On Wed, Dec 20, 2006 at 03:55:25PM -0800, Linus Torvalds wrote:
> On Thu, 21 Dec 2006, David Chinner wrote:
> >
> > XFS appears to call clear_page_dirty to get the mapping tree dirty
> > tag set correctly at the same time the page dirty flag is cleared. I
> > note that this can be done by set_page_writeback() if we clear the
> > dirty flag on the page first when we are writing back the entire page.
>
> Yes. I think the XFS routine should just use "clear_page_dirty_fir_io()",
> since that matches what it actually wants to do (surprise surprise, it's
> going to write it out).
Yup ;)
> HOWEVER. Why is it conditional? Can somebody who understands XFS tell me
> why "clear_dirty" would ever be 0? I can grep the sources, and I see that
> it's an unconditional 1 in one call-site, but then in the other one it
> does
>
> xfs_start_page_writeback(page, wbc, !page_dirty, count);
page dirty starts at the number of dirty buffers on the page, and as
we map each dirty buffer into the I/O we decrement the page dirty count.
Hence if we map all of the buffers into the I/O, we are cleaning
the entire page and hence we can clear the dirty state on the page.
> and that part just blows my mind. Why would you do a
> xfs_start_page_writeback() and _not_ write the page out? Is this for a
> partial-page-only case?
Yes, partial-page-only case when doing speculative write clustering. We'll hit
this when an extent boundary is not page aligned (fs block size < page size
case) and we need to issue at least two separate I/Os to clean the page.
Because we leave the page dirty and we are working ahead of the index in
generic_writepages() we'll get the rest of the page flushed when we return
back to generic_writepages() as the page is still dirty in the mapping
tree....
> Anyway, your patch looks fine. It seems to be the right thing to do.
Ok, thanks, Linus.
Cheers,
Dave.
--
Dave Chinner
Principal Engineer
SGI Australian Software Group
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