Re: [BUGFIX][PATCH -tip] x86: fix text_poke to handle highmem pages

[Masami Hiramatsu <mhiramat@redhat.com>]

Mathieu Desnoyers wrote:
> * Masami Hiramatsu (mhiramat@redhat.com) wrote:
>> Mathieu Desnoyers wrote:
>>> * Masami Hiramatsu (mhiramat@redhat.com) wrote:
>>>> Fix a bug in text_poke to handle highmem pages, because module
>>>> text pages are possible to be highmem pages on x86-32.
>>>> In that case, since fixmap can't handle those pages, text_poke
>>>> uses kmap_atomic.
>>>>
>>> Hrm, can you remind me what would be the downside of using kmap_atomic
>>> in every scenarios (highmem and non-highmem) then ?
>> kmap_atomic can handle only highmem pages. If you passes lowmem pages,
>> it returns just original vaddr of it. (because kmap is only for
>> highmem support)
>>
> 
> OK, and if we are doing a second kmap_atomic() of a module text page
> which is already mapped, does the second kmap_atomic return the vaddr of
> the page originally mapped or is it creating a second mapping ?

Good point, kmap_atomic creates a second mapping if the page is highmem.

in arch/x86/mm/highmem_32.c:

void *kmap_atomic_prot(struct page *page, enum km_type type, pgprot_t prot)
{
	...
        if (!PageHighMem(page))
                return page_address(page);
	...
}

> Because if we ever decide to enforce read-only page mapping for module
> text pages, touching highmem pages too, we will run into real trouble if
> those happen to be the same page.

So, as long as the page is in highmem, we can create a second mapping by
using kmap_atomic.

Thank you,

> 
> Mathieu
> 
>>> I would try to avoid "special cases" as much as possible, because they
>>> just make problems harder to reproduce.
>> Actually, this bug is a special case because it happens only on PAE kernel.
>>
>>> The idea would be to either add fixmap highmem support, or to simply use
>>> kmap_atomic() for all cases until we add fixmap highmem support.
>>>
>>
>> Thank you,
>>
>> -- 
>> Masami Hiramatsu
>>
>> Software Engineer
>> Hitachi Computer Products (America) Inc.
>> Software Solutions Division
>>
>> e-mail: mhiramat@redhat.com
>>
> 

-- 
Masami Hiramatsu

Software Engineer
Hitachi Computer Products (America) Inc.
Software Solutions Division

e-mail: mhiramat@redhat.com