Re: [RFC v7 06/10] mm/damon: Implement callbacks for physical memory monitoring

[Shakeel Butt <shakeelb@google.com>]

On Tue, Aug 18, 2020 at 12:27 AM SeongJae Park <sjpark@amazon.com> wrote:
>
> From: SeongJae Park <sjpark@amazon.de>
>
> This commit implements the four callbacks (->init_target_regions,
> ->update_target_regions, ->prepare_access_check, and ->check_accesses)
> for the basic access monitoring of the physical memory address space.
> By setting the callback pointers to point those, users can easily
> monitor the accesses to the physical memory.
>
> Internally, it uses the PTE Accessed bit, as similar to that of the
> virtual memory support.  Also, it supports only user memory pages, as
> idle page tracking also does, for the same reason.  If the monitoring
> target physical memory address range contains non-user memory pages,
> access check of the pages will do nothing but simply treat the pages as
> not accessed.
>
> Users who want to use other access check primitives and/or monitor the
> non-user memory regions could implement and use their own callbacks.
>
> Signed-off-by: SeongJae Park <sjpark@amazon.de>
[snip]
> +static void damon_phys_mkold(unsigned long paddr)
> +{
> +       struct page *page = damon_phys_get_page(PHYS_PFN(paddr));
> +       struct rmap_walk_control rwc = {
> +               .rmap_one = damon_page_mkold,
> +               .anon_lock = page_lock_anon_vma_read,
> +       };
> +       bool need_lock;
> +
> +       if (!page)
> +               return;
> +
> +       if (!page_mapped(page) || !page_rmapping(page))
> +               return;

I don't think you want to skip the unmapped pages. The point of
physical address space monitoring was to include the monitoring of
unmapped pages, so, skipping them invalidates the underlying
motivation.

> +
> +       need_lock = !PageAnon(page) || PageKsm(page);
> +       if (need_lock && !trylock_page(page))
> +               return;
> +
> +       rmap_walk(page, &rwc);
> +
> +       if (need_lock)
> +               unlock_page(page);
> +       put_page(page);
> +}
> +
[snip]
> +
> +static bool damon_phys_young(unsigned long paddr, unsigned long *page_sz)
> +{
> +       struct page *page = damon_phys_get_page(PHYS_PFN(paddr));
> +       struct damon_phys_access_chk_result result = {
> +               .page_sz = PAGE_SIZE,
> +               .accessed = false,
> +       };
> +       struct rmap_walk_control rwc = {
> +               .arg = &result,
> +               .rmap_one = damon_page_accessed,
> +               .anon_lock = page_lock_anon_vma_read,
> +       };
> +       bool need_lock;
> +
> +       if (!page)
> +               return false;
> +
> +       if (!page_mapped(page) || !page_rmapping(page))
> +               return false;

Same here.

> +
> +       need_lock = !PageAnon(page) || PageKsm(page);
> +       if (need_lock && !trylock_page(page))
> +               return false;
> +
> +       rmap_walk(page, &rwc);
> +
> +       if (need_lock)
> +               unlock_page(page);
> +       put_page(page);
> +
> +       *page_sz = result.page_sz;
> +       return result.accessed;
> +}
> +