From: Qu Wenruo <wqu@xxxxxxxx>
commit 1db7959aacd905e6487d0478ac01d89f86eb1e51 upstream.
[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.
Call memalloc_retry_wait() if it fails 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@xxxxxxxx>
Tested-by: Julian Taylor <julian.taylor@xxxxxxxx>
Link: https://lore.kernel.org/all/8966c095-cbe7-4d22-9784-a647d1bf27c3@xxxxxxxx/
Fixes: 395cb57e8560 ("btrfs: wait between incomplete batch memory allocations")
CC: stable@xxxxxxxxxxxxxxx # 6.1+
Reviewed-by: Sweet Tea Dorminy <sweettea-kernel@xxxxxxxxxx>
Reviewed-by: Filipe Manana <fdmanana@xxxxxxxx>
Signed-off-by: Qu Wenruo <wqu@xxxxxxxx>
Reviewed-by: David Sterba <dsterba@xxxxxxxx>
Signed-off-by: David Sterba <dsterba@xxxxxxxx>
---
fs/btrfs/extent_io.c | 14 ++------------
1 file changed, 2 insertions(+), 12 deletions(-)
diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index 5acb2cb79d4b..9fbffd84b16c 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -686,24 +686,14 @@ int btrfs_alloc_page_array(unsigned int nr_pages, struct page **page_array)
unsigned int last = allocated;
allocated = alloc_pages_bulk_array(GFP_NOFS, nr_pages, page_array);
-
- 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) {
+ if (unlikely(allocated == last)) {
+ /* No progress, fail and do cleanup. */
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;
}
--
2.45.0
