[PATCH v3 1/4] btrfs: refactor @nrptrs calculation of btrfs_buffered_write()

[Qu Wenruo <wqu@suse.com>]

@nrptrs of btrfs_bufferd_write() determines the up limit of pages we can
process in one batch.

Refactor that calculation into its own function, and fix a small page
alignment bug where the result can be one page less than ideal.

Despite that, no functionality change.

Signed-off-by: Qu Wenruo <wqu@suse.com>
---
 fs/btrfs/file.c | 24 ++++++++++++++++++++----
 1 file changed, 20 insertions(+), 4 deletions(-)

diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index 5a818ebcb01f..64f744989697 100644
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -1620,6 +1620,25 @@ void btrfs_check_nocow_unlock(struct btrfs_inode *inode)
 	btrfs_drew_write_unlock(&inode->root->snapshot_lock);
 }
 
+/* Helper to get how many pages we should alloc for the batch */
+static int calc_nr_pages(loff_t pos, struct iov_iter *iov)
+{
+	int nr_pages;
+
+	/*
+	 * Try to cover the full iov range, as btrfs metadata/data reserve
+	 * and release can be pretty slow, thus the more pages we process in
+	 * one batch the better.
+	 */
+	nr_pages = (round_up(pos + iov_iter_count(iov), PAGE_SIZE) -
+		    round_down(pos, PAGE_SIZE)) / PAGE_SIZE;
+
+	nr_pages = min(nr_pages, current->nr_dirtied_pause -
+				 current->nr_dirtied);
+	nr_pages = max(nr_pages, 8);
+	return nr_pages;
+}
+
 static noinline ssize_t btrfs_buffered_write(struct kiocb *iocb,
 					       struct iov_iter *i)
 {
@@ -1638,10 +1657,7 @@ static noinline ssize_t btrfs_buffered_write(struct kiocb *iocb,
 	bool only_release_metadata = false;
 	bool force_page_uptodate = false;
 
-	nrptrs = min(DIV_ROUND_UP(iov_iter_count(i), PAGE_SIZE),
-			PAGE_SIZE / (sizeof(struct page *)));
-	nrptrs = min(nrptrs, current->nr_dirtied_pause - current->nr_dirtied);
-	nrptrs = max(nrptrs, 8);
+	nrptrs = calc_nr_pages(pos, i);
 	pages = kmalloc_array(nrptrs, sizeof(struct page *), GFP_KERNEL);
 	if (!pages)
 		return -ENOMEM;
-- 
2.28.0