Page fault while link_path_walk for path_len > 4060 bytes

[Al Viro]

On Mon, Aug 28, 2017 at 09:53:00AM +0530, ankijain at codeaurora.org wrote:
> Hi Will Deacon/ Al viro
> 
> 
> -->Please find the attached kmsg.txt
> <3>[17620.275249] BUG: sleeping function called from invalid context at /local/mnt/workspace/lnxbuild/project/trees_in_use/free_tree_platform_manifest_refs_tags_AU_LINUX_ANDROID_LA.UM.5.7.07.01.01.287.725_sdm660_64_commander_26168534/checkout/kernel/msm-4.4/arch/arm64/mm/fault.c:313
> <3>[17620.276504] in_atomic(): 0, irqs_disabled(): 0, pid: 10290, name:
> stress-ng-dirde
> <6>[17620.298995] ------------[ cut here ]------------
> <2>[17620.299009] kernel BUG at /local/mnt/workspace/lnxbuild/project/trees_in_use/free_tree_platform_manifest_refs_tags_AU_LINUX_ANDROID_LA.UM.5.7.07.01.01.287.725_sdm660_64_commander_26168534/checkout/kernel/msm-4.4/kernel/sched/core.c:8528!
> <6>[17620.306372] ------------[ cut here ]------------
> <2>[17620.327239] kernel BUG at /local/mnt/workspace/lnxbuild/project/trees_in_use/free_tree_platform_manifest_refs_tags_AU_LINUX_ANDROID_LA.UM.5.7.07.01.01.287.725_sdm660_64_commander_26168534/checkout/kernel/msm-4.4/kernel/sched/core.c:8528!
> 
> 
> --> we are using arm64 machine with kernel 4.4.
> --> can you please guide us, how to capture ESR value while taking the
> fault?
> -->
> -    { do_page_fault,    SIGSEGV, SEGV_MAPERR,    "level 3 translation
> fault"    },
> +    { do_translation_fault,    SIGSEGV, SEGV_MAPERR,    "level 3
> translation fault"    },
> we will try with above changes and get back to you.
> 
> -> config and kmsg are attached.
> 
> Regards,
> Ankit Jain
> Qualcomm India Private Limited, on behalf of Qualcomm Innovation
> Center, Inc.
> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a
> Linux Foundation Collaborative Project

Umm...  Line numbers make no sense for 4.4.  Could you post a reference
to the actual tree used (repository + SHA1; again, it can't be vanilla
4.4, or stable/linux-4.4.y, for that matter) as well as your .config?

In any case, looks like in_atomic() is false there, so we need an explicit
pagefault_disable() to make sure it goes to no_context.

Looking through the callchains...
	* __d_lookup() -> d_same_name() -> dentry_cmp() -> dentry_string_cmp()
with rcu_read_lock() held by __d_lookup().
	* d_alloc_parallel() -> d_same_name(), etc.  rcu_read_lock() held by
d_alloc_parallel() in one case, dentry->d_lock in another.
	* d_exact_alias() -> d_same_name().  inode->i_lock held by d_exact_alias().
	* d_alloc_parallel() -> __d_lookup_rcu() -> dentry_cmp().
rcu_read_lock() held by d_alloc_parallel().
	* lookup_fast() -> __d_lookup_rcu(), etc.  rcu_read_lock() grabbed by
path_init().
	* full_name_hash().  Fuckloads.
	* hashlen_string().  Fewer, but...
	* link_path_walk() -> hash_name().  rcu_read_lock() held by path_init().

And then there's siphash(), but that one AFAICS should never see those faults.

Hell knows...  I'm somewhat tempted to slap pagefault_disable()/pagefault_enable()
in dentry_string_cmp(), full_name_hash(), hashlen_string() and hash_name().
Regardless of the locks held by callers.  Doing that in load_unaligned_zeropad()
itself would be ridiculously costly, but these 4 would probably be saner...

I still would like to see the details of config, though.