Is high_memory check in omap2.c for OneNAND is sufficient?

[Singh, Vimal]

Thanks Andrian... this helped me a lot...
________________________________________
From: Adrian Hunter [adrian.hunter at nokia.com]
Sent: Thursday, March 26, 2009 2:25 PM
To: Singh, Vimal
Cc: linux-mtd at lists.infradead.org
Subject: Re: Is high_memory check in omap2.c for OneNAND is sufficient?

ext Singh, Vimal wrote:
> From: Adrian Hunter [adrian.hunter at nokia.com]
>> Singh, Vimal wrote:
>>> From: Adrian Hunter [adrian.hunter at nokia.com]
>>>> Singh, Vimal wrote:
>>>>> From: Adrian Hunter [adrian.hunter at nokia.com]
>>>>>> Singh, Vimal wrote:
>>>>>>> There is check for 'high_memory' in 'drivers/mtd/onenand/omap2.c', always before doing 'dma_map_single'.
>>>>>>> Snippet:
>>>>>>> ----------------------
>>>>>>>         if (buf >= high_memory) {
>>>>>>>                 struct page *p1;
>>>>>>> ----------------------
>>>>>>> This check seems not sufficient. There should be a check for upper boundary too.
>>>>>>> Thinking scenario when 'buf' is less than 'high_memory', but somewhere near to it, and 'count' is big enough to beyond 'high_memory'.
>>>>>> AFAIK it is not possible to allocate memory that crosses the high_memory boundary.
>>>>> Do you mean 'buf' can not cross 'high_memory' boundary?
>>>> Yes.
>>> If so, then I wonder why above check is present in code...
>>>
>> Which check?
>>
>
> I mean, if 'buf' can cross 'high_memory' boundary then why do we nedd "if (buf >= high_memory)" check and re-manipulating 'buf' in code (in drivers/mtd/onenand/omap2.c).
>
> I am really confuse now... when this particular condition will occur.
>
>

buf can be below high_memory but in that case buf + count cannot be high_memory or above.

The addresses below high_memory are mapped differently to those above high_memory.

If you kmalloc, you get an address below high_memory.

If you vmalloc, you get an address above high_memory.

Never does a memory allocation cross above and below high_memory.

That is my understanding (on OMAP at least), but I am far from a guru, so please correct me if I am wrong.

>>>>> But then I have seen a case where it was crossing that and BUG was reported by function 'dma_cache_maint'.
>>>> Is it possible that 'buf' or 'count' is wrong?
>>> I do not think so...
>>>
>
>
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