Re: Any reason to use put_page in slub.c?

[Glauber Costa <glommer@parallels.com>]

On 08/01/2012 10:10 PM, Christoph Lameter wrote:
> On Wed, 1 Aug 2012, Glauber Costa wrote:
> 
>> I've audited all users of get_page() in the drivers/ directory for
>> patterns like this. In general, they kmalloc something like a table of
>> entries, and then get_page() the entries. The entries are either user
>> pages, pages allocated by the page allocator, or physical addresses
>> through their pfn (in 2 cases from the vga ones...)
>>
>> I took a look about some other instances where virt_to_page occurs
>> together with kmalloc as well, and they all seem to fall in the same
>> category.
> 
> The case that was notorious in the past was a scsi control structure
> allocated from slab that was then written to the device via DMA. And it
> was not on x86 but some esoteric platform (powerpc?),
> 
> A reference to the discussion of this issue in 2007:
> 
> http://lkml.indiana.edu/hypermail/linux/kernel/0706.3/0424.html
> 
Thanks.

So again, I've scanned across that thread, and found some very useful
excerpts from it, that can only argue in favor of my patch =)

"There are no kmalloced pages. There is only kmalloced memory. You
allocate pages from the page allocator. Its a layering violation to
expect a page struct operation on a slab object to work."

"So someone played loose ball with the slab, was successful and that
makes it right now?"

Looking at the code again, I see that page_mapping(), that ends up being
called to do the translation in those pathological cases now features a
VM_BUG_ON(), put in place by yourself. This dates back from 2007, giving
me enough reason to believe that whatever issue still existed back then
is already sorted out - or nobody really cares.


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