Re: [PATCH v22 06/18] mm/damon: Implement primitives for the virtual memory address spaces

[Shakeel Butt <shakeelb@google.com>]

On Tue, Oct 20, 2020 at 2:06 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 provided by
> this commit is designed 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
> it set again after one sampling period.  This could disturb other kernel
> subsystems using the Accessed bits, namely Idle page tracking and the
> reclaim logic.  To avoid such disturbances, DAMON makes it mutually
> exclusive with Idle page tracking and uses ``PG_idle`` and ``PG_young``
> page flags to solve the conflict with the reclaim logics, 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
> 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 |  14 +
>  mm/damon/Kconfig      |  10 +
>  mm/damon/Makefile     |   1 +
>  mm/damon/primitives.c | 582 ++++++++++++++++++++++++++++++++++++++++++
>  4 files changed, 607 insertions(+)
>  create mode 100644 mm/damon/primitives.c
>
> diff --git a/include/linux/damon.h b/include/linux/damon.h
> index b8562814751e..70cc4b54212e 100644
> --- a/include/linux/damon.h
> +++ b/include/linux/damon.h
> @@ -238,4 +238,18 @@ int damon_stop(struct damon_ctx **ctxs, int nr_ctxs);
>
>  #endif /* CONFIG_DAMON */
>
> +#ifdef CONFIG_DAMON_PRIMITIVES
> +
> +/* Reference callback implementations for virtual memory */
> +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(struct damon_target *t);
> +void damon_va_cleanup(struct damon_ctx *ctx);
> +void damon_va_set_primitives(struct damon_ctx *ctx);
> +
> +#endif /* CONFIG_DAMON_PRIMITIVES */
> +
> +
>  #endif
> diff --git a/mm/damon/Kconfig b/mm/damon/Kconfig
> index d00e99ac1a15..0d2a18ddb9d8 100644
> --- a/mm/damon/Kconfig
> +++ b/mm/damon/Kconfig
> @@ -12,4 +12,14 @@ config DAMON
>           See https://damonitor.github.io/doc/html/latest-damon/index.html for
>           more information.
>
> +config DAMON_PRIMITIVES

I would rather name this base on virtual address space monitoring
maybe like DAMON_VMA_OPTIMIZED_MONITORING or something similar.
PRIMITIVES does not seem like a good name for this.

> +       bool "Monitoring primitives for virtual address spaces monitoring"
> +       depends on DAMON && MMU && !IDLE_PAGE_TRACKING
> +       select PAGE_EXTENSION if !64BIT
> +       select PAGE_IDLE_FLAG
> +       help
> +         This builds the default data access monitoring primitives for DAMON.
> +         The primitives support only virtual address spaces.  If this cannot
> +         cover your use case, you can implement and use your own primitives.
> +
>  endmenu
> diff --git a/mm/damon/Makefile b/mm/damon/Makefile
> index 4fd2edb4becf..2f3235a52e5e 100644
> --- a/mm/damon/Makefile
> +++ b/mm/damon/Makefile
> @@ -1,3 +1,4 @@
>  # SPDX-License-Identifier: GPL-2.0
>
>  obj-$(CONFIG_DAMON)            := core.o
> +obj-$(CONFIG_DAMON_PRIMITIVES) += primitives.o
> diff --git a/mm/damon/primitives.c b/mm/damon/primitives.c
> new file mode 100644
> index 000000000000..9b603ac0077c
> --- /dev/null
> +++ b/mm/damon/primitives.c

Same with the filename.

> @@ -0,0 +1,582 @@
> +// SPDX-License-Identifier: GPL-2.0
[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;

Don't you need mmu_notifier_clear_young() here?

I am still looking more into this patch