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Date: Tue, 1 Dec 2020 11:49:35 +0800
From: Chunguang Xu <brookxu.cn@...il.com>
To: tytso@....edu, adilger.kernel@...ger.ca
Cc: bzzz@...mcloud.com, linux-ext4@...r.kernel.org
Subject: [PATCH] ext4: avoid s_mb_prefetch to be zero in individual scenarios
From: Chunguang Xu <brookxu@...cent.com>
patch cfd7323 introduces block bitmap prefetch, and expects to read
block bitmaps of flex_bg through an IO. However, it seems to ignore
the value range of s_log_groups_per_flex. In the scenario where the
value of s_log_groups_per_flex is greater than 27, s_mb_prefetch or
s_mb_prefetch_limit will overflow, cause a divide zero exception.
In addition, the logic of calculating nr maybe also flawed, because
the size of flexbg is fixed during a single mount, but s_mb_prefetch
can be modified, which causes nr to fail to meet the value condition
of [1, flexbg_size].
PID: 3873 TASK: ffff88800f11d880 CPU: 2 COMMAND: "executor"
#0 [ffff8880114a6ec0] __show_regs.cold.7 at ffffffff83cf29e2
#1 [ffff8880114a6f40] do_trap at ffffffff81065c61
#2 [ffff8880114a6f98] do_error_trap at ffffffff81065d65
#3 [ffff8880114a6fe0] exc_divide_error at ffffffff83dd2fd4
#4 [ffff8880114a7000] asm_exc_divide_error at ffffffff83e00872
[exception RIP: ext4_mb_regular_allocator+3885]
RIP: ffffffff8191258d RSP: ffff8880114a70b8 RFLAGS: 00010246
RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffffffff8191257a
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000005
RBP: 0000000000000200 R8: ffff88800f11d880 R9: ffffed1001e23b11
R10: ffff88800f11d887 R11: ffffed1001e23b10 R12: ffff888010147000
R13: 0000000000000000 R14: 0000000000000002 R15: dffffc0000000000
ORIG_RAX: ffffffffffffffff CS: 0010 SS: 0018
#5 [ffff8880114a7260] ext4_mb_new_blocks at ffffffff8191b6ba
#6 [ffff8880114a7420] ext4_new_meta_blocks at ffffffff81870d6f
#7 [ffff8880114a74e8] ext4_xattr_block_set at ffffffff819ced37
#8 [ffff8880114a7758] ext4_xattr_set_handle at ffffffff819d4776
#9 [ffff8880114a7928] ext4_xattr_set at ffffffff819d501b
RIP: 000000000045eb29 RSP: 00007ff74e97bc38 RFLAGS: 00000246
RAX: ffffffffffffffda RBX: 000000000055bf00 RCX: 000000000045eb29
RDX: 00000000200000c0 RSI: 0000000020000080 RDI: 0000000020000040
RBP: 00000000004b068e R8: 0000000000000001 R9: 0000000000000000
R10: 0000000000000002 R11: 0000000000000246 R12: 000000000055bf00
R13: 00007fff50fc111f R14: 00007ff74e97bdc0 R15: 0000000000022000
ORIG_RAX: 00000000000000bc CS: 0033 SS: 002b
The maximum size of a single IO will be limited by multiple factors,
such as max_hw_sectors, max_dev_sectors, BLK_DEF_MAX_SECTORS. The
max_hw_sectors, max_dev_sectors are determined by the device, and
BLK_DEF_MAX_SECTORS is a constant. In most scenarios, users will not
modify max_sectors. Therefore, we can safely assume that the maximum
size of a single IO is BLK_DEF_MAX_SECTORS. So far, we have determined
the number of blocks that a single IO can hold. Usually the file
system block is a multiple of the disk block, but we will ignore this
for now. According to the current value of BLK_DEF_MAX_SECTORS and
comprehensive considerations, the maximum number of bitmap blocks that
can be loaded by a single IO can be safely limited to 2^12. This maybe
a good choice to solve divide zero problem and avoiding performance
degradation.
Reported-by: Tosk Robot <tencent_os_robot@...cent.com>
Signed-off-by: Chunguang Xu <brookxu@...cent.com>
Reviewed-by: Samuel Liao <samuelliao@...cent.com>
---
fs/ext4/mballoc.c | 12 +++++++-----
1 file changed, 7 insertions(+), 5 deletions(-)
diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index 24af9ed..06af4ca 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -2395,9 +2395,10 @@ void ext4_mb_prefetch_fini(struct super_block *sb, ext4_group_t group,
nr = sbi->s_mb_prefetch;
if (ext4_has_feature_flex_bg(sb)) {
- nr = (group / sbi->s_mb_prefetch) *
- sbi->s_mb_prefetch;
- nr = nr + sbi->s_mb_prefetch - group;
+ nr = 1 << sbi->s_log_groups_per_flex;
+ if (group & (nr - 1))
+ nr -= group & (nr - 1);
+ nr = min(nr, sbi->s_mb_prefetch);
}
prefetch_grp = ext4_mb_prefetch(sb, group,
nr, &prefetch_ios);
@@ -2700,7 +2701,7 @@ static int ext4_mb_init_backend(struct super_block *sb)
ext4_group_t ngroups = ext4_get_groups_count(sb);
ext4_group_t i;
struct ext4_sb_info *sbi = EXT4_SB(sb);
- int err;
+ int err, log;
struct ext4_group_desc *desc;
struct ext4_group_info ***group_info;
struct kmem_cache *cachep;
@@ -2733,7 +2734,8 @@ static int ext4_mb_init_backend(struct super_block *sb)
if (ext4_has_feature_flex_bg(sb)) {
/* a single flex group is supposed to be read by a single IO */
- sbi->s_mb_prefetch = 1 << sbi->s_es->s_log_groups_per_flex;
+ log = min_t(unsigned char, 12, sbi->s_es->s_log_groups_per_flex);
+ sbi->s_mb_prefetch = 1 << log;
sbi->s_mb_prefetch *= 8; /* 8 prefetch IOs in flight at most */
} else {
sbi->s_mb_prefetch = 32;
--
1.8.3.1
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