On Wed, Jul 14, 2021 at 12:36 PM Joao Martins <[email protected]> wrote:
>
> Currently memmap_init_zone_device() ends up initializing 32768 pages
> when it only needs to initialize 128 given tail page reuse. That
> number is worse with 1GB compound page geometries, 262144 instead of
> 128. Update memmap_init_zone_device() to skip redundant
> initialization, detailed below.
>
> When a pgmap @geometry is set, all pages are mapped at a given huge page
> alignment and use compound pages to describe them as opposed to a
> struct per 4K.
>
> With @geometry > PAGE_SIZE and when struct pages are stored in ram
> (!altmap) most tail pages are reused. Consequently, the amount of unique
> struct pages is a lot smaller that the total amount of struct pages
> being mapped.
>
> The altmap path is left alone since it does not support memory savings
> based on compound pagemap geometries.
>
> Signed-off-by: Joao Martins <[email protected]>
> ---
> mm/page_alloc.c | 14 +++++++++++++-
> 1 file changed, 13 insertions(+), 1 deletion(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 188cb5f8c308..96975edac0a8 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -6600,11 +6600,23 @@ static void __ref __init_zone_device_page(struct page
> *page, unsigned long pfn,
> static void __ref memmap_init_compound(struct page *page, unsigned long pfn,
> unsigned long zone_idx, int nid,
> struct dev_pagemap *pgmap,
> + struct vmem_altmap *altmap,
> unsigned long nr_pages)
> {
> unsigned int order_align = order_base_2(nr_pages);
> unsigned long i;
>
> + /*
> + * With compound page geometry and when struct pages are stored in ram
> + * (!altmap) most tail pages are reused. Consequently, the amount of
> + * unique struct pages to initialize is a lot smaller that the total
> + * amount of struct pages being mapped.
> + * See vmemmap_populate_compound_pages().
> + */
> + if (!altmap)
> + nr_pages = min_t(unsigned long, nr_pages,
What's the scenario where nr_pages is < 128? Shouldn't alignment
already be guaranteed?
> + 2 * (PAGE_SIZE/sizeof(struct page)));
> +
> __SetPageHead(page);
>
> for (i = 1; i < nr_pages; i++) {
> @@ -6657,7 +6669,7 @@ void __ref memmap_init_zone_device(struct zone *zone,
> continue;
>
> memmap_init_compound(page, pfn, zone_idx, nid, pgmap,
> - pfns_per_compound);
> + altmap, pfns_per_compound);
This feels odd, memmap_init_compound() doesn't really care about
altmap, what do you think about explicitly calculating the parameters
that memmap_init_compound() needs and passing them in?
Not a strong requirement to change, but take another look at let me know.
> }
>
> pr_info("%s initialised %lu pages in %ums\n", __func__,
> --
> 2.17.1
>
