On Mon, Sep 17, 2018 at 12:28:54PM -0500, Eric Sandeen wrote: > > find_group_orlov() randomly selects block groups for root directories, > but AFAIK there is no explicit random behavior in the block allocator. > > It's more of a chaotic system than intentional random selection. :) There are places where we call prandom32(), and the dir_index hashing is influenced by randomly set s_hash_seed in the superblock. The checksum is also influenced by either the randomly set file system UUID or s_checksum_seed (if set). I had fooled around with a patch allow a benchmarker to initialize a fixed random state to try to stablize results from a benchmarking perspective. (See attached). However, I abandoned it when we discovered nearly all of the variability we saw in our benchmarks when we were working on the SMR-friendly ext4 paper[1] were caused by the fact that the writeback cache ratio was wandering all over the place, especially with drive-managed SMR disks, since the speed of these disks varied a huge amount depending on how much garbage collection was needed to clear out space in the HDD's media cache. So we fixed the problem via: echo 50 > /sys/class/bdi/<dev>/min_ratio echo 50 > /sys/class/bdi/<dev>/max_ratio and I didn't bother with the patch, since the prandom_u32() calls contributed almost nothing to the chaotic variability of the benchmark results. (This is something the Linux kernel would eventually need to address in some automatic way if Drive-Managed SMR were to become more than afterthought in the marketplace, but given that Drive-Managed SMR hasn't seen much market uptake, it's not been an issue I've thought deserved effort addressing in a more general/user-friendly way.) [1] https://www.usenix.org/system/files/conference/fast17/fast17-aghayev.pdf - Ted ext4: add ability to control the pseudo-random seed used by ext4 Ext4 uses a pseudo-random generator in a few places: to spread out directories when htree is not enabled; to randomize the wait times for MMP backoff and lazy inode table initialization. For benchmarking purposes, it's useful to control the psueorandom number seed, expose this via /sys/fs/ext4/<dev>/prandom_seed. Signed-off-by: Theodore Ts'o <tytso@xxxxxxx> --- fs/ext4/ext4.h | 4 ++++ fs/ext4/ialloc.c | 6 ++++-- fs/ext4/mmp.c | 6 +++--- fs/ext4/super.c | 3 ++- fs/ext4/sysfs.c | 20 ++++++++++++++++++++ include/linux/random.h | 1 + lib/random32.c | 10 +++++----- 7 files changed, 39 insertions(+), 11 deletions(-) diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h index ea31931..3dbb03a 100644 --- a/fs/ext4/ext4.h +++ b/fs/ext4/ext4.h @@ -35,6 +35,7 @@ #include <linux/fscrypto.h> #include <linux/falloc.h> #include <linux/percpu-rwsem.h> +#include <linux/random.h> #ifdef __KERNEL__ #include <linux/compat.h> #endif @@ -1491,6 +1492,9 @@ struct ext4_sb_info { /* Precomputed FS UUID checksum for seeding other checksums */ __u32 s_csum_seed; + /* RND state for the file system */ + struct rnd_state s_rnd_state; + /* Reclaim extents from extent status tree */ struct shrinker s_es_shrinker; struct list_head s_es_list; /* List of inodes with reclaimable extents */ diff --git a/fs/ext4/ialloc.c b/fs/ext4/ialloc.c index 35f3518..09a1458 100644 --- a/fs/ext4/ialloc.c +++ b/fs/ext4/ialloc.c @@ -485,8 +485,10 @@ static int find_group_orlov(struct super_block *sb, struct inode *parent, hinfo.seed = sbi->s_hash_seed; ext4fs_dirhash(qstr->name, qstr->len, &hinfo); grp = hinfo.hash; - } else - grp = prandom_u32(); + } else { + grp = prandom_u32_state(&sbi->s_rnd_state); + pr_err("ext4 random: %lu\n", grp); + } parent_group = (unsigned)grp % ngroups; for (i = 0; i < ngroups; i++) { g = (parent_group + i) % ngroups; diff --git a/fs/ext4/mmp.c b/fs/ext4/mmp.c index 23d436d..99f69dc 100644 --- a/fs/ext4/mmp.c +++ b/fs/ext4/mmp.c @@ -258,12 +258,12 @@ exit_thread: * Get a random new sequence number but make sure it is not greater than * EXT4_MMP_SEQ_MAX. */ -static unsigned int mmp_new_seq(void) +static unsigned int mmp_new_seq(struct ext4_sb_info *sbi) { u32 new_seq; do { - new_seq = prandom_u32(); + new_seq = prandom_u32_state(&sbi->s_rnd_state); } while (new_seq > EXT4_MMP_SEQ_MAX); return new_seq; @@ -342,7 +342,7 @@ skip: /* * write a new random sequence number. */ - seq = mmp_new_seq(); + seq = mmp_new_seq(EXT4_SB(sb)); mmp->mmp_seq = cpu_to_le32(seq); retval = write_mmp_block(sb, bh); diff --git a/fs/ext4/super.c b/fs/ext4/super.c index c13a4e4..2c86b98 100644 --- a/fs/ext4/super.c +++ b/fs/ext4/super.c @@ -2969,7 +2969,7 @@ static struct ext4_li_request *ext4_li_request_new(struct super_block *sb, * spread the inode table initialization requests * better. */ - elr->lr_next_sched = jiffies + (prandom_u32() % + elr->lr_next_sched = jiffies + (prandom_u32_state(&sbi->s_rnd_state) % (EXT4_DEF_LI_MAX_START_DELAY * HZ)); return elr; } @@ -3274,6 +3274,7 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent) if (sb->s_bdev->bd_part) sbi->s_sectors_written_start = part_stat_read(sb->s_bdev->bd_part, sectors[1]); + _prandom_seed(&sbi->s_rnd_state, 0, true); /* Cleanup superblock name */ strreplace(sb->s_id, '/', '!'); diff --git a/fs/ext4/sysfs.c b/fs/ext4/sysfs.c index 1420a3c..f74d34b 100644 --- a/fs/ext4/sysfs.c +++ b/fs/ext4/sysfs.c @@ -26,6 +26,7 @@ typedef enum { attr_feature, attr_pointer_ui, attr_pointer_atomic, + attr_prandom_seed, } attr_id_t; typedef enum { @@ -90,6 +91,21 @@ static ssize_t inode_readahead_blks_store(struct ext4_attr *a, return count; } +static ssize_t prandom_seed_store(struct ext4_attr *a, + struct ext4_sb_info *sbi, + const char *buf, size_t count) +{ + unsigned long t; + int ret; + + ret = kstrtoul(skip_spaces(buf), 0, &t); + if (ret) + return ret; + + _prandom_seed(&sbi->s_rnd_state, t, false); + return count; +} + static ssize_t reserved_clusters_store(struct ext4_attr *a, struct ext4_sb_info *sbi, const char *buf, size_t count) @@ -178,6 +194,7 @@ EXT4_RW_ATTR_SBI_UI(mb_stream_req, s_mb_stream_request); EXT4_RW_ATTR_SBI_UI(mb_group_prealloc, s_mb_group_prealloc); EXT4_RW_ATTR_SBI_UI(extent_max_zeroout_kb, s_extent_max_zeroout_kb); EXT4_ATTR(trigger_fs_error, 0200, trigger_test_error); +EXT4_ATTR(prandom_seed, 0200, prandom_seed); EXT4_RW_ATTR_SBI_UI(err_ratelimit_interval_ms, s_err_ratelimit_state.interval); EXT4_RW_ATTR_SBI_UI(err_ratelimit_burst, s_err_ratelimit_state.burst); EXT4_RW_ATTR_SBI_UI(warning_ratelimit_interval_ms, s_warning_ratelimit_state.interval); @@ -216,6 +233,7 @@ static struct attribute *ext4_attrs[] = { ATTR_LIST(errors_count), ATTR_LIST(first_error_time), ATTR_LIST(last_error_time), + ATTR_LIST(prandom_seed), NULL, }; @@ -313,6 +331,8 @@ static ssize_t ext4_attr_store(struct kobject *kobj, return inode_readahead_blks_store(a, sbi, buf, len); case attr_trigger_test_error: return trigger_test_error(a, sbi, buf, len); + case attr_prandom_seed: + return prandom_seed_store(a, sbi, buf, len); } return 0; } diff --git a/include/linux/random.h b/include/linux/random.h index e47e533..64b70a8 100644 --- a/include/linux/random.h +++ b/include/linux/random.h @@ -45,6 +45,7 @@ struct rnd_state { __u32 s1, s2, s3, s4; }; +void _prandom_seed(struct rnd_state *state, u32 seed, bool mix_with_hwseed); u32 prandom_u32_state(struct rnd_state *state); void prandom_bytes_state(struct rnd_state *state, void *buf, size_t nbytes); void prandom_seed_full_state(struct rnd_state __percpu *pcpu_state); diff --git a/lib/random32.c b/lib/random32.c index 510d1ce..5134111 100644 --- a/lib/random32.c +++ b/lib/random32.c @@ -157,8 +157,7 @@ static u32 __extract_hwseed(void) return val; } -static void prandom_seed_early(struct rnd_state *state, u32 seed, - bool mix_with_hwseed) +void _prandom_seed(struct rnd_state *state, u32 seed, bool mix_with_hwseed) { #define LCG(x) ((x) * 69069U) /* super-duper LCG */ #define HWSEED() (mix_with_hwseed ? __extract_hwseed() : 0) @@ -167,6 +166,7 @@ static void prandom_seed_early(struct rnd_state *state, u32 seed, state->s3 = __seed(HWSEED() ^ LCG(state->s2), 16U); state->s4 = __seed(HWSEED() ^ LCG(state->s3), 128U); } +EXPORT_SYMBOL(_prandom_seed); /** * prandom_seed - add entropy to pseudo random number generator @@ -204,7 +204,7 @@ static int __init prandom_init(void) struct rnd_state *state = &per_cpu(net_rand_state, i); u32 weak_seed = (i + jiffies) ^ random_get_entropy(); - prandom_seed_early(state, weak_seed, true); + _prandom_seed(state, weak_seed, true); prandom_warmup(state); } @@ -429,7 +429,7 @@ static void __init prandom_state_selftest(void) for (i = 0; i < ARRAY_SIZE(test1); i++) { struct rnd_state state; - prandom_seed_early(&state, test1[i].seed, false); + _prandom_seed(&state, test1[i].seed, false); prandom_warmup(&state); if (test1[i].result != prandom_u32_state(&state)) @@ -444,7 +444,7 @@ static void __init prandom_state_selftest(void) for (i = 0; i < ARRAY_SIZE(test2); i++) { struct rnd_state state; - prandom_seed_early(&state, test2[i].seed, false); + _prandom_seed(&state, test2[i].seed, false); prandom_warmup(&state); for (j = 0; j < test2[i].iteration - 1; j++)
