Re: [PATCH v1 09/11] mm/page_alloc: reuse tail struct pages for compound pagemaps

[Joao Martins <joao.m.martins@oracle.com>]

On 6/7/21 8:32 PM, Dan Williams wrote:
> On Mon, Jun 7, 2021 at 6:49 AM Joao Martins <joao.m.martins@oracle.com> wrote:
> [..]
>>> Given all of the above I'm wondering if there should be a new
>>> "compound specific" flavor of this routine rather than trying to
>>> cleverly inter mingle the old path with the new. This is easier
>>> because you have already done the work in previous patches to break
>>> this into helpers. So just have memmap_init_zone_device() do it the
>>> "old" way and memmap_init_compound() handle all the tail page init +
>>> optimizations.
>>>
>> I can separate it out, should be easier to follow.
>>
>> Albeit just a note, I think memmap_init_compound() should be the new normal as metadata
>> more accurately represents what goes on the page tables. That's regardless of
>> vmemmap-based gains, and hence why my train of thought was to not separate it.
>>
>> After this series, all devmap pages where @geometry matches @align will have compound
>> pages be used instead. And we enable that in device-dax as first user (in the next patch).
>> altmap or not so far just differentiates on the uniqueness of struct pages as the former
>> doesn't reuse base pages that only contain tail pages and consequently makes us initialize
>> all tail struct pages.
> 
> I think combining the cases into a common central routine makes the
> code that much harder to maintain. A small duplication cost is worth
> it in my opinion to help readability / maintainability.
> 
I am addressing this comment and taking a step back. By just moving the tail page init to
memmap_init_compound() this gets a lot more readable. Albeit now I think having separate
top-level loops over pfns, doesn't bring much improvement there.

Here's what I have by moving just tails init to a separate routine. See your original
suggestion after the scissors mark. I have a slight inclination towards the first one, but
no really strong preference. Thoughts?

[...]

static void __ref memmap_init_compound(struct page *page, unsigned long pfn,
                                        unsigned long zone_idx, int nid,
                                        struct dev_pagemap *pgmap,
                                        unsigned long nr_pages)
{
        unsigned int order_align = order_base_2(nr_pages);
        unsigned long i;

        __SetPageHead(page);

        for (i = 1; i < nr_pages; i++) {
                __init_zone_device_page(page + i, pfn + i, zone_idx,
                                        nid, pgmap);
                prep_compound_tail(page, i);

                /*
                 * The first and second tail pages need to
                 * initialized first, hence the head page is
                 * prepared last.
                 */
                if (i == 2)
                        prep_compound_head(page, order_align);
        }
}

void __ref memmap_init_zone_device(struct zone *zone,
                                   unsigned long start_pfn,
                                   unsigned long nr_pages,
                                   struct dev_pagemap *pgmap)
{
        unsigned long pfn, end_pfn = start_pfn + nr_pages;
        struct pglist_data *pgdat = zone->zone_pgdat;
        struct vmem_altmap *altmap = pgmap_altmap(pgmap);
        unsigned long pfns_per_compound = pgmap_pfn_geometry(pgmap);
        unsigned long zone_idx = zone_idx(zone);
        unsigned long start = jiffies;
        int nid = pgdat->node_id;

        if (WARN_ON_ONCE(!pgmap || zone_idx(zone) != ZONE_DEVICE))
                return;

        /*
         * The call to memmap_init_zone should have already taken care
         * of the pages reserved for the memmap, so we can just jump to
         * the end of that region and start processing the device pages.
         */
        if (altmap) {
                start_pfn = altmap->base_pfn + vmem_altmap_offset(altmap);
                nr_pages = end_pfn - start_pfn;
        }

        for (pfn = start_pfn; pfn < end_pfn; pfn += pfns_per_compound) {
                struct page *page = pfn_to_page(pfn);

                __init_zone_device_page(page, pfn, zone_idx, nid, pgmap);

                if (pfns_per_compound == 1)
                        continue;

                memmap_init_compound(page, pfn, zone_idx, nid, pgmap,
                                     pfns_per_compound);
        }

        pr_info("%s initialised %lu pages in %ums\n", __func__,
                nr_pages, jiffies_to_msecs(jiffies - start));
}


[...]

--->8-----
Whereas your original suggestion would look more like this:

[...]

static void __ref memmap_init_compound(unsigned long zone_idx, int nid,
                                        struct dev_pagemap *pgmap,
                                        unsigned long start_pfn,
                                        unsigned long end_pfn)
{
        unsigned long pfns_per_compound = pgmap_pfn_geometry(pgmap);
        unsigned int order_align = order_base_2(pfns_per_compound);
        unsigned long i;

        for (pfn = start_pfn; pfn < end_pfn; pfn += pfns_per_compound) {
                struct page *page = pfn_to_page(pfn);

                __init_zone_device_page(page, pfn, zone_idx, nid, pgmap);

                __SetPageHead(page);

                for (i = 1; i < pfns_per_compound; i++) {
                        __init_zone_device_page(page + i, pfn + i, zone_idx,
                                                nid, pgmap);
                        prep_compound_tail(page, i);

                        /*
                         * The first and second tail pages need to
                         * initialized first, hence the head page is
                         * prepared last.
                         */
                        if (i == 2)
                                prep_compound_head(page, order_align);
                }
        }
}

static void __ref memmap_init_base(unsigned long zone_idx, int nid,
                                         struct dev_pagemap *pgmap,
                                         unsigned long start_pfn,
                                         unsigned long end_pfn)
{
        for (pfn = start_pfn; pfn < end_pfn; pfn++) {
                struct page *page = pfn_to_page(pfn);

                __init_zone_device_page(page, pfn, zone_idx, nid, pgmap);
        }
}

void __ref memmap_init_zone_device(struct zone *zone,
                                   unsigned long start_pfn,
                                   unsigned long nr_pages,
                                   struct dev_pagemap *pgmap)
{
        unsigned long pfn, end_pfn = start_pfn + nr_pages;
        struct pglist_data *pgdat = zone->zone_pgdat;
        struct vmem_altmap *altmap = pgmap_altmap(pgmap);
        bool compound = pgmap_geometry(pgmap) > PAGE_SIZE;
        unsigned long zone_idx = zone_idx(zone);
        unsigned long start = jiffies;
        int nid = pgdat->node_id;

        if (WARN_ON_ONCE(!pgmap || zone_idx(zone) != ZONE_DEVICE))
                return;

        /*
         * The call to memmap_init_zone should have already taken care
         * of the pages reserved for the memmap, so we can just jump to
         * the end of that region and start processing the device pages.
         */
        if (altmap) {
                start_pfn = altmap->base_pfn + vmem_altmap_offset(altmap);
                nr_pages = end_pfn - start_pfn;
        }

        if (compound)
                memmap_init_compound(zone_idx, nid, pgmap, start_pfn, end_pfn);
        else
                memmap_init_base(zone_idx, nid, pgmap, start_pfn, end_pfn);

        pr_info("%s initialised %lu pages in %ums\n", __func__,
                nr_pages, jiffies_to_msecs(jiffies - start));
}