Messages in this thread | | | From | Arnd Bergmann <> | Subject | Re: [RFC][PATCH RESEND] mm: vmalloc: remove ioremap align constraint | Date | Sun, 04 Jan 2015 17:38:06 +0100 |
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On Saturday 03 January 2015 18:59:46 Sergey Dyasly wrote: > Hi Arnd, > > First, some background information. We originally encountered high fragmentation > issue in vmalloc area: > > 1. Total size of vmalloc area was 400 MB. > 2. 200 MB of vmalloc area was consumed by ioremaps of various sizes. > 3. Largest contiguous chunk of vmalloc area was 12 MB. > 4. ioremap of 10 MB failed due to 8 MB alignment requirement.
Interesting, can you describe how you end up with that many ioremap mappings? 200MB seems like a lot. Do you perhaps get a lot of duplicate entries for the same hardware registers, or maybe a leak?
Can you send the output of /proc/vmallocinfo?
> It was decided to further increase the size of vmalloc area to resolve the above > issue. And I don't like that solution because it decreases the amount of lowmem.
If all the mappings are in fact required, have you considered using CONFIG_VMSPLIT_2G split to avoid the use of highmem?
> Now let's see how ioremap uses supersections. Judging from current implementation > of __arm_ioremap_pfn_caller: > > #if !defined(CONFIG_SMP) && !defined(CONFIG_ARM_LPAE) > if (pfn >= 0x100000 && !((paddr | size | addr) & ~SUPERSECTION_MASK)) { > remap_area_supersections(); > } else if (!((paddr | size | addr) & ~PMD_MASK)) { > remap_area_sections(); > } else > #endif > err = ioremap_page_range(); > > supersections and sections mappings are used only in !SMP && !LPAE case. > Otherwise, mapping is created using the usual 4K pages (and we are using SMP). > The suggested patch removes alignment requirements for ioremap but it means that > sections will not be used in !SMP case. So another solution is required. > > __get_vm_area_node has align parameter, maybe it can be used to specify the > required alignment of ioremap operation? Because I find current generic fls > algorithm to be very restrictive in cases when it's not necessary to use such > a big alignment.
I think using next-power-of-two alignment generally helps limit the effects of fragmentation the same way that the buddy allocator works.
Since the section and supersection maps are only used with non-SMP non-LPAE (why is that the case btw?), it would however make sense to use the default (7 + PAGE_SHIFT) instead of the ARM-specific 24 here if one of them is set, I don't see any downsides to that.
Arnd
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