Re: [PATCH v31 05/13] mm/damon: Implement primitives for the virtual memory address spaces

[Shakeel Butt <shakeelb@google.com>]

On Mon, Jun 21, 2021 at 1:31 AM SeongJae Park <sj38.park@gmail.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 the next sampling target page and checks
> whether it is set again after one sampling period.  This could disturb
> the reclaim 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 spaces, 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>
> Reviewed-by: Fernand Sieber <sieberf@amazon.com>

Couple of nits below and one concern on the default value of
primitive_update_interval of virtual address space primitive.
Otherwise looks good to me.

[...]

> +
> +/*
> + * Size-evenly split a region into 'nr_pieces' small regions
> + *
> + * Returns 0 on success, or negative error code otherwise.
> + */
> +static int damon_va_evenly_split_region(struct damon_ctx *ctx,

I don't see ctx being used in this function.

> +               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, DAMON_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;
> +}

[...]

> +/*
> + * Get the three regions in the given target (task)
> + *
> + * Returns 0 on success, negative error code otherwise.
> + */
> +static int damon_va_three_regions(struct damon_target *t,
> +                               struct damon_addr_range regions[3])
> +{
> +       struct mm_struct *mm;
> +       int rc;
> +
> +       mm = damon_get_mm(t);
> +       if (!mm)
> +               return -EINVAL;
> +
> +       mmap_read_lock(mm);
> +       rc = __damon_va_three_regions(mm->mmap, regions);
> +       mmap_read_unlock(mm);

This is being called for each target every second by default. Seems
too aggressive. Applications don't change their address space every
second. I would recommend to default ctx->primitive_update_interval to
a higher default value.

> +
> +       mmput(mm);
> +       return rc;
> +}
> +

[...]

> +static void __damon_va_init_regions(struct damon_ctx *c,

Keep the convention of naming damon_ctx ctx.

> +                                    struct damon_target *t)
> +{
> +       struct damon_region *r;
> +       struct damon_addr_range regions[3];
> +       unsigned long sz = 0, nr_pieces;
> +       int i;
> +
> +       if (damon_va_three_regions(t, regions)) {
> +               pr_err("Failed to get three regions of target %lu\n", t->id);
> +               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 < DAMON_MIN_REGION)
> +               sz = DAMON_MIN_REGION;
> +
> +       /* Set the initial three regions of the target */
> +       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_va_evenly_split_region(c, r, nr_pieces);
> +       }
> +}

[...]

> +/*
> + * Update damon regions for the three big regions of the given target
> + *
> + * t           the given target
> + * bregions    the three big regions of the target
> + */
> +static void damon_va_apply_three_regions(struct damon_ctx *ctx,

ctx not used in this function.


> +               struct damon_target *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,
> +                                               DAMON_MIN_REGION),
> +                                       ALIGN(br->end, DAMON_MIN_REGION));
> +                       if (!newr)
> +                               continue;
> +                       damon_insert_region(newr, damon_prev_region(r), r);
> +               } else {
> +                       first->ar.start = ALIGN_DOWN(br->start,
> +                                       DAMON_MIN_REGION);
> +                       last->ar.end = ALIGN(br->end, DAMON_MIN_REGION);
> +               }
> +       }
> +}