Possible bio merging breakage in mp bio rework

[Nikolay Borisov]

Hello Ming, 

Following the mp biovec rework what is the maximum 
data that a bio could contain? Should it be PAGE_SIZE * bio_vec 
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. 
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.