Re: [PATCHv5 3/5] ext4: mballoc: Introduce pcpu seqcnt for freeing PA to improve ENOSPC handling

[Ritesh Harjani <ritesh.list@gmail.com>]

Hi Marek,

On 6/3/20 12:18 PM, Marek Szyprowski wrote:
> Hi Ritesh,
> 
> On 20.05.2020 08:40, Ritesh Harjani wrote:
>> There could be a race in function ext4_mb_discard_group_preallocations()
>> where the 1st thread may iterate through group's bb_prealloc_list and
>> remove all the PAs and add to function's local list head.
>> Now if the 2nd thread comes in to discard the group preallocations,
>> it will see that the group->bb_prealloc_list is empty and will return 0.
>>
>> Consider for a case where we have less number of groups
>> (for e.g. just group 0),
>> this may even return an -ENOSPC error from ext4_mb_new_blocks()
>> (where we call for ext4_mb_discard_group_preallocations()).
>> But that is wrong, since 2nd thread should have waited for 1st thread
>> to release all the PAs and should have retried for allocation.
>> Since 1st thread was anyway going to discard the PAs.
>>
>> The algorithm using this percpu seq counter goes below:
>> 1. We sample the percpu discard_pa_seq counter before trying for block
>>      allocation in ext4_mb_new_blocks().
>> 2. We increment this percpu discard_pa_seq counter when we either allocate
>>      or free these blocks i.e. while marking those blocks as used/free in
>>      mb_mark_used()/mb_free_blocks().
>> 3. We also increment this percpu seq counter when we successfully identify
>>      that the bb_prealloc_list is not empty and hence proceed for discarding
>>      of those PAs inside ext4_mb_discard_group_preallocations().
>>
>> Now to make sure that the regular fast path of block allocation is not
>> affected, as a small optimization we only sample the percpu seq counter
>> on that cpu. Only when the block allocation fails and when freed blocks
>> found were 0, that is when we sample percpu seq counter for all cpus using
>> below function ext4_get_discard_pa_seq_sum(). This happens after making
>> sure that all the PAs on grp->bb_prealloc_list got freed or if it's empty.
>>
>> It can be well argued that why don't just check for grp->bb_free to
>> see if there are any free blocks to be allocated. So here are the two
>> concerns which were discussed:-
>>
>> 1. If for some reason the blocks available in the group are not
>>      appropriate for allocation logic (say for e.g.
>>      EXT4_MB_HINT_GOAL_ONLY, although this is not yet implemented), then
>>      the retry logic may result into infinte looping since grp->bb_free is
>>      non-zero.
>>
>> 2. Also before preallocation was clubbed with block allocation with the
>>      same ext4_lock_group() held, there were lot of races where grp->bb_free
>>      could not be reliably relied upon.
>> Due to above, this patch considers discard_pa_seq logic to determine if
>> we should retry for block allocation. Say if there are are n threads
>> trying for block allocation and none of those could allocate or discard
>> any of the blocks, then all of those n threads will fail the block
>> allocation and return -ENOSPC error. (Since the seq counter for all of
>> those will match as no block allocation/discard was done during that
>> duration).
>>
>> Signed-off-by: Ritesh Harjani <riteshh@linux.ibm.com>
> 
> This patch landed in yesterday's linux-next and causes following
> WARNING/BUG on various Samsung Exynos-based boards:
> 
>    BUG: using smp_processor_id() in preemptible [00000000] code: logsave/552
>    caller is ext4_mb_new_blocks+0x404/0x1300

Yes, this is being discussed in the community.
I have submitted a patch which should help fix this warning msg.
Feel free to give this a try on your setup.

https://marc.info/?l=linux-ext4&m=159110574414645&w=2


-ritesh