Re: new mballoc patches.

["Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>]

Hi Alex,

Aneesh Kumar K.V wrote:
> 
> I checked the logs today for the fsstress run i found this in my dmesg 
> log. The
> same stack trace is repeated for many times
> 
> 
> uh! busy PA
> Call Trace:
> [c0000000efa72fc0] [c00000000000fe30] .show_stack+0x6c/0x1a0 (unreliable)
> [c0000000efa73060] [c0000000001954a0] 
> .ext4_mb_discard_group_preallocations+0x208/0x4fc
> [c0000000efa73180] [c0000000001957c8] 
> .ext4_mb_discard_preallocations+0x34/0x94
> [c0000000efa73220] [c000000000197fc8] .ext4_mb_new_blocks+0x1fc/0x2c0
> [c0000000efa733a0] [c00000000018dfb4] .ext4_ext_get_blocks+0x540/0x6f8
> [c0000000efa734d0] [c00000000017bfe8] .ext4_get_block+0x12c/0x1b4
> [c0000000efa73580] [c000000000109104] .__blockdev_direct_IO+0x554/0xb94
> [c0000000efa736b0] [c000000000179b28] .ext4_direct_IO+0x138/0x208
> [c0000000efa73790] [c00000000009a314] .generic_file_direct_IO+0x134/0x1a0
> [c0000000efa73840] [c00000000009a404] .generic_file_direct_write+0x84/0x150
> [c0000000efa73900] [c00000000009bf54] 
> .__generic_file_aio_write_nolock+0x2c4/0x3d4
> [c0000000efa73a00] [c00000000009c0e4] .generic_file_aio_write+0x80/0x114
> [c0000000efa73ac0] [c000000000175c90] .ext4_file_write+0x2c/0xd4
> [c0000000efa73b50] [c0000000000d0cf4] .do_sync_write+0xc4/0x124
> [c0000000efa73cf0] [c0000000000d16bc] .vfs_write+0x120/0x1f4
> [c0000000efa73d90] [c0000000000d21a8] .sys_write+0x4c/0x8c
> [c0000000efa73e30] [c00000000000852c] syscall_exit+0x0/0x40
> uh! busy PA


I think we can remove from the code this dump_stack.


list_for_each_entry_safe(pa, tmp,
                                &grp->bb_prealloc_list, pa_group_list) {
                spin_lock(&pa->pa_lock);
                if (atomic_read(&pa->pa_count)) {
                        spin_unlock(&pa->pa_lock);
                        printk(KERN_ERR "uh! busy PA\n");
                        dump_stack();
                        busy = 1;
                        continue;
                }

This happens during ext4_mb_discard_group_prealloction. It is quiet possible that
during the discard operation some other CPUs can use the preallocated space right ?
Infact down the code we see if we have skipped some of the PA (busy == 1 )and the 
free space retrieved is not enough they we loop again  

Can you let me know why we marked the dump_stack there ?


-aneesh