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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