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Subject: Re: [PATCH v5 4/7] hugetlb: disable region_add file_region coalescing
To:     Mina Almasry <almasrymina@google.com>
Cc:     shuah@kernel.org, rientjes@google.com, shakeelb@google.com,
        gthelen@google.com, akpm@linux-foundation.org,
        khalid.aziz@oracle.com, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, linux-kselftest@vger.kernel.org,
        cgroups@vger.kernel.org, aneesh.kumar@linux.vnet.ibm.com,
        mkoutny@suse.com
References: <20190919222421.27408-1-almasrymina@google.com>
 <20190919222421.27408-5-almasrymina@google.com>
From:   Mike Kravetz <mike.kravetz@oracle.com>
Message-ID: <62a2a742-1735-7272-3c6c-213efc7adb9f@oracle.com>
Date:   Fri, 27 Sep 2019 14:44:33 -0700
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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