-------|     ( |---| is a page )
> 
> then, writeback happens:
> 
>    |-------|-------|-------|-------|-------|
> 
> and the dirtier continues:
> 
>    |-------|-------|----***|*******|*****--|
>                    A       B
> 
> Next writeback should start from page A, not B.
  Yes, this is downside of our current scheme. Have you actually observed
it in practice or is it mostly a theoretic concern?
 
But as I'm thinking about it, it wouldn't harm our original aim to do
what you propose and it can help this relatively common case. So I think
it's a good idea. Fengguang, what do you think?

								Honza
-- 
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR
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