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SubjectRe: [PATCH v5 4/7] hugetlb: disable region_add file_region coalescing
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On 9/19/19 3:24 PM, Mina Almasry wrote:
> A follow up patch in this series adds hugetlb cgroup uncharge info the
> file_region entries in resv->regions. The cgroup uncharge info may
> differ for different regions, so they can no longer be coalesced at
> region_add time. So, disable region coalescing in region_add in this
> patch.
>
> Behavior change:
>
> Say a resv_map exists like this [0->1], [2->3], and [5->6].
>
> Then a region_chg/add call comes in region_chg/add(f=0, t=5).
>
> Old code would generate resv->regions: [0->5], [5->6].
> New code would generate resv->regions: [0->1], [1->2], [2->3], [3->5],
> [5->6].
>
> Special care needs to be taken to handle the resv->adds_in_progress
> variable correctly. In the past, only 1 region would be added for every
> region_chg and region_add call. But now, each call may add multiple
> regions, so we can no longer increment adds_in_progress by 1 in region_chg,
> or decrement adds_in_progress by 1 after region_add or region_abort. Instead,
> region_chg calls add_reservation_in_range() to count the number of regions
> needed and allocates those, and that info is passed to region_add and
> region_abort to decrement adds_in_progress correctly.
>
> Signed-off-by: Mina Almasry <almasrymina@google.com>
>
> ---
> mm/hugetlb.c | 273 +++++++++++++++++++++++++++++----------------------
> 1 file changed, 158 insertions(+), 115 deletions(-)
>
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index bac1cbdd027c..d03b048084a3 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -244,6 +244,12 @@ struct file_region {
> long to;
> };
>
> +/* Helper that removes a struct file_region from the resv_map cache and returns
> + * it for use.
> + */
> +static struct file_region *
> +get_file_region_entry_from_cache(struct resv_map *resv, long from, long to);
> +

Instead of the forward declaration, just put the function here.

> /* Must be called with resv->lock held. Calling this with count_only == true
> * will count the number of pages to be added but will not modify the linked
> * list.
> @@ -251,51 +257,61 @@ struct file_region {
> static long add_reservation_in_range(struct resv_map *resv, long f, long t,
> bool count_only)
> {
> - long chg = 0;
> + long add = 0;
> struct list_head *head = &resv->regions;
> + long last_accounted_offset = f;
> struct file_region *rg = NULL, *trg = NULL, *nrg = NULL;
>
> - /* Locate the region we are before or in. */
> - list_for_each_entry (rg, head, link)
> - if (f <= rg->to)
> - break;
> -
> - /* Round our left edge to the current segment if it encloses us. */
> - if (f > rg->from)
> - f = rg->from;
> -
> - chg = t - f;
> + /* In this loop, we essentially handle an entry for the range
> + * last_accounted_offset -> rg->from, at every iteration, with some
> + * bounds checking.
> + */
> + list_for_each_entry_safe(rg, trg, head, link) {
> + /* Skip irrelevant regions that start before our range. */
> + if (rg->from < f) {
> + /* If this region ends after the last accounted offset,
> + * then we need to update last_accounted_offset.
> + */
> + if (rg->to > last_accounted_offset)
> + last_accounted_offset = rg->to;
> + continue;
> + }
>
> - /* Check for and consume any regions we now overlap with. */
> - nrg = rg;
> - list_for_each_entry_safe (rg, trg, rg->link.prev, link) {
> - if (&rg->link == head)
> - break;
> + /* When we find a region that starts beyond our range, we've
> + * finished.
> + */
> if (rg->from > t)
> break;
>
> - /* We overlap with this area, if it extends further than
> - * us then we must extend ourselves. Account for its
> - * existing reservation.
> + /* Add an entry for last_accounted_offset -> rg->from, and
> + * update last_accounted_offset.
> */
> - if (rg->to > t) {
> - chg += rg->to - t;
> - t = rg->to;
> + if (rg->from > last_accounted_offset) {
> + add += rg->from - last_accounted_offset;
> + if (!count_only) {
> + nrg = get_file_region_entry_from_cache(
> + resv, last_accounted_offset, rg->from);
> + list_add(&nrg->link, rg->link.prev);
> + }
> }
> - chg -= rg->to - rg->from;
>
> - if (!count_only && rg != nrg) {
> - list_del(&rg->link);
> - kfree(rg);
> - }
> + last_accounted_offset = rg->to;
> }
>
> - if (!count_only) {
> - nrg->from = f;
> - nrg->to = t;
> + /* Handle the case where our range extends beyond
> + * last_accounted_offset.
> + */
> + if (last_accounted_offset < t) {
> + add += t - last_accounted_offset;
> + if (!count_only) {
> + nrg = get_file_region_entry_from_cache(
> + resv, last_accounted_offset, t);
> + list_add(&nrg->link, rg->link.prev);
> + }
> + last_accounted_offset = t;
> }
>
> - return chg;
> + return add;
> }
>
> /*
> @@ -305,46 +321,24 @@ static long add_reservation_in_range(struct resv_map *resv, long f, long t,

The start of this comment block says,

/*
* Add the huge page range represented by [f, t) to the reserve
* map. Existing regions will be expanded to accommodate the specified
* range, or a region will be taken from the cache.

We are no longer expanding existing regions. Correct?
As an optimization, I guess we could coalesce/combine reion entries as
long as they are for the same cgroup. However, it may not be worth the
effort.

> * must exist in the cache due to the previous call to region_chg with
> * the same range.
> *
> + * regions_needed is the out value provided by a previous
> + * call to region_chg.
> + *
> * Return the number of new huge pages added to the map. This
> * number is greater than or equal to zero.
> */
> -static long region_add(struct resv_map *resv, long f, long t)
> +static long region_add(struct resv_map *resv, long f, long t,
> + long regions_needed)
> {
> - struct list_head *head = &resv->regions;
> - struct file_region *rg, *nrg;
> long add = 0;
>
> spin_lock(&resv->lock);
> - /* Locate the region we are either in or before. */
> - list_for_each_entry(rg, head, link)
> - if (f <= rg->to)
> - break;
>
> - /*
> - * If no region exists which can be expanded to include the
> - * specified range, pull a region descriptor from the cache
> - * and use it for this range.
> - */
> - if (&rg->link == head || t < rg->from) {
> - VM_BUG_ON(resv->region_cache_count <= 0);
> -
> - resv->region_cache_count--;
> - nrg = list_first_entry(&resv->region_cache, struct file_region,
> - link);
> - list_del(&nrg->link);
> -
> - nrg->from = f;
> - nrg->to = t;
> - list_add(&nrg->link, rg->link.prev);
> -
> - add += t - f;
> - goto out_locked;
> - }
> + VM_BUG_ON(resv->region_cache_count < regions_needed);
>
> add = add_reservation_in_range(resv, f, t, false);
> + resv->adds_in_progress -= regions_needed;

Consider this example,

- region_chg(1,2)
adds_in_progress = 1
cache entries 1
- region_chg(3,4)
adds_in_progress = 2
cache entries 2
- region_chg(5,6)
adds_in_progress = 3
cache entries 3

At this point, no region descriptors are in the map because only
region_chg has been called.

- region_chg(0,6)
adds_in_progress = 4
cache entries 4

Is that correct so far?

Then the following sequence happens,

- region_add(1,2)
adds_in_progress = 3
cache entries 3
- region_add(3,4)
adds_in_progress = 2
cache entries 2
- region_add(5,6)
adds_in_progress = 1
cache entries 1

list of region descriptors is:
[1->2] [3->4] [5->6]

- region_add(0,6)
This is going to require 3 cache entries but only one is in the cache.
I think we are going to BUG in get_file_region_entry_from_cache() the
second time it is called from add_reservation_in_range().

I stopped looking at the code here as things will need to change if this
is a real issue.
--
Mike Kravetz

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