Message-ID: <55A952DB.9040003@gmail.com>
Date: Fri, 17 Jul 2015 15:09:15 -0400
From: Austin S Hemmelgarn <ahferroin7@gmail.com>
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To: Oleg Nesterov <oleg@redhat.com>
CC: Benjamin LaHaise <bcrl@kvack.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Joonsoo Kim <js1304@gmail.com>, Fengguang Wu <fengguang.wu@intel.com>,
        Jeff Moyer <jmoyer@redhat.com>, Johannes Weiner <hannes@cmpxchg.org>,
        Stephen Rothwell <sfr@canb.auug.org.au>, linux-next@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm-move-mremap-from-file_operations-to-vm_operations_struct-fix
References: <20150716231405.GA25147@redhat.com> <20150716162444.26425f5e227387f1166a6d16@linux-foundation.org> <20150716235227.GA26551@redhat.com> <20150717140615.GA2779@kvack.org> <20150717172726.GA30443@redhat.com> <20150717173757.GD2779@kvack.org> <20150717175542.GA31888@redhat.com> <55A94578.5050306@gmail.com> <20150717181940.GA946@redhat.com> <55A94BD2.5040603@gmail.com> <20150717185434.GA2290@redhat.com>
In-Reply-To: <20150717185434.GA2290@redhat.com>
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On 2015-07-17 14:54, Oleg Nesterov wrote:
> On 07/17, Austin S Hemmelgarn wrote:
>>
>> On 2015-07-17 14:19, Oleg Nesterov wrote:
>>> On 07/17, Austin S Hemmelgarn wrote:
>>>>
>>>> On 2015-07-17 13:55, Oleg Nesterov wrote:
>>>>> On 07/17, Benjamin LaHaise wrote:
>>>>>>
>>>>>> Don't add BUG().  It's the equivalent approach of saying "I think =
this code
>>>>>> isn't needed, but I'm lazy and not going to remove it properly."
>>>>>
>>>>> There is another interpretation: I think this code must be never ca=
lled,
>>>>> if it is actually called we have a serious problem which should be =
loudly
>>>>> reported.
>>>>>
>>>> And not compiling it at all _will_ loudly report it, it'll just repo=
rt
>>>> it during linking instead of at run-time, which is a much better tim=
e to
>>>> shout about it.
>>>
>>> And how can we do this?
>>>
>> If a function that isn't defined (for example, you use a #if block to
>> comment it out under certain circumstances), then the link will fail
>> rather noisily something references it.
>
> This is what we are trying to fix.
Ah, I misunderstood the intent of the patch, looking at it again this=20
makes perfect sense.
>
>> We already know during the
>> compile that it's a NOMMU kernel, so anything that calls it on a MMU
>> enabled kernel can have a compile time check added
>
> It already has. memory.c is not compiled if NOMMU.
>
> The problem is aio_ring_vm_ops which references this function. And btw
> filemap_fault() too.
>
> And just in case, I won't mind to add ifdef(CONFIG_MMU) there, I am
> waiting for reply from Benjamin.
>
>> instead of doing the
>> check at runtime (or even just calling it without checking), thus even=

>> further reducing code size.
>
> So what exactly do you suggest to fix the problem?
Some sort of COMPILE_BUG_ON usage (I think that's the name of the macro=20
that I'm thinking of, not certain though) possibly?  My only point is=20
that it's more useful if the function should never be called to find a=20
way to enforce this at build time instead of runtime (even more so for=20
NOMMU systems, they tend (in my experience at least) to be even more of=20
a pain to debug when they crash).  I do agree though that a helpful=20
message is much preferred to a link error.


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