Re: [Xen-devel] [PATCH v1] mm/page_alloc: fix MEMF_no_dma allocations for single NUMA

[Sergey Dyasli <sergey.dyasli@xxxxxxxxxx>]

On 07/01/2019 12:05, Jan Beulich wrote:
>>>> On 07.01.19 at 12:27, <sergey.dyasli@xxxxxxxxxx> wrote:
>> Currently dma_bitsize is zero by default on single NUMA node machines.
>> This makes all alloc_domheap_pages() calls with MEMF_no_dma return NULL.
>>
>> There is only 1 user of MEMF_no_dma: dom0_memflags, which are used
>> during memory allocation for Dom0. Failing allocation with default
>> dom0_memflags is especially severe for the PV Dom0 case: it makes
>> alloc_chunk() to use suboptimal 2MB allocation algorithm with a search
>> for higher memory addresses.
>>
>> This can lead to the NMI watchdog timeout during PV Dom0 construction
>> on some machines, which can be worked around by specifying "dma_bits"
>> in Xen's cmdline manually.
>>
>> Fix the issue by initialising dma_bitsize even on single NUMA machines.
> 
> I've not yet looked at why exactly this was done for multi-node
> systems only, but in any event this change renders somewhat
> stale the comment next to the dma_bitsize definition.
> 
>> --- a/xen/common/page_alloc.c
>> +++ b/xen/common/page_alloc.c
>> @@ -1863,7 +1863,7 @@ void __init end_boot_allocator(void)
>>      nr_bootmem_regions = 0;
>>      init_heap_pages(virt_to_page(bootmem_region_list), 1);
>>  
>> -    if ( !dma_bitsize && (num_online_nodes() > 1) )
>> +    if ( !dma_bitsize )
>>          dma_bitsize = arch_get_dma_bitsize();
> 
> Did you consider the alternative of leaving this code alone and
> instead doing
> 
>     if ( !dma_bitsize )
>         memflags &= ~MEMF_no_dma;
>     else if ( (dma_zone = bits_to_zone(dma_bitsize)) < zone_hi )
>         pg = alloc_heap_pages(dma_zone + 1, zone_hi, order, memflags, d);
> 
> in alloc_domheap_pages(), which would also address the same
> issue in the case of arch_get_dma_bitsize() returning zero?

I like this suggestion. There is no point in MEMF_no_dma if dma_zone
doesn't exist, so the flag can be ignored. The allocator should first
use higher memory addresses for memory allocation requests, and DMA (<4G)
memory shouldn't end up allocated for Dom0 RAM (given there is enough
mem >4G).

I'll prepare a v2 soon.

Thanks,
Sergey

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