linux-ext4 - [PATCH v3 2/3] ext4: correct the judgment of BUG in ext4_mb_normalize

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Date:   Sat, 28 May 2022 19:00:16 +0800
From:   Baokun Li <libaokun1@...wei.com>
To:     <linux-ext4@...r.kernel.org>
CC:     <tytso@....edu>, <adilger.kernel@...ger.ca>, <jack@...e.cz>,
        <ritesh.list@...il.com>, <lczerner@...hat.com>,
        <linux-kernel@...r.kernel.org>, <yi.zhang@...wei.com>,
        <yebin10@...wei.com>, <yukuai3@...wei.com>, <libaokun1@...wei.com>
Subject: [PATCH v3 2/3] ext4: correct the judgment of BUG in ext4_mb_normalize_request

ext4_mb_normalize_request() can move logical start of allocated blocks
to reduce fragmentation and better utilize preallocation. However logical
block requested as a start of allocation (ac->ac_o_ex.fe_logical) should
always be covered by allocated blocks so we should check that by
modifying and to or in the assertion.

Signed-off-by: Baokun Li <libaokun1@...wei.com>
---
V1->V2:
	Change Fixes from dfe076c106f6 to c9de560ded61.
V2->V3:
	Delete Fixes tag.
	Add more comments and commit logs to make the code easier to understand.

 fs/ext4/mballoc.c | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index 4d3740fdff90..9e06334771a3 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -4185,7 +4185,22 @@ ext4_mb_normalize_request(struct ext4_allocation_context *ac,
 	}
 	rcu_read_unlock();
 
-	if (start + size <= ac->ac_o_ex.fe_logical &&
+	/*
+	 * In this function "start" and "size" are normalized for better
+	 * alignment and length such that we could preallocate more blocks.
+	 * This normalization is done such that original request of
+	 * ac->ac_o_ex.fe_logical & fe_len should always lie within "start" and
+	 * "size" boundaries.
+	 * (Note fe_len can be relaxed since FS block allocation API does not
+	 * provide gurantee on number of contiguous blocks allocation since that
+	 * depends upon free space left, etc).
+	 * In case of inode pa, later we use the allocated blocks
+	 * [pa_start + fe_logical - pa_lstart, fe_len/size] from the preallocated
+	 * range of goal/best blocks [start, size] to put it at the
+	 * ac_o_ex.fe_logical extent of this inode.
+	 * (See ext4_mb_use_inode_pa() for more details)
+	 */
+	if (start + size <= ac->ac_o_ex.fe_logical ||
 			start > ac->ac_o_ex.fe_logical) {
 		ext4_msg(ac->ac_sb, KERN_ERR,
 			 "start %lu, size %lu, fe_logical %lu",
-- 
2.31.1