Re: [PATCH 08/13] fs: limit filesystem stacking depth

Miklos Szeredi
2012-09-03T15:04:28Z

Sedat Dilek <sedat.dilek@gmail.com> writes:

>>
>> Yes, setting it to 2 is defensive, it would need more stack depth
>> analysis to see what an acceptable number would be.
>>
>
> Can you describe such an analysis method (in case you need help for
> testing it)?

I attached a systemtap script (x86-64 specific) which prints stack use
for stacked overlayfs filesystems. Interpret output like this:

7288 0 ovl_lookup (bash/5721)
^ ^
| |
| +- function name (process/pid)
|
+------------ current stack use

7080 208 ovl_permission (bash/5721)
^ ^
| |
| +- stacking depth indicated by indentation
|
+------ stack increase from previous stacking level

You can try it on various setups (overlayfs being used as the lower
and/or upper level) and executing various filesystem operations.

Looks like "copy up" is the most stack hungry operation, it may be worth
trying to reduce its stack usage.

Thanks,
Miklos
---

global rec_level, stacks

probe
kernel.function("ovl_permission"),
kernel.function("ovl_getattr"),
kernel.function("ovl_dir_getattr"),
kernel.function("ovl_setattr"),
kernel.function("ovl_setxattr"),
kernel.function("ovl_listxattr"),
kernel.function("ovl_removexattr"),
kernel.function("ovl_dentry_open"),
kernel.function("ovl_lookup"),
kernel.function("ovl_mkdir"),
kernel.function("ovl_symlink"),
kernel.function("ovl_unlink"),
kernel.function("ovl_rmdir"),
kernel.function("ovl_rename"),
kernel.function("ovl_link"),
kernel.function("ovl_create"),
kernel.function("ovl_mknod"),
kernel.function("ovl_follow_link"),
kernel.function("ovl_put_link"),
kernel.function("ovl_readlink"),
kernel.function("ovl_dir_open"),
kernel.function("ovl_readdir"),
kernel.function("ovl_dir_llseek"),
kernel.function("ovl_dir_fsync"),
kernel.function("ovl_dir_release"),
kernel.function("ovl_dentry_release"),
kernel.function("ovl_put_super"),
kernel.function("ovl_statfs")
{
tid = tid();
i = rec_level[tid]++;
stack_rem = u_register("rbp") & 0x1fff;
stacks[tid, i] = stack_rem;
delta = i > 0 ? stacks[tid, i - 1] - stack_rem : 0;
printf("%4i %4i %-*s %s (%s/%i)\n", stack_rem, delta, i * 8, "", probefunc(), execname(), tid);
}

probe
kernel.function("ovl_permission").return,
kernel.function("ovl_getattr").return,
kernel.function("ovl_dir_getattr").return,
kernel.function("ovl_setattr").return,
kernel.function("ovl_setxattr").return,
kernel.function("ovl_listxattr").return,
kernel.function("ovl_removexattr").return,
kernel.function("ovl_dentry_open").return,
kernel.function("ovl_lookup").return,
kernel.function("ovl_mkdir").return,
kernel.function("ovl_symlink").return,
kernel.function("ovl_unlink").return,
kernel.function("ovl_rmdir").return,
kernel.function("ovl_rename").return,
kernel.function("ovl_link").return,
kernel.function("ovl_create").return,
kernel.function("ovl_mknod").return,
kernel.function("ovl_follow_link").return,
kernel.function("ovl_put_link").return,
kernel.function("ovl_readlink").return,
kernel.function("ovl_dir_open").return,
kernel.function("ovl_readdir").return,
kernel.function("ovl_dir_llseek").return,
kernel.function("ovl_dir_fsync").return,
kernel.function("ovl_dir_release").return,
kernel.function("ovl_dentry_release").return,
kernel.function("ovl_put_super").return,
kernel.function("ovl_statfs").return
{
rec_level[tid()]--;
}