Re: Possible bio merging breakage in mp bio rework

[Nikolay Borisov]

On 6.04.19 Ð. 3:16 Ñ., Ming Lei wrote:
> Hi Nikolay,
> 
> On Fri, Apr 05, 2019 at 07:04:18PM +0300, Nikolay Borisov wrote:
>> Hello Ming, 
>>
>> Following the mp biovec rework what is the maximum 
>> data that a bio could contain? Should it be PAGE_SIZE * bio_vec 
> 
> There isn't any maximum data limit on the bio submitted from fs,
> and block layer will make the final bio sent to driver correct
> by applying all kinds of queue limit, such as max segment size,
> max segment number, max sectors, ...
> 
>> or something else? Currently I can see bios as large as 127 megs 
>> on sequential workloads, I got prompted to this since btrfs has a 
>> memory allocation that is dependent on the data in the bio and this 
>> particular memory allocation started failing with order 6 allocs. 
> 
> Could you share us the code? I don't see why order 6 allocs is a must.

When a bio is submitted btrfs has to calculate the checksum for it, this
happens in btrfs_csum_one_bio. Said checksums are stored in an
kmalloc'ed array, whose size is calculated as:

32 + bio_size / btrfs' block size (usually 4k). So for a 127mb bio that
would be: 32 * ((134184960Ã4096) * 4) = 127k. We'd make an order 3
allocation. Admittedly the code in btrfs should know better rather than
make unbounded allocations without a fallback, but bio suddenly becoming
rather unbounded in their size caught us offhand.


> 
>> Further debugging showed that with the following xfs_io command line: 
>>
>>
>> xfs_io -f -c "pwrite -S 0x61 -b 4m 0 10g" /media/scratch/file1
>>
>> I can easily see very large bios: 
>>
>> [  188.366540] kworker/-7       3.... 34847519us : btrfs_submit_bio_hook: bio: ffff8dffe9940bb0 bi_iter.bi_size = 134184960 bi_vcn: 28 bi_vcnt_max: 256
>> [  188.367129] kworker/-658     2.... 34946536us : btrfs_submit_bio_hook: bio: ffff8dffe9940370 bi_iter.bi_size = 134246400 bi_vcn: 28 bi_vcnt_max: 256
>> [  188.367714] kworker/-7       3.... 35107967us : btrfs_submit_bio_hook: bio: ffff8dffe9940bb0 bi_iter.bi_size = 134184960 bi_vcn: 30 bi_vcnt_max: 256
>> [  188.368319] kworker/-658     2.... 35229894us : btrfs_submit_bio_hook: bio: ffff8dffe9940370 bi_iter.bi_size = 134246400 bi_vcn: 32 bi_vcnt_max: 256
>> [  188.368909] kworker/-7       3.... 35374809us : btrfs_submit_bio_hook: bio: ffff8dffe9940bb0 bi_iter.bi_size = 134184960 bi_vcn: 25 bi_vcnt_max: 256
>> [  188.369498] kworker/-658     2.... 35516194us : btrfs_submit_bio_hook: bio: ffff8dffe9940370 bi_iter.bi_size = 134246400 bi_vcn: 31 bi_vcnt_max: 256
>> [  188.370086] kworker/-7       3.... 35663669us : btrfs_submit_bio_hook: bio: ffff8dffe9940bb0 bi_iter.bi_size = 134184960 bi_vcn: 32 bi_vcnt_max: 256
>> [  188.370696] kworker/-658     2.... 35791006us : btrfs_submit_bio_hook: bio: ffff8dffe9940370 bi_iter.bi_size = 100655104 bi_vcn: 24 bi_vcnt_max: 256
>> [  188.371335] kworker/-658     2.... 35816114us : btrfs_submit_bio_hook: bio: ffff8dffe99434f0 bi_iter.bi_size = 33591296 bi_vcn: 5 bi_vcnt_max: 256
>>
>>
>> So that's 127 megs in a single bio? This stems from the new merging logic. 
>> 07173c3ec276 ("block: enable multipage bvecs") made it so that physically 
>> contiguous pages added to the bio would just modify bi_iter.bi_size and the 
>> initial page's bio_vec's bv_len. There's no longer the 
>> page == bv->bv_page portion of the check. 
> 
> bio_add_page() tries best to put physically contiguous pages into one bvec, and
> I don't see anything is wrong in the log.
> 
> Could you show us what the real problem is?
> 
> Thanks,
> Ming
>