linux-kernel - Re: [PATCH] i386: fix vmalloc_sync

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Date:	Fri, 20 Jun 2008 07:58:20 +0100
From:	"Jan Beulich" <jbeulich@...ell.com>
To:	"Jeremy Fitzhardinge" <jeremy@...p.org>
Cc:	<mingo@...e.hu>, <tglx@...utronix.de>,
	<linux-kernel@...r.kernel.org>, <hpa@...or.com>
Subject: Re: [PATCH] i386: fix vmalloc_sync_all() for Xen

>I think it would be better to separately test whether the vmalloc 
>mapping is present in the init_mm and skip the syncing loop in that 
>case, rather than this somewhat convoluted logic to overload the test in 
>vmalloc_sync_one.

That's what the x86-64 code does. When I wrote this originally, I tried
to keep the pre-existing logic as much as possible, so I split out
vmalloc_sync_one() by mostly moving existing code. I certainly agree
that this has room for cleaning up (and then possibly including unification
with x86-64).

>>>> This is a replacement of the BUG_ON() that an earlier patch from you
>>>> removed: Failure of vmalloc_sync_one() must happen on the first
>>>> entry or never, and this is what is being checked for here.
>>>>   
>>>>       
>>> Could you add a comment?
>>>     
>>
>> Sure, though there was none originally, and the intention seemed
>> quite clear to me.
>
>Well, looks to me like vmalloc_sync_one can only return NULL iff the 
>vmalloc mapping is absent in init_mm, so that's going to be invariant 

Correct.

>with respect to any other pgd you pass in.  So I don't think the BUG_ON 
>will ever fire, and it's unclear what actual logical property it's 
>testing for.

My point of adding the BUG_ON() is that in vmalloc_sync_all() it is not
clear that vmalloc_sync_one() can fail only due to init_mm's page table
not being appropriately populated. So yes, this BUG_ON() is not
expected to ever fire - but isn't that a property of all BUG_ON()'s?

Jan

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