Re: [PATCH v23 05/15] mm/damon: Implement primitives for the virtual memory address spaces

[Shakeel Butt <shakeelb@google.com>]

On Tue, Dec 15, 2020 at 3:58 AM SeongJae Park <sjpark@amazon.com> wrote:
>
> From: SeongJae Park <sjpark@amazon.de>
>
> This commit introduces a reference implementation of the address space
> specific low level primitives for the virtual address space, so that
> users of DAMON can easily monitor the data accesses on virtual address
> spaces of specific processes by simply configuring the implementation to
> be used by DAMON.
>
> The low level primitives for the fundamental access monitoring are
> defined in two parts:
>
> 1. Identification of the monitoring target address range for the address
>    space.
> 2. Access check of specific address range in the target space.
>
> The reference implementation for the virtual address space does the
> works as below.
>
> PTE Accessed-bit Based Access Check
> -----------------------------------
>
> The implementation uses PTE Accessed-bit for basic access checks.  That
> is, it clears the bit for next sampling target page and checks whether

'for the next'

> it set again after one sampling period.  This could disturb the reclaim

'it is set'

> logic.  DAMON uses ``PG_idle`` and ``PG_young`` page flags to solve the
> conflict, as Idle page tracking does.
>
> VMA-based Target Address Range Construction
> -------------------------------------------
>
> Only small parts in the super-huge virtual address space of the
> processes are mapped to physical memory and accessed.  Thus, tracking
> the unmapped address regions is just wasteful.  However, because DAMON
> can deal with some level of noise using the adaptive regions adjustment
> mechanism, tracking every mapping is not strictly required but could
> even incur a high overhead in some cases.  That said, too huge unmapped
> areas inside the monitoring target should be removed to not take the
> time for the adaptive mechanism.
>
> For the reason, this implementation converts the complex mappings to
> three distinct regions that cover every mapped area of the address
> space.  Also, the two gaps between the three regions are the two biggest
> unmapped areas in the given address space.  The two biggest unmapped
> areas would be the gap between the heap and the uppermost mmap()-ed
> region, and the gap between the lowermost mmap()-ed region and the stack
> in most of the cases.  Because these gaps are exceptionally huge in
> usual address spacees, excluding these will be sufficient to make a

*spaces

> reasonable trade-off.  Below shows this in detail::
>
>     <heap>
>     <BIG UNMAPPED REGION 1>
>     <uppermost mmap()-ed region>
>     (small mmap()-ed regions and munmap()-ed regions)
>     <lowermost mmap()-ed region>
>     <BIG UNMAPPED REGION 2>
>     <stack>
>
> Signed-off-by: SeongJae Park <sjpark@amazon.de>
> Reviewed-by: Leonard Foerster <foersleo@amazon.de>
> ---
>  include/linux/damon.h |  13 +
>  mm/damon/Kconfig      |   9 +
>  mm/damon/Makefile     |   1 +
>  mm/damon/vaddr.c      | 579 ++++++++++++++++++++++++++++++++++++++++++
>  4 files changed, 602 insertions(+)
>  create mode 100644 mm/damon/vaddr.c
>
> diff --git a/include/linux/damon.h b/include/linux/damon.h
> index f446f8433599..39b4d6d3ddee 100644
> --- a/include/linux/damon.h
> +++ b/include/linux/damon.h
> @@ -274,4 +274,17 @@ int damon_stop(struct damon_ctx **ctxs, int nr_ctxs);
>
>  #endif /* CONFIG_DAMON */
>
> +#ifdef CONFIG_DAMON_VADDR
> +
> +/* Monitoring primitives for virtual memory address spaces */
> +void damon_va_init_regions(struct damon_ctx *ctx);
> +void damon_va_update_regions(struct damon_ctx *ctx);
> +void damon_va_prepare_access_checks(struct damon_ctx *ctx);
> +unsigned int damon_va_check_accesses(struct damon_ctx *ctx);
> +bool damon_va_target_valid(void *t);
> +void damon_va_cleanup(struct damon_ctx *ctx);
> +void damon_va_set_primitives(struct damon_ctx *ctx);

Any reason for these to be in the header?

> +
[snip]
> +
> +static bool damon_va_young(struct mm_struct *mm, unsigned long addr,
> +                       unsigned long *page_sz)
> +{
> +       pte_t *pte = NULL;
> +       pmd_t *pmd = NULL;
> +       spinlock_t *ptl;
> +       bool young = false;
> +
> +       if (follow_pte_pmd(mm, addr, NULL, &pte, &pmd, &ptl))
> +               return false;
> +
> +       *page_sz = PAGE_SIZE;
> +       if (pte) {
> +               young = pte_young(*pte);
> +               if (!young)
> +                       young = !page_is_idle(pte_page(*pte));
> +               pte_unmap_unlock(pte, ptl);
> +               return young;
> +       }
> +
> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> +       young = pmd_young(*pmd);
> +       if (!young)
> +               young = !page_is_idle(pmd_page(*pmd));
> +       spin_unlock(ptl);
> +       *page_sz = ((1UL) << HPAGE_PMD_SHIFT);
> +#endif /* CONFIG_TRANSPARENT_HUGEPAGE */
> +
> +       return young;

You need mmu_notifier_test_young() here. Hmm I remember mentioning
this in some previous version as well.

BTW have you tested this on a VM?

The patch looks good overall.