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Subject: Re: [PATCH v18 06/14] mm/damon: Implement callbacks for the virtual
 memory address spaces
From:   Greg Thelen <gthelen@google.com>
To:     SeongJae Park <sjpark@amazon.com>, akpm@linux-foundation.org
Cc:     SeongJae Park <sjpark@amazon.de>, Jonathan.Cameron@Huawei.com,
        aarcange@redhat.com, acme@kernel.org,
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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.  To avoid disturbing other
> Accessed bit users such as the reclamation logic, the implementation
> adjusts the ``PG_Idle`` and ``PG_Young`` appropriately, as same to the
> 'Idle Page Tracking'.
>
> 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 |   6 +
>  mm/damon.c            | 474 ++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 480 insertions(+)
>
> diff --git a/include/linux/damon.h b/include/linux/damon.h
> index 3c0b92a679e8..310d36d123b3 100644
> --- a/include/linux/damon.h
> +++ b/include/linux/damon.h
> @@ -144,6 +144,12 @@ struct damon_ctx {
>  	void (*aggregate_cb)(struct damon_ctx *context);
>  };
>  
> +/* Reference callback implementations for virtual memory */
> +void kdamond_init_vm_regions(struct damon_ctx *ctx);
> +void kdamond_update_vm_regions(struct damon_ctx *ctx);
> +void kdamond_prepare_vm_access_checks(struct damon_ctx *ctx);
> +unsigned int kdamond_check_vm_accesses(struct damon_ctx *ctx);
> +
>  int damon_set_pids(struct damon_ctx *ctx, int *pids, ssize_t nr_pids);
>  int damon_set_attrs(struct damon_ctx *ctx, unsigned long sample_int,
>  		unsigned long aggr_int, unsigned long regions_update_int,
> diff --git a/mm/damon.c b/mm/damon.c
> index b844924b9fdb..386780739007 100644
> --- a/mm/damon.c
> +++ b/mm/damon.c
> @@ -9,6 +9,9 @@
>   * This file is constructed in below parts.
>   *
>   * - Functions and macros for DAMON data structures
> + * - Functions for the initial monitoring target regions construction
> + * - Functions for the dynamic monitoring target regions update
> + * - Functions for the access checking of the regions
>   * - Functions for DAMON core logics and features
>   * - Functions for the DAMON programming interface
>   * - Functions for the module loading/unloading
> @@ -196,6 +199,477 @@ static unsigned long damon_region_sz_limit(struct damon_ctx *ctx)
>  	return sz;
>  }
>  
> +/*
> + * Get the mm_struct of the given task
> + *
> + * Caller _must_ put the mm_struct after use, unless it is NULL.
> + *
> + * Returns the mm_struct of the task on success, NULL on failure
> + */
> +static struct mm_struct *damon_get_mm(struct damon_task *t)
> +{
> +	struct task_struct *task;
> +	struct mm_struct *mm;
> +
> +	task = damon_get_task_struct(t);
> +	if (!task)
> +		return NULL;
> +
> +	mm = get_task_mm(task);
> +	put_task_struct(task);
> +	return mm;
> +}
> +
> +/*
> + * Functions for the initial monitoring target regions construction
> + */
> +
> +/*
> + * Size-evenly split a region into 'nr_pieces' small regions
> + *
> + * Returns 0 on success, or negative error code otherwise.
> + */
> +static int damon_split_region_evenly(struct damon_ctx *ctx,
> +		struct damon_region *r, unsigned int nr_pieces)
> +{
> +	unsigned long sz_orig, sz_piece, orig_end;
> +	struct damon_region *n = NULL, *next;
> +	unsigned long start;
> +
> +	if (!r || !nr_pieces)
> +		return -EINVAL;
> +
> +	orig_end = r->ar.end;
> +	sz_orig = r->ar.end - r->ar.start;
> +	sz_piece = ALIGN_DOWN(sz_orig / nr_pieces, MIN_REGION);
> +
> +	if (!sz_piece)
> +		return -EINVAL;
> +
> +	r->ar.end = r->ar.start + sz_piece;
> +	next = damon_next_region(r);
> +	for (start = r->ar.end; start + sz_piece <= orig_end;
> +			start += sz_piece) {
> +		n = damon_new_region(start, start + sz_piece);
> +		if (!n)
> +			return -ENOMEM;
> +		damon_insert_region(n, r, next);
> +		r = n;
> +	}
> +	/* complement last region for possible rounding error */
> +	if (n)
> +		n->ar.end = orig_end;
> +
> +	return 0;
> +}
> +
> +static unsigned long sz_range(struct damon_addr_range *r)
> +{
> +	return r->end - r->start;
> +}
> +
> +static void swap_ranges(struct damon_addr_range *r1,
> +			struct damon_addr_range *r2)
> +{
> +	struct damon_addr_range tmp;
> +
> +	tmp = *r1;
> +	*r1 = *r2;
> +	*r2 = tmp;
> +}
> +
> +/*
> + * Find three regions separated by two biggest unmapped regions
> + *
> + * vma		the head vma of the target address space
> + * regions	an array of three address ranges that results will be saved
> + *
> + * This function receives an address space and finds three regions in it which
> + * separated by the two biggest unmapped regions in the space.  Please refer to
> + * below comments of 'damon_init_vm_regions_of()' function to know why this is
> + * necessary.
> + *
> + * Returns 0 if success, or negative error code otherwise.
> + */
> +static int damon_three_regions_in_vmas(struct vm_area_struct *vma,
> +				       struct damon_addr_range regions[3])
> +{
> +	struct damon_addr_range gap = {0}, first_gap = {0}, second_gap = {0};
> +	struct vm_area_struct *last_vma = NULL;
> +	unsigned long start = 0;
> +	struct rb_root rbroot;
> +
> +	/* Find two biggest gaps so that first_gap > second_gap > others */
> +	for (; vma; vma = vma->vm_next) {
> +		if (!last_vma) {
> +			start = vma->vm_start;
> +			goto next;
> +		}
> +
> +		if (vma->rb_subtree_gap <= sz_range(&second_gap)) {
> +			rbroot.rb_node = &vma->vm_rb;
> +			vma = rb_entry(rb_last(&rbroot),
> +					struct vm_area_struct, vm_rb);
> +			goto next;
> +		}
> +
> +		gap.start = last_vma->vm_end;
> +		gap.end = vma->vm_start;
> +		if (sz_range(&gap) > sz_range(&second_gap)) {
> +			swap_ranges(&gap, &second_gap);
> +			if (sz_range(&second_gap) > sz_range(&first_gap))
> +				swap_ranges(&second_gap, &first_gap);
> +		}
> +next:
> +		last_vma = vma;
> +	}
> +
> +	if (!sz_range(&second_gap) || !sz_range(&first_gap))
> +		return -EINVAL;
> +
> +	/* Sort the two biggest gaps by address */
> +	if (first_gap.start > second_gap.start)
> +		swap_ranges(&first_gap, &second_gap);
> +
> +	/* Store the result */
> +	regions[0].start = ALIGN(start, MIN_REGION);
> +	regions[0].end = ALIGN(first_gap.start, MIN_REGION);
> +	regions[1].start = ALIGN(first_gap.end, MIN_REGION);
> +	regions[1].end = ALIGN(second_gap.start, MIN_REGION);
> +	regions[2].start = ALIGN(second_gap.end, MIN_REGION);
> +	regions[2].end = ALIGN(last_vma->vm_end, MIN_REGION);
> +
> +	return 0;
> +}
> +
> +/*
> + * Get the three regions in the given task
> + *
> + * Returns 0 on success, negative error code otherwise.
> + */
> +static int damon_three_regions_of(struct damon_task *t,
> +				struct damon_addr_range regions[3])
> +{
> +	struct mm_struct *mm;
> +	int rc;
> +
> +	mm = damon_get_mm(t);
> +	if (!mm)
> +		return -EINVAL;
> +
> +	down_read(&mm->mmap_sem);
> +	rc = damon_three_regions_in_vmas(mm->mmap, regions);
> +	up_read(&mm->mmap_sem);
> +
> +	mmput(mm);
> +	return rc;
> +}
> +
> +/*
> + * Initialize the monitoring target regions for the given task
> + *
> + * t	the given target task
> + *
> + * Because only a number of small portions of the entire address space
> + * is actually mapped to the memory and accessed, monitoring the unmapped
> + * regions is wasteful.  That said, because we can deal with small noises,
> + * tracking every mapping is not strictly required but could even incur a high
> + * overhead if the mapping frequently changes or the number of mappings is
> + * high.  The adaptive regions adjustment mechanism will further help to deal
> + * with the noise by simply identifying the unmapped areas as a region that
> + * has no access.  Moreover, applying the real mappings that would have many
> + * unmapped areas inside will make the adaptive mechanism quite complex.  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, we convert 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.  In detail, this function first identifies the start and the
> + * end of the mappings and the two biggest unmapped areas of the address space.
> + * Then, it constructs the three regions as below:
> + *
> + *     [mappings[0]->start, big_two_unmapped_areas[0]->start)
> + *     [big_two_unmapped_areas[0]->end, big_two_unmapped_areas[1]->start)
> + *     [big_two_unmapped_areas[1]->end, mappings[nr_mappings - 1]->end)
> + *
> + * As usual memory map of processes is as below, the gap between the heap and
> + * the uppermost mmap()-ed region, and the gap between the lowermost mmap()-ed
> + * region and the stack will be two biggest unmapped regions.  Because these
> + * gaps are exceptionally huge areas in usual address space, excluding these
> + * two biggest unmapped regions will be sufficient to make a trade-off.
> + *
> + *   <heap>
> + *   <BIG UNMAPPED REGION 1>
> + *   <uppermost mmap()-ed region>
> + *   (other mmap()-ed regions and small unmapped regions)
> + *   <lowermost mmap()-ed region>
> + *   <BIG UNMAPPED REGION 2>
> + *   <stack>
> + */
> +static void damon_init_vm_regions_of(struct damon_ctx *c, struct damon_task *t)
> +{
> +	struct damon_region *r;
> +	struct damon_addr_range regions[3];
> +	unsigned long sz = 0, nr_pieces;
> +	int i;
> +
> +	if (damon_three_regions_of(t, regions)) {
> +		pr_err("Failed to get three regions of task %d\n", t->pid);
> +		return;
> +	}
> +
> +	for (i = 0; i < 3; i++)
> +		sz += regions[i].end - regions[i].start;
> +	if (c->min_nr_regions)
> +		sz /= c->min_nr_regions;
> +	if (sz < MIN_REGION)
> +		sz = MIN_REGION;
> +
> +	/* Set the initial three regions of the task */
> +	for (i = 0; i < 3; i++) {
> +		r = damon_new_region(regions[i].start, regions[i].end);
> +		if (!r) {
> +			pr_err("%d'th init region creation failed\n", i);
> +			return;
> +		}
> +		damon_add_region(r, t);
> +
> +		nr_pieces = (regions[i].end - regions[i].start) / sz;
> +		damon_split_region_evenly(c, r, nr_pieces);
> +	}
> +}
> +
> +/* Initialize '->regions_list' of every task */
> +void kdamond_init_vm_regions(struct damon_ctx *ctx)
> +{
> +	struct damon_task *t;
> +
> +	damon_for_each_task(t, ctx) {
> +		/* the user may set the target regions as they want */
> +		if (!nr_damon_regions(t))
> +			damon_init_vm_regions_of(ctx, t);
> +	}
> +}
> +
> +/*
> + * Functions for the dynamic monitoring target regions update
> + */
> +
> +/*
> + * Check whether a region is intersecting an address range
> + *
> + * Returns true if it is.
> + */
> +static bool damon_intersect(struct damon_region *r, struct damon_addr_range *re)
> +{
> +	return !(r->ar.end <= re->start || re->end <= r->ar.start);
> +}
> +
> +/*
> + * Update damon regions for the three big regions of the given task
> + *
> + * t		the given task
> + * bregions	the three big regions of the task
> + */
> +static void damon_apply_three_regions(struct damon_ctx *ctx,
> +		struct damon_task *t, struct damon_addr_range bregions[3])
> +{
> +	struct damon_region *r, *next;
> +	unsigned int i = 0;
> +
> +	/* Remove regions which are not in the three big regions now */
> +	damon_for_each_region_safe(r, next, t) {
> +		for (i = 0; i < 3; i++) {
> +			if (damon_intersect(r, &bregions[i]))
> +				break;
> +		}
> +		if (i == 3)
> +			damon_destroy_region(r);
> +	}
> +
> +	/* Adjust intersecting regions to fit with the three big regions */
> +	for (i = 0; i < 3; i++) {
> +		struct damon_region *first = NULL, *last;
> +		struct damon_region *newr;
> +		struct damon_addr_range *br;
> +
> +		br = &bregions[i];
> +		/* Get the first and last regions which intersects with br */
> +		damon_for_each_region(r, t) {
> +			if (damon_intersect(r, br)) {
> +				if (!first)
> +					first = r;
> +				last = r;
> +			}
> +			if (r->ar.start >= br->end)
> +				break;
> +		}
> +		if (!first) {
> +			/* no damon_region intersects with this big region */
> +			newr = damon_new_region(
> +					ALIGN_DOWN(br->start, MIN_REGION),
> +					ALIGN(br->end, MIN_REGION));
> +			if (!newr)
> +				continue;
> +			damon_insert_region(newr, damon_prev_region(r), r);
> +		} else {
> +			first->ar.start = ALIGN_DOWN(br->start, MIN_REGION);
> +			last->ar.end = ALIGN(br->end, MIN_REGION);
> +		}
> +	}
> +}
> +
> +/*
> + * Update regions for current memory mappings
> + */
> +void kdamond_update_vm_regions(struct damon_ctx *ctx)
> +{
> +	struct damon_addr_range three_regions[3];
> +	struct damon_task *t;
> +
> +	damon_for_each_task(t, ctx) {
> +		if (damon_three_regions_of(t, three_regions))
> +			continue;
> +		damon_apply_three_regions(ctx, t, three_regions);
> +	}
> +}
> +
> +/*
> + * Functions for the access checking of the regions
> + */
> +
> +static void damon_mkold(struct mm_struct *mm, unsigned long addr)
> +{
> +	pte_t *pte = NULL;
> +	pmd_t *pmd = NULL;
> +	spinlock_t *ptl;
> +
> +	if (follow_pte_pmd(mm, addr, NULL, &pte, &pmd, &ptl))
> +		return;
> +
> +	if (pte) {
> +		if (pte_young(*pte)) {
> +			clear_page_idle(pte_page(*pte));
> +			set_page_young(pte_page(*pte));

While this compiles without support for PG_young and PG_idle, I assume
it won't work well because it'd clear pte.young without setting
PG_young.  And this would mess with vmscan.

So this code appears to depend on PG_young and PG_idle, which are
currently only available via CONFIG_IDLE_PAGE_TRACKING.  DAMON could
depend on CONFIG_IDLE_PAGE_TRACKING via Kconfig.  But I assume that
CONFIG_IDLE_PAGE_TRACKING and CONFIG_DAMON cannot be concurrently used
because they'll stomp on each other's use of pte.young, PG_young,
PG_idle.
So I suspect we want:
1. CONFIG_DAMON to depend on !CONFIG_IDLE_PAGE_TRACKING and vise-versa.
2. PG_young,PG_idle and related helpers to depend on
   CONFIG_DAMON||CONFIG_IDLE_PAGE_TRACKING.

> +		}
> +		*pte = pte_mkold(*pte);
> +		pte_unmap_unlock(pte, ptl);
> +		return;
> +	}
> +
> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> +	if (pmd_young(*pmd)) {
> +		clear_page_idle(pmd_page(*pmd));
> +		set_page_young(pmd_page(*pmd));
> +	}
> +	*pmd = pmd_mkold(*pmd);
> +	spin_unlock(ptl);
> +#endif /* CONFIG_TRANSPARENT_HUGEPAGE */
> +}
> +
> +static void damon_prepare_vm_access_check(struct damon_ctx *ctx,
> +			struct mm_struct *mm, struct damon_region *r)
> +{
> +	r->sampling_addr = damon_rand(r->ar.start, r->ar.end);
> +
> +	damon_mkold(mm, r->sampling_addr);
> +}
> +
> +void kdamond_prepare_vm_access_checks(struct damon_ctx *ctx)
> +{
> +	struct damon_task *t;
> +	struct mm_struct *mm;
> +	struct damon_region *r;
> +
> +	damon_for_each_task(t, ctx) {
> +		mm = damon_get_mm(t);
> +		if (!mm)
> +			continue;
> +		damon_for_each_region(r, t)
> +			damon_prepare_vm_access_check(ctx, mm, r);
> +		mmput(mm);
> +	}
> +}
> +
> +static bool damon_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);
> +		pte_unmap_unlock(pte, ptl);
> +		return young;
> +	}
> +
> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> +	young = pmd_young(*pmd);
> +	spin_unlock(ptl);
> +	*page_sz = ((1UL) << HPAGE_PMD_SHIFT);
> +#endif	/* CONFIG_TRANSPARENT_HUGEPAGE */
> +
> +	return young;
> +}
> +
> +/*
> + * Check whether the region was accessed after the last preparation
> + *
> + * mm	'mm_struct' for the given virtual address space
> + * r	the region to be checked
> + */
> +static void damon_check_vm_access(struct damon_ctx *ctx,
> +			       struct mm_struct *mm, struct damon_region *r)
> +{
> +	static struct mm_struct *last_mm;
> +	static unsigned long last_addr;
> +	static unsigned long last_page_sz = PAGE_SIZE;
> +	static bool last_accessed;
> +
> +	/* If the region is in the last checked page, reuse the result */
> +	if (mm == last_mm && (ALIGN_DOWN(last_addr, last_page_sz) ==
> +				ALIGN_DOWN(r->sampling_addr, last_page_sz))) {
> +		if (last_accessed)
> +			r->nr_accesses++;
> +		return;
> +	}
> +
> +	last_accessed = damon_young(mm, r->sampling_addr, &last_page_sz);
> +	if (last_accessed)
> +		r->nr_accesses++;
> +
> +	last_mm = mm;
> +	last_addr = r->sampling_addr;
> +}
> +
> +unsigned int kdamond_check_vm_accesses(struct damon_ctx *ctx)
> +{
> +	struct damon_task *t;
> +	struct mm_struct *mm;
> +	struct damon_region *r;
> +	unsigned int max_nr_accesses = 0;
> +
> +	damon_for_each_task(t, ctx) {
> +		mm = damon_get_mm(t);
> +		if (!mm)
> +			continue;
> +		damon_for_each_region(r, t) {
> +			damon_check_vm_access(ctx, mm, r);
> +			max_nr_accesses = max(r->nr_accesses, max_nr_accesses);
> +		}
> +		mmput(mm);
> +	}
> +
> +	return max_nr_accesses;
> +}
> +
>  /*
>   * Functions for DAMON core logics and features
>   */