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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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?

Jan



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