From: Qu Wenruo wqu@suse.com

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@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") CC: stable@vger.kernel.org # 6.1+ Reviewed-by: Sweet Tea Dorminy sweettea-kernel@dorminy.me Reviewed-by: Filipe Manana fdmanana@suse.com Signed-off-by: Qu Wenruo wqu@suse.com Reviewed-by: David Sterba dsterba@suse.com Signed-off-by: David Sterba dsterba@suse.com --- 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; }