linux-ext4 - [PATCH] ext4: Don't use CR_BEST_AVAIL

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Date:   Sun, 16 Jul 2023 19:33:34 +0530
From:   "Ritesh Harjani (IBM)" <ritesh.list@...il.com>
To:     linux-ext4@...r.kernel.org
Cc:     Ojaswin Mujoo <ojaswin@...ux.ibm.com>,
        "Ritesh Harjani (IBM)" <ritesh.list@...il.com>,
        Eric Whitney <enwlinux@...il.com>
Subject: [PATCH] ext4: Don't use CR_BEST_AVAIL_LEN for non-regular files

Using CR_BEST_AVAIL_LEN only make sense for regular files, as for
non-regular files we never normalize the allocation request length i.e.
goal len is same as original length (ac_g_ex.fe_len == ac_o_ex.fe_len).

Hence there is no scope of trimming the goal length to make it
satisfy original request len. Thus this patch avoids using
CR_BEST_AVAIL_LEN criteria for non-regular files request.

Fixes: 33122aa930f1 ("ext4: Add allocation criteria 1.5 (CR1_5)")
Reported-by: Eric Whitney <enwlinux@...il.com>
Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@...il.com>
---
 fs/ext4/mballoc.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index 3ab37533349f..bc004f5d3f3c 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -975,7 +975,18 @@ static void ext4_mb_choose_next_group_goal_fast(struct ext4_allocation_context *
 		*group = grp->bb_group;
 		ac->ac_flags |= EXT4_MB_CR_GOAL_LEN_FAST_OPTIMIZED;
 	} else {
-		*new_cr = CR_BEST_AVAIL_LEN;
+		/*
+		 * CR_BEST_AVAIL_LEN works based on the concept that we have
+		 * a larger normalized goal len request which can be trimmed to
+		 * a smaller goal len such that it can still satisfy original
+		 * request len. However, allocation request for non-regular
+		 * files never gets normalized.
+		 * See function ext4_mb_normalize_request() (EXT4_MB_HINT_DATA).
+		 */
+		if (ac->ac_flags & EXT4_MB_HINT_DATA)
+			*new_cr = CR_BEST_AVAIL_LEN;
+		else
+			*new_cr = CR_GOAL_LEN_SLOW;
 	}
 }

--
2.40.1