[PATCH v2] btrfs: do not wait for short bulk allocation

[Qu Wenruo <wqu@suse.com>]

[BUG]
There is a recent report that when memory pressure is high (including
cached pages), btrfs can spend most of its time on memory allocation in
btrfs_alloc_page_array() for compressed read/write.

[CAUSE]
For btrfs_alloc_page_array() we always go alloc_pages_bulk_array(), and
even if the bulk allocation failed (fell back to single page
allocation) we still retry but with extra memalloc_retry_wait().

If the bulk alloc only returned one page a time, we would spend a lot of
time on the retry wait.

The behavior was introduced in commit 395cb57e8560 ("btrfs: wait between
incomplete batch memory allocations").

[FIX]
Although the commit mentioned that other filesystems do the wait, it's
not the case at least nowadays.

All the mainlined filesystems only call memalloc_retry_wait() if they
failed to allocate any page (not only for bulk allocation).
If there is any progress, they won't call memalloc_retry_wait() at all.

For example, xfs_buf_alloc_pages() would only call memalloc_retry_wait()
if there is no allocation progress at all, and the call is not for
metadata readahead.

So I don't believe we should call memalloc_retry_wait() unconditionally
for short allocation.

This patch would only call memalloc_retry_wait() if failed to allocate
any page for tree block allocation (which goes with __GFP_NOFAIL and may
not need the special handling anyway), and reduce the latency for
btrfs_alloc_page_array().

Reported-by: Julian Taylor <julian.taylor@1und1.de>
Tested-by: Julian Taylor <julian.taylor@1und1.de>
Link: https://lore.kernel.org/all/8966c095-cbe7-4d22-9784-a647d1bf27c3@1und1.de/
Fixes: 395cb57e8560 ("btrfs: wait between incomplete batch memory allocations")
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
---
Changelog:
v2:
- Still use bulk allocation function
  Since alloc_pages_bulk_array() would fall back to single page
  allocation by itself, there is no need to go alloc_page() manually.

- Update the commit message to indicate other fses do not call
  memalloc_retry_wait() unconditionally
  In fact, they only call it when they need to retry hard and can not
  really fail.
---
 fs/btrfs/extent_io.c | 22 +++++++++-------------
 1 file changed, 9 insertions(+), 13 deletions(-)

diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index 7441245b1ceb..c96089b6f388 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -681,31 +681,27 @@ static void end_bbio_data_read(struct btrfs_bio *bbio)
 int btrfs_alloc_page_array(unsigned int nr_pages, struct page **page_array,
 			   gfp_t extra_gfp)
 {
+	const gfp_t gfp = GFP_NOFS | extra_gfp;
 	unsigned int allocated;
 
 	for (allocated = 0; allocated < nr_pages;) {
 		unsigned int last = allocated;
 
-		allocated = alloc_pages_bulk_array(GFP_NOFS | extra_gfp,
-						   nr_pages, page_array);
+		allocated = alloc_pages_bulk_array(gfp, nr_pages, page_array);
+		if (unlikely(allocated == last)) {
+			/* Can not fail, wait and retry. */
+			if (extra_gfp & __GFP_NOFAIL) {
+				memalloc_retry_wait(GFP_NOFS);
+				continue;
+			}
 
-		if (allocated == nr_pages)
-			return 0;
-
-		/*
-		 * During this iteration, no page could be allocated, even
-		 * though alloc_pages_bulk_array() falls back to alloc_page()
-		 * if  it could not bulk-allocate. So we must be out of memory.
-		 */
-		if (allocated == last) {
+			/* Allowed to fail, error out. */
 			for (int i = 0; i < allocated; i++) {
 				__free_page(page_array[i]);
 				page_array[i] = NULL;
 			}
 			return -ENOMEM;
 		}
-
-		memalloc_retry_wait(GFP_NOFS);
 	}
 	return 0;
 }
-- 
2.44.0