linux-kernel - Re: [RFC][Patch v12 1/2] mm: page

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Date:   Fri, 11 Oct 2019 07:02:56 -0400
From:   Nitesh Narayan Lal <nitesh@...hat.com>
To:     Alexander Duyck <alexander.duyck@...il.com>
Cc:     kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>, virtio-dev@...ts.oasis-open.org,
        Paolo Bonzini <pbonzini@...hat.com>, lcapitulino@...hat.com,
        Pankaj Gupta <pagupta@...hat.com>,
        "Wang, Wei W" <wei.w.wang@...el.com>,
        Yang Zhang <yang.zhang.wz@...il.com>,
        Rik van Riel <riel@...riel.com>,
        David Hildenbrand <david@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>, dodgen@...gle.com,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        dhildenb@...hat.com, Andrea Arcangeli <aarcange@...hat.com>,
        john.starks@...rosoft.com, Dave Hansen <dave.hansen@...el.com>,
        Michal Hocko <mhocko@...e.com>, cohuck@...hat.com
Subject: Re: [RFC][Patch v12 1/2] mm: page_reporting: core infrastructure

On 10/10/19 4:36 PM, Alexander Duyck wrote:
> On Mon, Aug 12, 2019 at 6:13 AM Nitesh Narayan Lal <nitesh@...hat.com> wrote:
> <snip>
>
>> +static int process_free_page(struct page *page,
>> +                            struct page_reporting_config *phconf, int count)
>> +{
>> +       int mt, order, ret = 0;
>> +
>> +       mt = get_pageblock_migratetype(page);
>> +       order = page_private(page);
>> +       ret = __isolate_free_page(page, order);
>> +
>> +       if (ret) {
>> +               /*
>> +                * Preserving order and migratetype for reuse while
>> +                * releasing the pages back to the buddy.
>> +                */
>> +               set_pageblock_migratetype(page, mt);
>> +               set_page_private(page, order);
>> +
>> +               sg_set_page(&phconf->sg[count++], page,
>> +                           PAGE_SIZE << order, 0);
>> +       }
>> +
>> +       return count;
>> +}
>> +
>> +/**
>> + * scan_zone_bitmap - scans the bitmap for the requested zone.
>> + * @phconf: page reporting configuration object initialized by the backend.
>> + * @zone: zone for which page reporting is requested.
>> + *
>> + * For every page marked in the bitmap it checks if it is still free if so it
>> + * isolates and adds them to a scatterlist. As soon as the number of isolated
>> + * pages reach the threshold set by the backend, they are reported to the
>> + * hypervisor by the backend. Once the hypervisor responds after processing
>> + * they are returned back to the buddy for reuse.
>> + */
>> +static void scan_zone_bitmap(struct page_reporting_config *phconf,
>> +                            struct zone *zone)
>> +{
>> +       unsigned long setbit;
>> +       struct page *page;
>> +       int count = 0;
>> +
>> +       sg_init_table(phconf->sg, phconf->max_pages);
>> +
>> +       for_each_set_bit(setbit, zone->bitmap, zone->nbits) {
>> +               /* Process only if the page is still online */
>> +               page = pfn_to_online_page((setbit << PAGE_REPORTING_MIN_ORDER) +
>> +                                         zone->base_pfn);
>> +               if (!page)
>> +                       continue;
>> +
>> +               spin_lock(&zone->lock);
>> +
>> +               /* Ensure page is still free and can be processed */
>> +               if (PageBuddy(page) && page_private(page) >=
>> +                   PAGE_REPORTING_MIN_ORDER)
>> +                       count = process_free_page(page, phconf, count);
>> +
>> +               spin_unlock(&zone->lock);
>> +               /* Page has been processed, adjust its bit and zone counter */
>> +               clear_bit(setbit, zone->bitmap);
>> +               atomic_dec(&zone->free_pages);
>> +
>> +               if (count == phconf->max_pages) {
>> +                       /* Report isolated pages to the hypervisor */
>> +                       phconf->report(phconf, count);
>> +
>> +                       /* Return processed pages back to the buddy */
>> +                       return_isolated_page(zone, phconf);
>> +
>> +                       /* Reset for next reporting */
>> +                       sg_init_table(phconf->sg, phconf->max_pages);
>> +                       count = 0;
>> +               }
>> +       }
>> +       /*
>> +        * If the number of isolated pages does not meet the max_pages
>> +        * threshold, we would still prefer to report them as we have already
>> +        * isolated them.
>> +        */
>> +       if (count) {
>> +               sg_mark_end(&phconf->sg[count - 1]);
>> +               phconf->report(phconf, count);
>> +
>> +               return_isolated_page(zone, phconf);
>> +       }
>> +}
>> +
> So one thing that occurred to me is that this code is missing checks
> so that it doesn't try to hint isolated pages. With the bitmap
> approach you need an additional check so that you aren't pulling
> isolated pages out and reporting them.

I think you mean that we should not report pages of type MIGRATE_ISOLATE.

The current code on which I am working, I have added the
is_migrate_isolate_page() check
to ensure that I am not processing these pages.

-- 
Thanks
Nitesh