Re: [PATCH 2/3 v2] mm: Fix writeback_in_progress()

[Jens Axboe <jaxboe@fusionio.com>]

On 08/05/2010 08:53 PM, Jan Kara wrote:
> Commit 83ba7b071f30f7c01f72518ad72d5cd203c27502 broke writeback_in_progress()
> as in that commit we started to remove work items from the list at the
> moment we start working on them and not at the moment they are finished.
> Thus if the flusher thread was doing some work but there was no other
> work queued, writeback_in_progress() returned false. This could in
> particular cause unnecessary queueing of background writeback from
> balance_dirty_pages() or writeout work from writeback_sb_if_idle().
> 
> This patch fixes the problem by introducing a bit in the bdi state which
> indicates that the flusher thread is processing some work and uses this
> bit for writeback_in_progress() test.
> 
> NOTE: Both callsites of writeback_in_progress() (namely,
> writeback_inodes_sb_if_idle() and balance_dirty_pages()) would actually
> need a different information than what writeback_in_progress() provides.
> They would need to know whether *the kind of writeback they are going
> to submit* is already queued. But this information isn't that simple
> to provide so let's fix writeback_in_progress() for the time being.

I left this deliberatly vague, as there's no good way to answer the
question that the caller wants to know about.

Alternatively, augment the list_empty() check with a task running
check. I guess that would require the wb lock, so your bit set/check
is the best solution I think.

Acked-by: Jens Axboe <jaxboe@fusionio.com>

-- 
Jens Axboe


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