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Commit f85d7d6c authored by Chris Mason's avatar Chris Mason
Browse files

Btrfs: properly honor wbc->nr_to_write changes 



When btrfs fills a delayed allocation, it tries to increase
the wbc nr_to_write to cover a big part of allocation.  The
theory is that we're doing contiguous IO and writing a few
more blocks will save seeks overall at a very low cost.

The problem is that extent_write_cache_pages could ignore
the new higher nr_to_write if nr_to_write had already gone
down to zero.  We fix that by rechecking the nr_to_write
for every page that is processed in the pagevec.

This updates the math around bumping the nr_to_write value
to make sure we don't leave a tiny amount of IO hanging
around for the very end of a new extent.

Signed-off-by: default avatarChris Mason <chris.mason@oracle.com>
parent 11833d66

fs/btrfs/extent_io.c

+27 −11
Original line number Diff line number Diff line
@@ -2182,7 +2182,7 @@ static int __extent_writepage(struct page *page, struct writeback_control *wbc, 
	delalloc_end = 0;

	page_started = 0;

	if (!epd->extent_locked) {

		u64 delalloc_to_write;

		u64 delalloc_to_write = 0;

		/*

		 * make sure the wbc mapping index is at least updated

		 * to this page.
@@ -2202,16 +2202,24 @@ static int __extent_writepage(struct page *page, struct writeback_control *wbc, 
			tree->ops->fill_delalloc(inode, page, delalloc_start,

						 delalloc_end, &page_started,

						 &nr_written);

			delalloc_to_write = (delalloc_end -

					max_t(u64, page_offset(page),

					      delalloc_start) + 1) >>

			/*

			 * delalloc_end is already one less than the total

			 * length, so we don't subtract one from

			 * PAGE_CACHE_SIZE

			 */

			delalloc_to_write += (delalloc_end - delalloc_start +

					      PAGE_CACHE_SIZE) >>

					      PAGE_CACHE_SHIFT;

			if (wbc->nr_to_write < delalloc_to_write) {

				wbc->nr_to_write = min_t(long, 8192,

						 delalloc_to_write);

			}

			delalloc_start = delalloc_end + 1;

		}

		if (wbc->nr_to_write < delalloc_to_write) {

			int thresh = 8192;



			if (delalloc_to_write < thresh * 2)

				thresh = delalloc_to_write;

			wbc->nr_to_write = min_t(u64, delalloc_to_write,

						 thresh);

		}



		/* did the fill delalloc function already unlock and start

		 * the IO?
@@ -2388,6 +2396,7 @@ static int extent_write_cache_pages(struct extent_io_tree *tree, 
{

	int ret = 0;

	int done = 0;

	int nr_to_write_done = 0;

	struct pagevec pvec;

	int nr_pages;

	pgoff_t index;
@@ -2407,7 +2416,7 @@ static int extent_write_cache_pages(struct extent_io_tree *tree, 
		scanned = 1;

	}

retry:

	while (!done && (index <= end) &&

	while (!done && !nr_to_write_done && (index <= end) &&

	       (nr_pages = pagevec_lookup_tag(&pvec, mapping, &index,

			      PAGECACHE_TAG_DIRTY, min(end - index,

				  (pgoff_t)PAGEVEC_SIZE-1) + 1))) {
@@ -2458,8 +2467,15 @@ static int extent_write_cache_pages(struct extent_io_tree *tree, 
				unlock_page(page);

				ret = 0;

			}

			if (ret || wbc->nr_to_write <= 0)

			if (ret)

				done = 1;



			/*

			 * the filesystem may choose to bump up nr_to_write.

			 * We have to make sure to honor the new nr_to_write

			 * at any time

			 */

			nr_to_write_done = wbc->nr_to_write <= 0;

		}

		pagevec_release(&pvec);

		cond_resched();