On Thu, Jan 9, 2025 at 12:31 AM <yangge1116@xxxxxxx> wrote:
>
> From: yangge <yangge1116@xxxxxxx>
>
> There are 4 NUMA nodes on my machine, and each NUMA node has 32GB
> of memory. I have configured 16GB of CMA memory on each NUMA node,
> and starting a 32GB virtual machine with device passthrough is
> extremely slow, taking almost an hour.
>
> During the start-up of the virtual machine, it will call
> pin_user_pages_remote(..., FOLL_LONGTERM, ...) to allocate memory.
> Long term GUP cannot allocate memory from CMA area, so a maximum of
> 16 GB of no-CMA memory on a NUMA node can be used as virtual machine
> memory. There is 16GB of free CMA memory on a NUMA node, which is
> sufficient to pass the order-0 watermark check, causing the
> __compaction_suitable() function to consistently return true.
> However, if there aren't enough migratable pages available, performing
> memory compaction is also meaningless. Besides checking whether
> the order-0 watermark is met, __compaction_suitable() also needs
> to determine whether there are sufficient migratable pages available
> for memory compaction.
>
> For costly allocations, because __compaction_suitable() always
> returns true, __alloc_pages_slowpath() can't exit at the appropriate
> place, resulting in excessively long virtual machine startup times.
> Call trace:
> __alloc_pages_slowpath
> if (compact_result == COMPACT_SKIPPED ||
> compact_result == COMPACT_DEFERRED)
> goto nopage; // should exit __alloc_pages_slowpath() from here
>
> When the 16G of non-CMA memory on a single node is exhausted, we will
> fallback to allocating memory on other nodes. In order to quickly
> fallback to remote nodes, we should skip memory compaction when
> migratable pages are insufficient. After this fix, it only takes a
> few tens of seconds to start a 32GB virtual machine with device
> passthrough functionality.
>
> Signed-off-by: yangge <yangge1116@xxxxxxx>
> ---
>
> V3:
> - fix build error
>
> V2:
> - consider unevictable folios
>
> mm/compaction.c | 20 ++++++++++++++++++++
> 1 file changed, 20 insertions(+)
>
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 07bd227..a9f1261 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -2383,7 +2383,27 @@ static bool __compaction_suitable(struct zone *zone, int order,
> int highest_zoneidx,
> unsigned long wmark_target)
> {
> + pg_data_t __maybe_unused *pgdat = zone->zone_pgdat;
> + unsigned long sum, nr_pinned;
> unsigned long watermark;
> +
> + sum = node_page_state(pgdat, NR_INACTIVE_FILE) +
> + node_page_state(pgdat, NR_INACTIVE_ANON) +
> + node_page_state(pgdat, NR_ACTIVE_FILE) +
> + node_page_state(pgdat, NR_ACTIVE_ANON) +
> + node_page_state(pgdat, NR_UNEVICTABLE);
> +
> + nr_pinned = node_page_state(pgdat, NR_FOLL_PIN_ACQUIRED) -
> + node_page_state(pgdat, NR_FOLL_PIN_RELEASED);
> +
Does the sum of all LRU pages equal non-CMA memory?
I'm quite confused for two reasons:
1. CMA pages can be LRU pages.
2. Free pages might not belong to any LRUs.
> + /*
> + * Gup-pinned pages are non-migratable. After subtracting these pages,
> + * we need to check if the remaining pages are sufficient for memory
> + * compaction.
> + */
> + if ((sum - nr_pinned) < (1 << order))
> + return false;
> +
> /*
> * Watermarks for order-0 must be met for compaction to be able to
> * isolate free pages for migration targets. This means that the
> --
> 2.7.4
>
>
Thanks
barry
