On Fri, Aug 01, 2008 at 10:24:12AM +0530, Aneesh Kumar K.V wrote:
> On Thu, Jul 31, 2008 at 02:10:55PM -0600, Andreas Dilger wrote:
> > On Jul 31, 2008 23:03 +0530, Aneesh Kumar wrote:
> > > With the below changes we reserve credit needed to insert only one extent
> > > resulting from a call to single get_block. That make sure we don't take
> > > too much journal credits during writeout. We also don't limit the pages
> > > to write. That means we loop through the dirty pages building largest
> > > possible contiguous block request. Then we issue a single get_block request.
> > > We may get less block that we requested. If so we would end up not mapping
> > > some of the buffer_heads. That means those buffer_heads are still marked delay.
> > > Later in the writepage callback via __mpage_writepage we redirty those pages.
> >
> > Can you please clarify this? Does this mean we take one pass through the
> > dirty pages, but possibly do not allocate some subset of the pages. Then,
> > at some later time these holes are written out separately? This seems
> > like it would produce fragmentation if we do not work to ensure the pages
> > are allocated in sequence. Maybe I'm misunderstanding your comment and
> > the unmapped pages are immediately mapped on the next loop?
>
> We take multiple pass through the dirty pages until wbc->nr_to_write is
> <= 0 or we don't have anything more to write. But if get_block doesn't
> return the requested number of blocks we may possibly not writeout
> some of the pages. Whether this can result in a disk layout worse than
> the current, I am not sure. I haven't looked at the layout yet.
> But these pages which are skipped are redirtied again via
> reditry_pages_for_writepage and will be forced for writeout. Well
> we can do better by setting wbc->encountered_congestion = 1; even
> though we are not really congested. That would cause most of the pdflush
> work func to retry writeback_indoes.
>
> for(;;) {
> ...
> wbc.pages_skipped = 0;
> writeback_inodes(&wbc);
> ...
>
> if (wbc.nr_to_write > 0 || wbc.pages_skipped > 0) {
> /* Wrote less than expected */
> if (wbc.encountered_congestion || wbc.more_io)
> congestion_wait(WRITE, HZ/10);
> else
> break;
> }
>
> }
>
like below ?
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 53a8fc7..6fd527c 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -1773,6 +1773,14 @@ static void mpage_da_map_blocks(struct mpage_da_data *mpd)
return;
BUG_ON(new.b_size == 0);
+ if (new.b_size < lbh->b_size) {
+ /*
+ * allocated less blocks. force writepages
+ * to be called again
+ */
+ mpd->wbc->more_io = 1;
+ }
+
if (buffer_new(&new))
__unmap_underlying_blocks(mpd->inode, &new);
@@ -1876,6 +1884,8 @@ static int __mpage_da_writepage(struct page *page,
* skip rest of the page in the page_vec
*/
mpd->io_done = 1;
+ /* We want writepages to be called again */
+ wbc->more_io = 1;
redirty_page_for_writepage(wbc, page);
unlock_page(page);
return MPAGE_DA_EXTENT_TAIL;
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