Messages in this thread | | | Date | Fri, 07 Jul 2006 10:01:11 +0200 | Subject | Re: [PATCH 1/1] Only use ARCH_PFN_OFFSET once during boot | From | Franck Bui-Huu <> |
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Mel Gorman wrote: > On Thu, 6 Jul 2006, Franck Bui-Huu wrote: > >> 2006/7/6, Mel Gorman <mel@csn.ul.ie>: >>> I think my patch does the job of moving ARCH_PFN_OFFSET out of the hot >>> path in a less risky fashion. However, if you are sure that callers to >>> free_area_init() and ARCH_PFN_OFFSET are ok after your patch, I'd be happy >>> to go with it. If you're not sure, I reckon my patch would be the way to >>> go. >>> >> Ok I try to explain better what I have in mind. Your patch changes the >> behaviour of free_area_init_node() in the sense that it doesn't work >> as expected if its fourth parameter is different from ARCH_PFN_OFFSET, >> it even becomes boggus IMHO. And I think it's valid to use it when >> FLATMEM model is selected. > > I'm missing something silly here. > > Before the patch, we have the following > o Call free_area_initSOMETHING() > o Set mem_map to NODE_DATA(0)->node_mem_map > o At each call to page_to_pfn() or pfn_to_page(), offset mem_map by > ARCH_PFN_OFFSET > > After the patch, we have > > o Call free_area_initSOMETHING() > o Set mem_map to NODE_DATA(0)->node_mem_map - ARCH_PFN_OFFSET > o At each call to page_to_pfn() or pfn_to_page(), use mem_map without > any additional offset > > I don't see how free_area_init_node() changed except for callers > using mem_map directly. >
you're right the behaviour is the same with the old code and with your patch that is:
If CONFIG_FLATMEM then free_area_init_node must be called:
free_area_init_node(..., ..., ..., ARCH_PFN_OFFSET, ...);
And it's quite dangerous because a user of this function must know the implementation of pfn_to_page() or alloc_node_mem_map() to know that.
Therefore, what I proposed was to let free_area_init_node() work as expected, so whatever the value of ARCH_PFN_OFFSET, this use
free_area_init_node(..., ..., ..., whatever, ...);
will define the start of mem as 'whatever' value. And if the user wants to use the default start mem value then he can do both:
free_area_init_node(..., ..., ..., ARCH_PFN_OFFSET, ...);
or (equivalent):
free_area_init(...);
> .... > > using mem_map directly. uh uh > > Both of our patches are broken. > > page_to_pfn() and pfn_to_page() both need ARCH_PFN_OFFSET to get PFNs, > that's fine. However, I forgot that another assumption of the FLATMEM memory > model is that mem_map[0] is the first valid struct page in the system. A
I would say that the first valid struct page in the system is
mem_map[PFN_UP(__pa(PAGE_OFFSET))] == mem_map[ARCH_PFN_OFFSET]
> number of architectures walk mem_map[] directly (cris and frv are examples) > without offsetting based on this assumption. >
but they do have ARCH_PFN_OFFSET = 0, no ?
Walking mem_map[] directly should be avoid.
If the mem start is different from 0 and ARCH_PFN_OFFSET is not set then all patches are correct and mem_map[0] is valid.
But if the user set ARCH_PFN_OFFSET != 0, he tells to the system that the start of memory is not 0, and mem_map[0] is now forbidden since no page exist in this area. BTW the problem exists with the old code, if the user do pfn_to_page(0), he will get an invalid page pointer.
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