Re: [PATCH v5 5/6] ext4: improve cr 0 / cr 1 group scanning

[Andreas Dilger <adilger@dilger.ca>]

On Mar 30, 2021, at 09:20, harshad shirwadkar <harshadshirwadkar@gmail.com> wrote:
> 
> I ran some experiments this morning and here are the results:
> 
> 1) Config 1:
>     - add change to skip CR0 / CR1 optimizations and fall back to
> linear search when all groups are not loaded
>     - prefetch block bitmaps mount option turned off
>     - first_write_latency: 6m 56s
>     - number_of_groups_loaded_at_the_end_of_first_write: 30726 out of
> 491520 total
> 
> 2) Config 2:
>     - add change to skip CR0 / CR1 optimizations and fall back to
> linear search when all groups are not loaded
>     - prefetch block bitmaps mount option turned on
>     - first_write_latency:4m42s
>     - number_of_groups_loaded_at_the_end_of_first_write: 490497 out
> of 491520 total
> 
> 3) Config 3:
>    - don't fallback to linear search
>    - prefetch block bitmaps turned off
>    - first_write_latency: 0.13s
>    - number_of_groups_loaded_at_the_end_of_first_write: 7 out of 491520 total
> 
> 4) Config 4:
>    - don't fallback to linear search
>    - prefetch block bitmaps turned on
>    - first_write_latency: 0.15s
>    - number_of_groups_loaded_at_the_end_of_first_write: 48384 out of
> 491520 total
> 
> Based on the above numbers, I think we should go for config 4, which
> means we should have prefetch_block_bitmaps turned on when
> mb_optimize_scan is on. What do you think about turning
> prefetch_block_bitmaps mount option on by default?

I think that makes sense. 

> 
> - Harshad
> 
>> On Mon, Mar 29, 2021 at 9:10 PM Andreas Dilger <adilger@dilger.ca> wrote:
>> 
>>> On Mar 29, 2021, at 17:03, harshad shirwadkar <harshadshirwadkar@gmail.com> wrote:
>>> 
>>> Another problem that I found in the patch was that when all groups are
>>> not loaded, the optimization concludes that CR0 and CR1 cannot be used
>>> for allocation (since these structures aren't populated). The problem
>>> goes away if we also mount with prefetch_block_bitmaps mount option.
>>> But I think if all the groups are not loaded, we should at least fall
>>> back to linear search at CR1. Maybe skipping CR0 is okay.
>> 
>> Alex's patch to skip cr0/1 if no groups are loaded is OK, as long as there is
>> some group prefetch is triggered in the background, IMHO, and is there
>> for later allocations.
>> 
>> That avoids lots of scanning at mount time if the filesystem is mostly full,
>> and the first allocation is not critical as long as this triggers group reads.
>> 
>> Do you have any idea what the performance impact of this is (eg.
>> latency to first write)?  Ideally there would be some group prefetch done
>> right at mount time (itable zero thread?) so that if writes are not
>> done *right* after mount there would not be any noticeable latency.
>> 
>> Cheers, Andreas
>> 
>>>> On Mon, Mar 29, 2021 at 12:32 PM harshad shirwadkar
>>>> <harshadshirwadkar@gmail.com> wrote:
>>>> 
>>>> Okay sounds good. I'll do that!
>>>> 
>>>>> On Mon, Mar 29, 2021 at 12:14 PM Andreas Dilger <adilger@dilger.ca> wrote:
>>>>> 
>>>>> On Mar 24, 2021, at 5:19 PM, Harshad Shirwadkar <harshadshirwadkar@gmail.com> wrote:
>>>>>> 
>>>>>> Instead of traversing through groups linearly, scan groups in specific
>>>>>> orders at cr 0 and cr 1. At cr 0, we want to find groups that have the
>>>>>> largest free order >= the order of the request. So, with this patch,
>>>>>> we maintain lists for each possible order and insert each group into a
>>>>>> list based on the largest free order in its buddy bitmap. During cr 0
>>>>>> allocation, we traverse these lists in the increasing order of largest
>>>>>> free orders. This allows us to find a group with the best available cr
>>>>>> 0 match in constant time. If nothing can be found, we fallback to cr 1
>>>>>> immediately.
>>>>>> 
>>>>>> At CR1, the story is slightly different. We want to traverse in the
>>>>>> order of increasing average fragment size. For CR1, we maintain a rb
>>>>>> tree of groupinfos which is sorted by average fragment size. Instead
>>>>>> of traversing linearly, at CR1, we traverse in the order of increasing
>>>>>> average fragment size, starting at the most optimal group. This brings
>>>>>> down cr 1 search complexity to log(num groups).
>>>>>> 
>>>>>> For cr >= 2, we just perform the linear search as before. Also, in
>>>>>> case of lock contention, we intermittently fallback to linear search
>>>>>> even in CR 0 and CR 1 cases. This allows us to proceed during the
>>>>>> allocation path even in case of high contention.
>>>>> 
>>>>> If you are refreshing this patch anyway, could you rename "mb_linear_limit"
>>>>> to "mb_linear_groups" or "mb_max_linear_groups" or similar?  It otherwise
>>>>> isn't clear what units the "limit" is in (could be MB/GB or something else).
>>>>> 
>>>>> Cheers, Andreas
>>>>> 
>>>>>> There is an opportunity to do optimization at CR2 too. That's because
>>>>>> at CR2 we only consider groups where bb_free counter (number of free
>>>>>> blocks) is greater than the request extent size. That's left as future
>>>>>> work.
>>>>>> 
>>>>>> All the changes introduced in this patch are protected under a new
>>>>>> mount option "mb_optimize_scan".
>>>>>> 
>>>>>> With this patchset, following experiment was performed:
>>>>>> 
>>>>>> Created a highly fragmented disk of size 65TB. The disk had no
>>>>>> contiguous 2M regions. Following command was run consecutively for 3
>>>>>> times:
>>>>>> 
>>>>>> time dd if=/dev/urandom of=file bs=2M count=10
>>>>>> 
>>>>>> Here are the results with and without cr 0/1 optimizations introduced
>>>>>> in this patch:
>>>>>> 
>>>>>> |---------+------------------------------+---------------------------|
>>>>>> |         | Without CR 0/1 Optimizations | With CR 0/1 Optimizations |
>>>>>> |---------+------------------------------+---------------------------|
>>>>>> | 1st run | 5m1.871s                     | 2m47.642s                 |
>>>>>> | 2nd run | 2m28.390s                    | 0m0.611s                  |
>>>>>> | 3rd run | 2m26.530s                    | 0m1.255s                  |
>>>>>> |---------+------------------------------+---------------------------|
>>>>>> 
>>>>>> Signed-off-by: Harshad Shirwadkar <harshadshirwadkar@gmail.com>
>>>>>> Reported-by: kernel test robot <lkp@intel.com>
>>>>>> Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
>>>>>> Reviewed-by: Andreas Dilger <adilger@dilger.ca>
>>>>>> ---
>>>>>> fs/ext4/ext4.h    |  19 ++-
>>>>>> fs/ext4/mballoc.c | 381 ++++++++++++++++++++++++++++++++++++++++++++--
>>>>>> fs/ext4/mballoc.h |  17 ++-
>>>>>> fs/ext4/super.c   |  28 +++-
>>>>>> fs/ext4/sysfs.c   |   2 +
>>>>>> 5 files changed, 432 insertions(+), 15 deletions(-)
>>>>>> 
>>>>>> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
>>>>>> index 85eeeba3bca3..5930c8cb5159 100644
>>>>>> --- a/fs/ext4/ext4.h
>>>>>> +++ b/fs/ext4/ext4.h
>>>>>> @@ -162,7 +162,10 @@ enum SHIFT_DIRECTION {
>>>>>> #define EXT4_MB_USE_RESERVED          0x2000
>>>>>> /* Do strict check for free blocks while retrying block allocation */
>>>>>> #define EXT4_MB_STRICT_CHECK          0x4000
>>>>>> -
>>>>>> +/* Large fragment size list lookup succeeded at least once for cr = 0 */
>>>>>> +#define EXT4_MB_CR0_OPTIMIZED                0x8000
>>>>>> +/* Avg fragment size rb tree lookup succeeded at least once for cr = 1 */
>>>>>> +#define EXT4_MB_CR1_OPTIMIZED                0x00010000
>>>>>> struct ext4_allocation_request {
>>>>>>     /* target inode for block we're allocating */
>>>>>>     struct inode *inode;
>>>>>> @@ -1247,7 +1250,9 @@ struct ext4_inode_info {
>>>>>> #define EXT4_MOUNT2_JOURNAL_FAST_COMMIT       0x00000010 /* Journal fast commit */
>>>>>> #define EXT4_MOUNT2_DAX_NEVER         0x00000020 /* Do not allow Direct Access */
>>>>>> #define EXT4_MOUNT2_DAX_INODE         0x00000040 /* For printing options only */
>>>>>> -
>>>>>> +#define EXT4_MOUNT2_MB_OPTIMIZE_SCAN 0x00000080 /* Optimize group
>>>>>> +                                                 * scanning in mballoc
>>>>>> +                                                 */
>>>>>> 
>>>>>> #define clear_opt(sb, opt)            EXT4_SB(sb)->s_mount_opt &= \
>>>>>>                                             ~EXT4_MOUNT_##opt
>>>>>> @@ -1527,9 +1532,14 @@ struct ext4_sb_info {
>>>>>>     unsigned int s_mb_free_pending;
>>>>>>     struct list_head s_freed_data_list;     /* List of blocks to be freed
>>>>>>                                                after commit completed */
>>>>>> +     struct rb_root s_mb_avg_fragment_size_root;
>>>>>> +     rwlock_t s_mb_rb_lock;
>>>>>> +     struct list_head *s_mb_largest_free_orders;
>>>>>> +     rwlock_t *s_mb_largest_free_orders_locks;
>>>>>> 
>>>>>>     /* tunables */
>>>>>>     unsigned long s_stripe;
>>>>>> +     unsigned int s_mb_linear_limit;
>>>>>>     unsigned int s_mb_stream_request;
>>>>>>     unsigned int s_mb_max_to_scan;
>>>>>>     unsigned int s_mb_min_to_scan;
>>>>>> @@ -1553,6 +1563,8 @@ struct ext4_sb_info {
>>>>>>     atomic_t s_bal_goals;   /* goal hits */
>>>>>>     atomic_t s_bal_breaks;  /* too long searches */
>>>>>>     atomic_t s_bal_2orders; /* 2^order hits */
>>>>>> +     atomic_t s_bal_cr0_bad_suggestions;
>>>>>> +     atomic_t s_bal_cr1_bad_suggestions;
>>>>>>     atomic64_t s_bal_cX_groups_considered[4];
>>>>>>     atomic64_t s_bal_cX_hits[4];
>>>>>>     atomic64_t s_bal_cX_failed[4];          /* cX loop didn't find blocks */
>>>>>> @@ -3309,11 +3321,14 @@ struct ext4_group_info {
>>>>>>     ext4_grpblk_t   bb_free;        /* total free blocks */
>>>>>>     ext4_grpblk_t   bb_fragments;   /* nr of freespace fragments */
>>>>>>     ext4_grpblk_t   bb_largest_free_order;/* order of largest frag in BG */
>>>>>> +     ext4_group_t    bb_group;       /* Group number */
>>>>>>     struct          list_head bb_prealloc_list;
>>>>>> #ifdef DOUBLE_CHECK
>>>>>>     void            *bb_bitmap;
>>>>>> #endif
>>>>>>     struct rw_semaphore alloc_sem;
>>>>>> +     struct rb_node  bb_avg_fragment_size_rb;
>>>>>> +     struct list_head bb_largest_free_order_node;
>>>>>>     ext4_grpblk_t   bb_counters[];  /* Nr of free power-of-two-block
>>>>>>                                      * regions, index is order.
>>>>>>                                      * bb_counters[3] = 5 means
>>>>>> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
>>>>>> index 15127d815461..cbf9a89c0ef5 100644
>>>>>> --- a/fs/ext4/mballoc.c
>>>>>> +++ b/fs/ext4/mballoc.c
>>>>>> @@ -127,11 +127,50 @@
>>>>>> * smallest multiple of the stripe value (sbi->s_stripe) which is
>>>>>> * greater than the default mb_group_prealloc.
>>>>>> *
>>>>>> + * If "mb_optimize_scan" mount option is set, we maintain in memory group info
>>>>>> + * structures in two data structures:
>>>>>> + *
>>>>>> + * 1) Array of largest free order lists (sbi->s_mb_largest_free_orders)
>>>>>> + *
>>>>>> + *    Locking: sbi->s_mb_largest_free_orders_locks(array of rw locks)
>>>>>> + *
>>>>>> + *    This is an array of lists where the index in the array represents the
>>>>>> + *    largest free order in the buddy bitmap of the participating group infos of
>>>>>> + *    that list. So, there are exactly MB_NUM_ORDERS(sb) (which means total
>>>>>> + *    number of buddy bitmap orders possible) number of lists. Group-infos are
>>>>>> + *    placed in appropriate lists.
>>>>>> + *
>>>>>> + * 2) Average fragment size rb tree (sbi->s_mb_avg_fragment_size_root)
>>>>>> + *
>>>>>> + *    Locking: sbi->s_mb_rb_lock (rwlock)
>>>>>> + *
>>>>>> + *    This is a red black tree consisting of group infos and the tree is sorted
>>>>>> + *    by average fragment sizes (which is calculated as ext4_group_info->bb_free
>>>>>> + *    / ext4_group_info->bb_fragments).
>>>>>> + *
>>>>>> + * When "mb_optimize_scan" mount option is set, mballoc consults the above data
>>>>>> + * structures to decide the order in which groups are to be traversed for
>>>>>> + * fulfilling an allocation request.
>>>>>> + *
>>>>>> + * At CR = 0, we look for groups which have the largest_free_order >= the order
>>>>>> + * of the request. We directly look at the largest free order list in the data
>>>>>> + * structure (1) above where largest_free_order = order of the request. If that
>>>>>> + * list is empty, we look at remaining list in the increasing order of
>>>>>> + * largest_free_order. This allows us to perform CR = 0 lookup in O(1) time.
>>>>>> + *
>>>>>> + * At CR = 1, we only consider groups where average fragment size > request
>>>>>> + * size. So, we lookup a group which has average fragment size just above or
>>>>>> + * equal to request size using our rb tree (data structure 2) in O(log N) time.
>>>>>> + *
>>>>>> + * If "mb_optimize_scan" mount option is not set, mballoc traverses groups in
>>>>>> + * linear order which requires O(N) search time for each CR 0 and CR 1 phase.
>>>>>> + *
>>>>>> * The regular allocator (using the buddy cache) supports a few tunables.
>>>>>> *
>>>>>> * /sys/fs/ext4/<partition>/mb_min_to_scan
>>>>>> * /sys/fs/ext4/<partition>/mb_max_to_scan
>>>>>> * /sys/fs/ext4/<partition>/mb_order2_req
>>>>>> + * /sys/fs/ext4/<partition>/mb_linear_limit
>>>>>> *
>>>>>> * The regular allocator uses buddy scan only if the request len is power of
>>>>>> * 2 blocks and the order of allocation is >= sbi->s_mb_order2_reqs. The
>>>>>> @@ -149,6 +188,16 @@
>>>>>> * can be used for allocation. ext4_mb_good_group explains how the groups are
>>>>>> * checked.
>>>>>> *
>>>>>> + * When "mb_optimize_scan" is turned on, as mentioned above, the groups may not
>>>>>> + * get traversed linearly. That may result in subsequent allocations being not
>>>>>> + * close to each other. And so, the underlying device may get filled up in a
>>>>>> + * non-linear fashion. While that may not matter on non-rotational devices, for
>>>>>> + * rotational devices that may result in higher seek times. "mb_linear_limit"
>>>>>> + * tells mballoc how many groups mballoc should search linearly before
>>>>>> + * performing consulting above data structures for more efficient lookups. For
>>>>>> + * non rotational devices, this value defaults to 0 and for rotational devices
>>>>>> + * this is set to MB_DEFAULT_LINEAR_LIMIT.
>>>>>> + *
>>>>>> * Both the prealloc space are getting populated as above. So for the first
>>>>>> * request we will hit the buddy cache which will result in this prealloc
>>>>>> * space getting filled. The prealloc space is then later used for the
>>>>>> @@ -299,6 +348,8 @@
>>>>>> *  - bitlock on a group      (group)
>>>>>> *  - object (inode/locality) (object)
>>>>>> *  - per-pa lock             (pa)
>>>>>> + *  - cr0 lists lock         (cr0)
>>>>>> + *  - cr1 tree lock          (cr1)
>>>>>> *
>>>>>> * Paths:
>>>>>> *  - new pa
>>>>>> @@ -328,6 +379,9 @@
>>>>>> *    group
>>>>>> *        object
>>>>>> *
>>>>>> + *  - allocation path (ext4_mb_regular_allocator)
>>>>>> + *    group
>>>>>> + *    cr0/cr1
>>>>>> */
>>>>>> static struct kmem_cache *ext4_pspace_cachep;
>>>>>> static struct kmem_cache *ext4_ac_cachep;
>>>>>> @@ -351,6 +405,9 @@ static void ext4_mb_generate_from_freelist(struct super_block *sb, void *bitmap,
>>>>>>                                             ext4_group_t group);
>>>>>> static void ext4_mb_new_preallocation(struct ext4_allocation_context *ac);
>>>>>> 
>>>>>> +static bool ext4_mb_good_group(struct ext4_allocation_context *ac,
>>>>>> +                            ext4_group_t group, int cr);
>>>>>> +
>>>>>> /*
>>>>>> * The algorithm using this percpu seq counter goes below:
>>>>>> * 1. We sample the percpu discard_pa_seq counter before trying for block
>>>>>> @@ -744,6 +801,251 @@ static void ext4_mb_mark_free_simple(struct super_block *sb,
>>>>>>     }
>>>>>> }
>>>>>> 
>>>>>> +static void ext4_mb_rb_insert(struct rb_root *root, struct rb_node *new,
>>>>>> +                     int (*cmp)(struct rb_node *, struct rb_node *))
>>>>>> +{
>>>>>> +     struct rb_node **iter = &root->rb_node, *parent = NULL;
>>>>>> +
>>>>>> +     while (*iter) {
>>>>>> +             parent = *iter;
>>>>>> +             if (cmp(new, *iter) > 0)
>>>>>> +                     iter = &((*iter)->rb_left);
>>>>>> +             else
>>>>>> +                     iter = &((*iter)->rb_right);
>>>>>> +     }
>>>>>> +
>>>>>> +     rb_link_node(new, parent, iter);
>>>>>> +     rb_insert_color(new, root);
>>>>>> +}
>>>>>> +
>>>>>> +static int
>>>>>> +ext4_mb_avg_fragment_size_cmp(struct rb_node *rb1, struct rb_node *rb2)
>>>>>> +{
>>>>>> +     struct ext4_group_info *grp1 = rb_entry(rb1,
>>>>>> +                                             struct ext4_group_info,
>>>>>> +                                             bb_avg_fragment_size_rb);
>>>>>> +     struct ext4_group_info *grp2 = rb_entry(rb2,
>>>>>> +                                             struct ext4_group_info,
>>>>>> +                                             bb_avg_fragment_size_rb);
>>>>>> +     int num_frags_1, num_frags_2;
>>>>>> +
>>>>>> +     num_frags_1 = grp1->bb_fragments ?
>>>>>> +             grp1->bb_free / grp1->bb_fragments : 0;
>>>>>> +     num_frags_2 = grp2->bb_fragments ?
>>>>>> +             grp2->bb_free / grp2->bb_fragments : 0;
>>>>>> +
>>>>>> +     return (num_frags_2 - num_frags_1);
>>>>>> +}
>>>>>> +
>>>>>> +/*
>>>>>> + * Reinsert grpinfo into the avg_fragment_size tree with new average
>>>>>> + * fragment size.
>>>>>> + */
>>>>>> +static void
>>>>>> +mb_update_avg_fragment_size(struct super_block *sb, struct ext4_group_info *grp)
>>>>>> +{
>>>>>> +     struct ext4_sb_info *sbi = EXT4_SB(sb);
>>>>>> +
>>>>>> +     if (!test_opt2(sb, MB_OPTIMIZE_SCAN) || grp->bb_free == 0)
>>>>>> +             return;
>>>>>> +
>>>>>> +     write_lock(&sbi->s_mb_rb_lock);
>>>>>> +     if (!RB_EMPTY_NODE(&grp->bb_avg_fragment_size_rb)) {
>>>>>> +             rb_erase(&grp->bb_avg_fragment_size_rb,
>>>>>> +                             &sbi->s_mb_avg_fragment_size_root);
>>>>>> +             RB_CLEAR_NODE(&grp->bb_avg_fragment_size_rb);
>>>>>> +     }
>>>>>> +
>>>>>> +     ext4_mb_rb_insert(&sbi->s_mb_avg_fragment_size_root,
>>>>>> +             &grp->bb_avg_fragment_size_rb,
>>>>>> +             ext4_mb_avg_fragment_size_cmp);
>>>>>> +     write_unlock(&sbi->s_mb_rb_lock);
>>>>>> +}
>>>>>> +
>>>>>> +/*
>>>>>> + * Choose next group by traversing largest_free_order lists. Return 0 if next
>>>>>> + * group was selected optimally. Return 1 if next group was not selected
>>>>>> + * optimally. Updates *new_cr if cr level needs an update.
>>>>>> + */
>>>>>> +static int ext4_mb_choose_next_group_cr0(struct ext4_allocation_context *ac,
>>>>>> +             int *new_cr, ext4_group_t *group, ext4_group_t ngroups)
>>>>>> +{
>>>>>> +     struct ext4_sb_info *sbi = EXT4_SB(ac->ac_sb);
>>>>>> +     struct ext4_group_info *iter, *grp;
>>>>>> +     int i;
>>>>>> +
>>>>>> +     if (ac->ac_status == AC_STATUS_FOUND)
>>>>>> +             return 1;
>>>>>> +
>>>>>> +     if (unlikely(sbi->s_mb_stats && ac->ac_flags & EXT4_MB_CR0_OPTIMIZED))
>>>>>> +             atomic_inc(&sbi->s_bal_cr0_bad_suggestions);
>>>>>> +
>>>>>> +     grp = NULL;
>>>>>> +     for (i = ac->ac_2order; i < MB_NUM_ORDERS(ac->ac_sb); i++) {
>>>>>> +             if (list_empty(&sbi->s_mb_largest_free_orders[i]))
>>>>>> +                     continue;
>>>>>> +             read_lock(&sbi->s_mb_largest_free_orders_locks[i]);
>>>>>> +             if (list_empty(&sbi->s_mb_largest_free_orders[i])) {
>>>>>> +                     read_unlock(&sbi->s_mb_largest_free_orders_locks[i]);
>>>>>> +                     continue;
>>>>>> +             }
>>>>>> +             grp = NULL;
>>>>>> +             list_for_each_entry(iter, &sbi->s_mb_largest_free_orders[i],
>>>>>> +                                 bb_largest_free_order_node) {
>>>>>> +                     if (sbi->s_mb_stats)
>>>>>> +                             atomic64_inc(&sbi->s_bal_cX_groups_considered[0]);
>>>>>> +                     if (likely(ext4_mb_good_group(ac, iter->bb_group, 0))) {
>>>>>> +                             grp = iter;
>>>>>> +                             break;
>>>>>> +                     }
>>>>>> +             }
>>>>>> +             read_unlock(&sbi->s_mb_largest_free_orders_locks[i]);
>>>>>> +             if (grp)
>>>>>> +                     break;
>>>>>> +     }
>>>>>> +
>>>>>> +     if (!grp) {
>>>>>> +             /* Increment cr and search again */
>>>>>> +             *new_cr = 1;
>>>>>> +     } else {
>>>>>> +             *group = grp->bb_group;
>>>>>> +             ac->ac_last_optimal_group = *group;
>>>>>> +             ac->ac_flags |= EXT4_MB_CR0_OPTIMIZED;
>>>>>> +     }
>>>>>> +     return 0;
>>>>>> +}
>>>>>> +
>>>>>> +/*
>>>>>> + * Choose next group by traversing average fragment size tree. Return 0 if next
>>>>>> + * group was selected optimally. Return 1 if next group could not selected
>>>>>> + * optimally (due to lock contention). Updates *new_cr if cr lvel needs an
>>>>>> + * update.
>>>>>> + */
>>>>>> +static int ext4_mb_choose_next_group_cr1(struct ext4_allocation_context *ac,
>>>>>> +             int *new_cr, ext4_group_t *group, ext4_group_t ngroups)
>>>>>> +{
>>>>>> +     struct ext4_sb_info *sbi = EXT4_SB(ac->ac_sb);
>>>>>> +     int avg_fragment_size, best_so_far;
>>>>>> +     struct rb_node *node, *found;
>>>>>> +     struct ext4_group_info *grp;
>>>>>> +
>>>>>> +     /*
>>>>>> +      * If there is contention on the lock, instead of waiting for the lock
>>>>>> +      * to become available, just continue searching lineraly. We'll resume
>>>>>> +      * our rb tree search later starting at ac->ac_last_optimal_group.
>>>>>> +      */
>>>>>> +     if (!read_trylock(&sbi->s_mb_rb_lock))
>>>>>> +             return 1;
>>>>>> +
>>>>>> +     if (unlikely(ac->ac_flags & EXT4_MB_CR1_OPTIMIZED)) {
>>>>>> +             if (sbi->s_mb_stats)
>>>>>> +                     atomic_inc(&sbi->s_bal_cr1_bad_suggestions);
>>>>>> +             /* We have found something at CR 1 in the past */
>>>>>> +             grp = ext4_get_group_info(ac->ac_sb, ac->ac_last_optimal_group);
>>>>>> +             for (found = rb_next(&grp->bb_avg_fragment_size_rb); found != NULL;
>>>>>> +                  found = rb_next(found)) {
>>>>>> +                     grp = rb_entry(found, struct ext4_group_info,
>>>>>> +                                    bb_avg_fragment_size_rb);
>>>>>> +                     if (sbi->s_mb_stats)
>>>>>> +                             atomic64_inc(&sbi->s_bal_cX_groups_considered[1]);
>>>>>> +                     if (likely(ext4_mb_good_group(ac, grp->bb_group, 1)))
>>>>>> +                             break;
>>>>>> +             }
>>>>>> +
>>>>>> +             goto done;
>>>>>> +     }
>>>>>> +
>>>>>> +     node = sbi->s_mb_avg_fragment_size_root.rb_node;
>>>>>> +     best_so_far = 0;
>>>>>> +     found = NULL;
>>>>>> +
>>>>>> +     while (node) {
>>>>>> +             grp = rb_entry(node, struct ext4_group_info,
>>>>>> +                            bb_avg_fragment_size_rb);
>>>>>> +             avg_fragment_size = 0;
>>>>>> +             /*
>>>>>> +              * Perform this check without locking, we'll lock later to confirm.
>>>>>> +              */
>>>>>> +             if (ext4_mb_good_group(ac, grp->bb_group, 1)) {
>>>>>> +                     avg_fragment_size = grp->bb_fragments ?
>>>>>> +                             grp->bb_free / grp->bb_fragments : 0;
>>>>>> +                     if (!best_so_far || avg_fragment_size < best_so_far) {
>>>>>> +                             best_so_far = avg_fragment_size;
>>>>>> +                             found = node;
>>>>>> +                     }
>>>>>> +             }
>>>>>> +             if (avg_fragment_size > ac->ac_g_ex.fe_len)
>>>>>> +                     node = node->rb_right;
>>>>>> +             else
>>>>>> +                     node = node->rb_left;
>>>>>> +     }
>>>>>> +
>>>>>> +done:
>>>>>> +     if (found) {
>>>>>> +             grp = rb_entry(found, struct ext4_group_info,
>>>>>> +                            bb_avg_fragment_size_rb);
>>>>>> +             *group = grp->bb_group;
>>>>>> +             ac->ac_flags |= EXT4_MB_CR1_OPTIMIZED;
>>>>>> +     } else {
>>>>>> +             *new_cr = 2;
>>>>>> +     }
>>>>>> +
>>>>>> +     read_unlock(&sbi->s_mb_rb_lock);
>>>>>> +     ac->ac_last_optimal_group = *group;
>>>>>> +     return 0;
>>>>>> +}
>>>>>> +
>>>>>> +/*
>>>>>> + * ext4_mb_choose_next_group: choose next group for allocation.
>>>>>> + *
>>>>>> + * @ac        Allocation Context
>>>>>> + * @new_cr    This is an output parameter. If the there is no good group
>>>>>> + *            available at current CR level, this field is updated to indicate
>>>>>> + *            the new cr level that should be used.
>>>>>> + * @group     This is an input / output parameter. As an input it indicates the
>>>>>> + *            last group used for allocation. As output, this field indicates
>>>>>> + *            the next group that should be used.
>>>>>> + * @ngroups   Total number of groups
>>>>>> + */
>>>>>> +static void ext4_mb_choose_next_group(struct ext4_allocation_context *ac,
>>>>>> +             int *new_cr, ext4_group_t *group, ext4_group_t ngroups)
>>>>>> +{
>>>>>> +     int ret;
>>>>>> +
>>>>>> +     *new_cr = ac->ac_criteria;
>>>>>> +
>>>>>> +     if (!test_opt2(ac->ac_sb, MB_OPTIMIZE_SCAN) ||
>>>>>> +         *new_cr >= 2 ||
>>>>>> +         !ext4_test_inode_flag(ac->ac_inode, EXT4_INODE_EXTENTS))
>>>>>> +             goto inc_and_return;
>>>>>> +
>>>>>> +     if (ac->ac_groups_linear_remaining) {
>>>>>> +             ac->ac_groups_linear_remaining--;
>>>>>> +             goto inc_and_return;
>>>>>> +     }
>>>>>> +
>>>>>> +     if (*new_cr == 0) {
>>>>>> +             ret = ext4_mb_choose_next_group_cr0(ac, new_cr, group, ngroups);
>>>>>> +             if (ret)
>>>>>> +                     goto inc_and_return;
>>>>>> +     }
>>>>>> +     if (*new_cr == 1) {
>>>>>> +             ret = ext4_mb_choose_next_group_cr1(ac, new_cr, group, ngroups);
>>>>>> +             if (ret)
>>>>>> +                     goto inc_and_return;
>>>>>> +     }
>>>>>> +     return;
>>>>>> +
>>>>>> +inc_and_return:
>>>>>> +     /*
>>>>>> +      * Artificially restricted ngroups for non-extent
>>>>>> +      * files makes group > ngroups possible on first loop.
>>>>>> +      */
>>>>>> +     *group = *group + 1;
>>>>>> +     if (*group >= ngroups)
>>>>>> +             *group = 0;
>>>>>> +}
>>>>>> +
>>>>>> /*
>>>>>> * Cache the order of the largest free extent we have available in this block
>>>>>> * group.
>>>>>> @@ -751,18 +1053,33 @@ static void ext4_mb_mark_free_simple(struct super_block *sb,
>>>>>> static void
>>>>>> mb_set_largest_free_order(struct super_block *sb, struct ext4_group_info *grp)
>>>>>> {
>>>>>> +     struct ext4_sb_info *sbi = EXT4_SB(sb);
>>>>>>     int i;
>>>>>> -     int bits;
>>>>>> 
>>>>>> +     if (test_opt2(sb, MB_OPTIMIZE_SCAN) && grp->bb_largest_free_order >= 0) {
>>>>>> +             write_lock(&sbi->s_mb_largest_free_orders_locks[
>>>>>> +                                           grp->bb_largest_free_order]);
>>>>>> +             list_del_init(&grp->bb_largest_free_order_node);
>>>>>> +             write_unlock(&sbi->s_mb_largest_free_orders_locks[
>>>>>> +                                           grp->bb_largest_free_order]);
>>>>>> +     }
>>>>>>     grp->bb_largest_free_order = -1; /* uninit */
>>>>>> 
>>>>>> -     bits = MB_NUM_ORDERS(sb) - 1;
>>>>>> -     for (i = bits; i >= 0; i--) {
>>>>>> +     for (i = MB_NUM_ORDERS(sb) - 1; i >= 0; i--) {
>>>>>>             if (grp->bb_counters[i] > 0) {
>>>>>>                     grp->bb_largest_free_order = i;
>>>>>>                     break;
>>>>>>             }
>>>>>>     }
>>>>>> +     if (test_opt2(sb, MB_OPTIMIZE_SCAN) &&
>>>>>> +         grp->bb_largest_free_order >= 0 && grp->bb_free) {
>>>>>> +             write_lock(&sbi->s_mb_largest_free_orders_locks[
>>>>>> +                                           grp->bb_largest_free_order]);
>>>>>> +             list_add_tail(&grp->bb_largest_free_order_node,
>>>>>> +                   &sbi->s_mb_largest_free_orders[grp->bb_largest_free_order]);
>>>>>> +             write_unlock(&sbi->s_mb_largest_free_orders_locks[
>>>>>> +                                           grp->bb_largest_free_order]);
>>>>>> +     }
>>>>>> }
>>>>>> 
>>>>>> static noinline_for_stack
>>>>>> @@ -818,6 +1135,7 @@ void ext4_mb_generate_buddy(struct super_block *sb,
>>>>>>     period = get_cycles() - period;
>>>>>>     atomic_inc(&sbi->s_mb_buddies_generated);
>>>>>>     atomic64_add(period, &sbi->s_mb_generation_time);
>>>>>> +     mb_update_avg_fragment_size(sb, grp);
>>>>>> }
>>>>>> 
>>>>>> /* The buddy information is attached the buddy cache inode
>>>>>> @@ -1517,6 +1835,7 @@ static void mb_free_blocks(struct inode *inode, struct ext4_buddy *e4b,
>>>>>> 
>>>>>> done:
>>>>>>     mb_set_largest_free_order(sb, e4b->bd_info);
>>>>>> +     mb_update_avg_fragment_size(sb, e4b->bd_info);
>>>>>>     mb_check_buddy(e4b);
>>>>>> }
>>>>>> 
>>>>>> @@ -1653,6 +1972,7 @@ static int mb_mark_used(struct ext4_buddy *e4b, struct ext4_free_extent *ex)
>>>>>>     }
>>>>>>     mb_set_largest_free_order(e4b->bd_sb, e4b->bd_info);
>>>>>> 
>>>>>> +     mb_update_avg_fragment_size(e4b->bd_sb, e4b->bd_info);
>>>>>>     ext4_set_bits(e4b->bd_bitmap, ex->fe_start, len0);
>>>>>>     mb_check_buddy(e4b);
>>>>>> 
>>>>>> @@ -2347,17 +2667,21 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
>>>>>>              * from the goal value specified
>>>>>>              */
>>>>>>             group = ac->ac_g_ex.fe_group;
>>>>>> +             ac->ac_last_optimal_group = group;
>>>>>> +             ac->ac_groups_linear_remaining = sbi->s_mb_linear_limit;
>>>>>>             prefetch_grp = group;
>>>>>> 
>>>>>> -             for (i = 0; i < ngroups; group++, i++) {
>>>>>> -                     int ret = 0;
>>>>>> +             for (i = 0; i < ngroups; i++) {
>>>>>> +                     int ret = 0, new_cr;
>>>>>> +
>>>>>>                     cond_resched();
>>>>>> -                     /*
>>>>>> -                      * Artificially restricted ngroups for non-extent
>>>>>> -                      * files makes group > ngroups possible on first loop.
>>>>>> -                      */
>>>>>> -                     if (group >= ngroups)
>>>>>> -                             group = 0;
>>>>>> +
>>>>>> +                     ext4_mb_choose_next_group(ac, &new_cr, &group, ngroups);
>>>>>> +
>>>>>> +                     if (new_cr != cr) {
>>>>>> +                             cr = new_cr;
>>>>>> +                             goto repeat;
>>>>>> +                     }
>>>>>> 
>>>>>>                     /*
>>>>>>                      * Batch reads of the block allocation bitmaps
>>>>>> @@ -2578,6 +2902,8 @@ int ext4_seq_mb_stats_show(struct seq_file *seq, void *offset)
>>>>>>                atomic64_read(&sbi->s_bal_cX_groups_considered[0]));
>>>>>>     seq_printf(seq, "\t\tuseless_loops: %llu\n",
>>>>>>                atomic64_read(&sbi->s_bal_cX_failed[0]));
>>>>>> +     seq_printf(seq, "\t\tbad_suggestions: %u\n",
>>>>>> +                atomic_read(&sbi->s_bal_cr0_bad_suggestions));
>>>>>> 
>>>>>>     seq_puts(seq, "\tcr1_stats:\n");
>>>>>>     seq_printf(seq, "\t\thits: %llu\n", atomic64_read(&sbi->s_bal_cX_hits[1]));
>>>>>> @@ -2585,6 +2911,8 @@ int ext4_seq_mb_stats_show(struct seq_file *seq, void *offset)
>>>>>>                atomic64_read(&sbi->s_bal_cX_groups_considered[1]));
>>>>>>     seq_printf(seq, "\t\tuseless_loops: %llu\n",
>>>>>>                atomic64_read(&sbi->s_bal_cX_failed[1]));
>>>>>> +     seq_printf(seq, "\t\tbad_suggestions: %u\n",
>>>>>> +                atomic_read(&sbi->s_bal_cr1_bad_suggestions));
>>>>>> 
>>>>>>     seq_puts(seq, "\tcr2_stats:\n");
>>>>>>     seq_printf(seq, "\t\thits: %llu\n", atomic64_read(&sbi->s_bal_cX_hits[2]));
>>>>>> @@ -2719,7 +3047,10 @@ int ext4_mb_add_groupinfo(struct super_block *sb, ext4_group_t group,
>>>>>>     INIT_LIST_HEAD(&meta_group_info[i]->bb_prealloc_list);
>>>>>>     init_rwsem(&meta_group_info[i]->alloc_sem);
>>>>>>     meta_group_info[i]->bb_free_root = RB_ROOT;
>>>>>> +     INIT_LIST_HEAD(&meta_group_info[i]->bb_largest_free_order_node);
>>>>>> +     RB_CLEAR_NODE(&meta_group_info[i]->bb_avg_fragment_size_rb);
>>>>>>     meta_group_info[i]->bb_largest_free_order = -1;  /* uninit */
>>>>>> +     meta_group_info[i]->bb_group = group;
>>>>>> 
>>>>>>     mb_group_bb_bitmap_alloc(sb, meta_group_info[i], group);
>>>>>>     return 0;
>>>>>> @@ -2909,6 +3240,26 @@ int ext4_mb_init(struct super_block *sb)
>>>>>>             i++;
>>>>>>     } while (i < MB_NUM_ORDERS(sb));
>>>>>> 
>>>>>> +     sbi->s_mb_avg_fragment_size_root = RB_ROOT;
>>>>>> +     sbi->s_mb_largest_free_orders =
>>>>>> +             kmalloc_array(MB_NUM_ORDERS(sb), sizeof(struct list_head),
>>>>>> +                     GFP_KERNEL);
>>>>>> +     if (!sbi->s_mb_largest_free_orders) {
>>>>>> +             ret = -ENOMEM;
>>>>>> +             goto out;
>>>>>> +     }
>>>>>> +     sbi->s_mb_largest_free_orders_locks =
>>>>>> +             kmalloc_array(MB_NUM_ORDERS(sb), sizeof(rwlock_t),
>>>>>> +                     GFP_KERNEL);
>>>>>> +     if (!sbi->s_mb_largest_free_orders_locks) {
>>>>>> +             ret = -ENOMEM;
>>>>>> +             goto out;
>>>>>> +     }
>>>>>> +     for (i = 0; i < MB_NUM_ORDERS(sb); i++) {
>>>>>> +             INIT_LIST_HEAD(&sbi->s_mb_largest_free_orders[i]);
>>>>>> +             rwlock_init(&sbi->s_mb_largest_free_orders_locks[i]);
>>>>>> +     }
>>>>>> +     rwlock_init(&sbi->s_mb_rb_lock);
>>>>>> 
>>>>>>     spin_lock_init(&sbi->s_md_lock);
>>>>>>     sbi->s_mb_free_pending = 0;
>>>>>> @@ -2961,6 +3312,10 @@ int ext4_mb_init(struct super_block *sb)
>>>>>>             spin_lock_init(&lg->lg_prealloc_lock);
>>>>>>     }
>>>>>> 
>>>>>> +     if (blk_queue_nonrot(bdev_get_queue(sb->s_bdev)))
>>>>>> +             sbi->s_mb_linear_limit = 0;
>>>>>> +     else
>>>>>> +             sbi->s_mb_linear_limit = MB_DEFAULT_LINEAR_LIMIT;
>>>>>>     /* init file for buddy data */
>>>>>>     ret = ext4_mb_init_backend(sb);
>>>>>>     if (ret != 0)
>>>>>> @@ -2972,6 +3327,8 @@ int ext4_mb_init(struct super_block *sb)
>>>>>>     free_percpu(sbi->s_locality_groups);
>>>>>>     sbi->s_locality_groups = NULL;
>>>>>> out:
>>>>>> +     kfree(sbi->s_mb_largest_free_orders);
>>>>>> +     kfree(sbi->s_mb_largest_free_orders_locks);
>>>>>>     kfree(sbi->s_mb_offsets);
>>>>>>     sbi->s_mb_offsets = NULL;
>>>>>>     kfree(sbi->s_mb_maxs);
>>>>>> @@ -3028,6 +3385,8 @@ int ext4_mb_release(struct super_block *sb)
>>>>>>             kvfree(group_info);
>>>>>>             rcu_read_unlock();
>>>>>>     }
>>>>>> +     kfree(sbi->s_mb_largest_free_orders);
>>>>>> +     kfree(sbi->s_mb_largest_free_orders_locks);
>>>>>>     kfree(sbi->s_mb_offsets);
>>>>>>     kfree(sbi->s_mb_maxs);
>>>>>>     iput(sbi->s_buddy_cache);
>>>>>> diff --git a/fs/ext4/mballoc.h b/fs/ext4/mballoc.h
>>>>>> index 68111a10cfee..02585e3cbcad 100644
>>>>>> --- a/fs/ext4/mballoc.h
>>>>>> +++ b/fs/ext4/mballoc.h
>>>>>> @@ -78,6 +78,18 @@
>>>>>> */
>>>>>> #define MB_DEFAULT_MAX_INODE_PREALLOC 512
>>>>>> 
>>>>>> +/*
>>>>>> + * Number of groups to search linearly before performing group scanning
>>>>>> + * optimization.
>>>>>> + */
>>>>>> +#define MB_DEFAULT_LINEAR_LIMIT              4
>>>>>> +
>>>>>> +/*
>>>>>> + * Minimum number of groups that should be present in the file system to perform
>>>>>> + * group scanning optimizations.
>>>>>> + */
>>>>>> +#define MB_DEFAULT_LINEAR_SCAN_THRESHOLD     16
>>>>>> +
>>>>>> /*
>>>>>> * Number of valid buddy orders
>>>>>> */
>>>>>> @@ -166,11 +178,14 @@ struct ext4_allocation_context {
>>>>>>     /* copy of the best found extent taken before preallocation efforts */
>>>>>>     struct ext4_free_extent ac_f_ex;
>>>>>> 
>>>>>> +     ext4_group_t ac_last_optimal_group;
>>>>>> +     __u32 ac_groups_considered;
>>>>>> +     __u32 ac_flags;         /* allocation hints */
>>>>>>     __u16 ac_groups_scanned;
>>>>>> +     __u16 ac_groups_linear_remaining;
>>>>>>     __u16 ac_found;
>>>>>>     __u16 ac_tail;
>>>>>>     __u16 ac_buddy;
>>>>>> -     __u16 ac_flags;         /* allocation hints */
>>>>>>     __u8 ac_status;
>>>>>>     __u8 ac_criteria;
>>>>>>     __u8 ac_2order;         /* if request is to allocate 2^N blocks and
>>>>>> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
>>>>>> index 3a2cd9fb7e73..6116640081c0 100644
>>>>>> --- a/fs/ext4/super.c
>>>>>> +++ b/fs/ext4/super.c
>>>>>> @@ -1687,7 +1687,7 @@ enum {
>>>>>>     Opt_dioread_nolock, Opt_dioread_lock,
>>>>>>     Opt_discard, Opt_nodiscard, Opt_init_itable, Opt_noinit_itable,
>>>>>>     Opt_max_dir_size_kb, Opt_nojournal_checksum, Opt_nombcache,
>>>>>> -     Opt_prefetch_block_bitmaps,
>>>>>> +     Opt_prefetch_block_bitmaps, Opt_mb_optimize_scan,
>>>>>> #ifdef CONFIG_EXT4_DEBUG
>>>>>>     Opt_fc_debug_max_replay, Opt_fc_debug_force
>>>>>> #endif
>>>>>> @@ -1788,6 +1788,7 @@ static const match_table_t tokens = {
>>>>>>     {Opt_nombcache, "nombcache"},
>>>>>>     {Opt_nombcache, "no_mbcache"},  /* for backward compatibility */
>>>>>>     {Opt_prefetch_block_bitmaps, "prefetch_block_bitmaps"},
>>>>>> +     {Opt_mb_optimize_scan, "mb_optimize_scan=%d"},
>>>>>>     {Opt_removed, "check=none"},    /* mount option from ext2/3 */
>>>>>>     {Opt_removed, "nocheck"},       /* mount option from ext2/3 */
>>>>>>     {Opt_removed, "reservation"},   /* mount option from ext2/3 */
>>>>>> @@ -1820,6 +1821,8 @@ static ext4_fsblk_t get_sb_block(void **data)
>>>>>> }
>>>>>> 
>>>>>> #define DEFAULT_JOURNAL_IOPRIO (IOPRIO_PRIO_VALUE(IOPRIO_CLASS_BE, 3))
>>>>>> +#define DEFAULT_MB_OPTIMIZE_SCAN     (-1)
>>>>>> +
>>>>>> static const char deprecated_msg[] =
>>>>>>     "Mount option \"%s\" will be removed by %s\n"
>>>>>>     "Contact linux-ext4@vger.kernel.org if you think we should keep it.\n";
>>>>>> @@ -2008,6 +2011,7 @@ static const struct mount_opts {
>>>>>>     {Opt_nombcache, EXT4_MOUNT_NO_MBCACHE, MOPT_SET},
>>>>>>     {Opt_prefetch_block_bitmaps, EXT4_MOUNT_PREFETCH_BLOCK_BITMAPS,
>>>>>>      MOPT_SET},
>>>>>> +     {Opt_mb_optimize_scan, EXT4_MOUNT2_MB_OPTIMIZE_SCAN, MOPT_GTE0},
>>>>>> #ifdef CONFIG_EXT4_DEBUG
>>>>>>     {Opt_fc_debug_force, EXT4_MOUNT2_JOURNAL_FAST_COMMIT,
>>>>>>      MOPT_SET | MOPT_2 | MOPT_EXT4_ONLY},
>>>>>> @@ -2092,6 +2096,7 @@ static int ext4_set_test_dummy_encryption(struct super_block *sb,
>>>>>> struct ext4_parsed_options {
>>>>>>     unsigned long journal_devnum;
>>>>>>     unsigned int journal_ioprio;
>>>>>> +     int mb_optimize_scan;
>>>>>> };
>>>>>> 
>>>>>> static int handle_mount_opt(struct super_block *sb, char *opt, int token,
>>>>>> @@ -2388,6 +2393,13 @@ static int handle_mount_opt(struct super_block *sb, char *opt, int token,
>>>>>>             sbi->s_mount_opt |= m->mount_opt;
>>>>>>     } else if (token == Opt_data_err_ignore) {
>>>>>>             sbi->s_mount_opt &= ~m->mount_opt;
>>>>>> +     } else if (token == Opt_mb_optimize_scan) {
>>>>>> +             if (arg != 0 && arg != 1) {
>>>>>> +                     ext4_msg(sb, KERN_WARNING,
>>>>>> +                              "mb_optimize_scan should be set to 0 or 1.");
>>>>>> +                     return -1;
>>>>>> +             }
>>>>>> +             parsed_opts->mb_optimize_scan = arg;
>>>>>>     } else {
>>>>>>             if (!args->from)
>>>>>>                     arg = 1;
>>>>>> @@ -4034,6 +4046,7 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
>>>>>>     /* Set defaults for the variables that will be set during parsing */
>>>>>>     parsed_opts.journal_ioprio = DEFAULT_JOURNAL_IOPRIO;
>>>>>>     parsed_opts.journal_devnum = 0;
>>>>>> +     parsed_opts.mb_optimize_scan = DEFAULT_MB_OPTIMIZE_SCAN;
>>>>>> 
>>>>>>     if ((data && !orig_data) || !sbi)
>>>>>>             goto out_free_base;
>>>>>> @@ -4984,6 +4997,19 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
>>>>>>     ext4_fc_replay_cleanup(sb);
>>>>>> 
>>>>>>     ext4_ext_init(sb);
>>>>>> +
>>>>>> +     /*
>>>>>> +      * Enable optimize_scan if number of groups is > threshold. This can be
>>>>>> +      * turned off by passing "mb_optimize_scan=0". This can also be
>>>>>> +      * turned on forcefully by passing "mb_optimize_scan=1".
>>>>>> +      */
>>>>>> +     if (parsed_opts.mb_optimize_scan == 1)
>>>>>> +             set_opt2(sb, MB_OPTIMIZE_SCAN);
>>>>>> +     else if (parsed_opts.mb_optimize_scan == 0)
>>>>>> +             clear_opt2(sb, MB_OPTIMIZE_SCAN);
>>>>>> +     else if (sbi->s_groups_count >= MB_DEFAULT_LINEAR_SCAN_THRESHOLD)
>>>>>> +             set_opt2(sb, MB_OPTIMIZE_SCAN);
>>>>>> +
>>>>>>     err = ext4_mb_init(sb);
>>>>>>     if (err) {
>>>>>>             ext4_msg(sb, KERN_ERR, "failed to initialize mballoc (%d)",
>>>>>> diff --git a/fs/ext4/sysfs.c b/fs/ext4/sysfs.c
>>>>>> index 59ca9d73b42f..16b8a838f631 100644
>>>>>> --- a/fs/ext4/sysfs.c
>>>>>> +++ b/fs/ext4/sysfs.c
>>>>>> @@ -213,6 +213,7 @@ EXT4_RW_ATTR_SBI_UI(mb_order2_req, s_mb_order2_reqs);
>>>>>> 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(mb_max_inode_prealloc, s_mb_max_inode_prealloc);
>>>>>> +EXT4_RW_ATTR_SBI_UI(mb_linear_limit, s_mb_linear_limit);
>>>>>> EXT4_RW_ATTR_SBI_UI(extent_max_zeroout_kb, s_extent_max_zeroout_kb);
>>>>>> EXT4_ATTR(trigger_fs_error, 0200, trigger_test_error);
>>>>>> EXT4_RW_ATTR_SBI_UI(err_ratelimit_interval_ms, s_err_ratelimit_state.interval);
>>>>>> @@ -260,6 +261,7 @@ static struct attribute *ext4_attrs[] = {
>>>>>>     ATTR_LIST(mb_stream_req),
>>>>>>     ATTR_LIST(mb_group_prealloc),
>>>>>>     ATTR_LIST(mb_max_inode_prealloc),
>>>>>> +     ATTR_LIST(mb_linear_limit),
>>>>>>     ATTR_LIST(max_writeback_mb_bump),
>>>>>>     ATTR_LIST(extent_max_zeroout_kb),
>>>>>>     ATTR_LIST(trigger_fs_error),
>>>>>> --
>>>>>> 2.31.0.291.g576ba9dcdaf-goog
>>>>>> 
>>>>> 
>>>>> 
>>>>> Cheers, Andreas
>>>>> 
>>>>> 
>>>>> 
>>>>> 
>>>>>