Re: [PATCH] remove BUG() in possible but rare condition

[Don Morris <don.morris@hp.com>]

On 04/11/2012 12:20 PM, Michal Hocko wrote:
> On Wed 11-04-12 11:57:56, Linus Torvalds wrote:
>> On Wed, Apr 11, 2012 at 11:48 AM, Michal Hocko <mhocko@suse.cz> wrote:
>>>
>>> I am not familiar with the code much but a trivial call chain walk up to
>>> write_dev_supers (in btrfs) shows that we do not check for the return value
>>> from __getblk so we would nullptr and there might be more.
>>> I guess these need some treat before the BUG might be removed, right?
>>
>> Well, realistically, isn't BUG() as bad as a NULL pointer dereference?
>>
>> Do you care about the exact message on the screen when your machine dies?
> 
> I personally do not care as I do not allow anything to map at that area.
> 
> It just seems that there are some callers who do not expect that the
> allocation fails. BUG at the allocation failure which dates back when it
> replaced buffer_error might have let to some assumptions (not good of
> course but we should better fix them.
> 
> That being said I am not against the patch. BUG on an allocation failure
> just doesn't feel right...

Apologies in advance for the relatively wide distribution for what may
be an obvious/stupid question, but if this is in a Documentation/vm
file, I don't see it.

Unless I'm really missing something, the allocation in question uses
GFP_NOFS flags, resulting in a "Wait Ok" request to the underlying
allocators. Other than gotchas like force failures returning a fail
even for Wait-safe allocations... is it actually expected for Wait-safe
kernel allocations to fail? Certainly page requests for user memory
can run afoul of OOM or other mechanisms, and certainly any request
for special/hard memory could be failed... but wouldn't a general small
kernel allocation should generally sleep until satisfied?

That's what the current code looks like to me -- but I could easily
be missing something in the reading, so I'm mainly asking if there's
a general policy to the API here. If it is that non-special kernel
allocations wait until they're satisfied... I'm wondering, why the
test/force mode that seems guaranteed to light off BUG() statements
such as this which are simply validating the allocation contract?

Don Morris

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