Re: [PATCH] mm: Do not boost watermarks to avoid fragmentation for the DISCONTIG memory model

[Vlastimil Babka]

On 4/19/19 11:43 AM, Mel Gorman wrote:
> Mikulas Patocka reported that 1c30844d2dfe ("mm: reclaim small amounts
> of memory when an external fragmentation event occurs") "broke" memory
> management on parisc. The machine is not NUMA but the DISCONTIG model
> creates three pgdats even though it's a UMA machine for the following
> ranges
> 
>         0) Start 0x0000000000000000 End 0x000000003fffffff Size   1024 MB
>         1) Start 0x0000000100000000 End 0x00000001bfdfffff Size   3070 MB
>         2) Start 0x0000004040000000 End 0x00000040ffffffff Size   3072 MB
> 
> From his own report
> 
> 	With the patch 1c30844d2, the kernel will incorrectly reclaim the
> 	first zone when it fills up, ignoring the fact that there are two
> 	completely free zones. Basiscally, it limits cache size to 1GiB.
> 
> 	For example, if I run:
> 	# dd if=/dev/sda of=/dev/null bs=1M count=2048
> 
> 	- with the proper kernel, there should be "Buffers - 2GiB"
> 	when this command finishes. With the patch 1c30844d2, buffers
> 	will consume just 1GiB or slightly more, because the kernel was
> 	incorrectly reclaiming them.
> 
> The page allocator and reclaim makes assumptions that pgdats really
> represent NUMA nodes and zones represent ranges and makes decisions
> on that basis. Watermark boosting for small pgdats leads to unexpected
> results even though this would have behaved reasonably on SPARSEMEM.
> 
> DISCONTIG is essentially deprecated and even parisc plans to move to
> SPARSEMEM so there is no need to be fancy, this patch simply disables
> watermark boosting by default on DISCONTIGMEM.
> 
> Fixes: 1c30844d2dfe ("mm: reclaim small amounts of memory when an external fragmentation event occurs")
> Reported-by: Mikulas Patocka <mpatocka@xxxxxxxxxx>
> Tested-by: Mikulas Patocka <mpatocka@xxxxxxxxxx>
> Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

> ---
>  Documentation/sysctl/vm.txt | 16 ++++++++--------
>  mm/page_alloc.c             | 13 +++++++++++++
>  2 files changed, 21 insertions(+), 8 deletions(-)
> 
> diff --git a/Documentation/sysctl/vm.txt b/Documentation/sysctl/vm.txt
> index 6af24cdb25cc..3f13d8599337 100644
> --- a/Documentation/sysctl/vm.txt
> +++ b/Documentation/sysctl/vm.txt
> @@ -866,14 +866,14 @@ The intent is that compaction has less work to do in the future and to
>  increase the success rate of future high-order allocations such as SLUB
>  allocations, THP and hugetlbfs pages.
>  
> -To make it sensible with respect to the watermark_scale_factor parameter,
> -the unit is in fractions of 10,000. The default value of 15,000 means
> -that up to 150% of the high watermark will be reclaimed in the event of
> -a pageblock being mixed due to fragmentation. The level of reclaim is
> -determined by the number of fragmentation events that occurred in the
> -recent past. If this value is smaller than a pageblock then a pageblocks
> -worth of pages will be reclaimed (e.g.  2MB on 64-bit x86). A boost factor
> -of 0 will disable the feature.
> +To make it sensible with respect to the watermark_scale_factor
> +parameter, the unit is in fractions of 10,000. The default value of
> +15,000 on !DISCONTIGMEM configurations means that up to 150% of the high
> +watermark will be reclaimed in the event of a pageblock being mixed due
> +to fragmentation. The level of reclaim is determined by the number of
> +fragmentation events that occurred in the recent past. If this value is
> +smaller than a pageblock then a pageblocks worth of pages will be reclaimed
> +(e.g.  2MB on 64-bit x86). A boost factor of 0 will disable the feature.
>  
>  =============================================================
>  
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index cfaba3889fa2..86c3806f1070 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -266,7 +266,20 @@ compound_page_dtor * const compound_page_dtors[] = {
>  
>  int min_free_kbytes = 1024;
>  int user_min_free_kbytes = -1;
> +#ifdef CONFIG_DISCONTIGMEM
> +/*
> + * DiscontigMem defines memory ranges as separate pg_data_t even if the ranges
> + * are not on separate NUMA nodes. Functionally this works but with
> + * watermark_boost_factor, it can reclaim prematurely as the ranges can be
> + * quite small. By default, do not boost watermarks on discontigmem as in
> + * many cases very high-order allocations like THP are likely to be
> + * unsupported and the premature reclaim offsets the advantage of long-term
> + * fragmentation avoidance.
> + */
> +int watermark_boost_factor __read_mostly;
> +#else
>  int watermark_boost_factor __read_mostly = 15000;
> +#endif
>  int watermark_scale_factor = 10;
>  
>  static unsigned long nr_kernel_pages __initdata;
>