Re: [PATCH RFC] hugetlb_cgroup: fix unbalanced css_put for shared mappings

[Mike Kravetz]

On 1/23/21 1:31 AM, Miaohe Lin wrote:
> The current implementation of hugetlb_cgroup for shared mappings could have
> different behavior. Consider the following two scenarios:
> 
> 1.Assume initial css reference count of hugetlb_cgroup is 1:
>   1.1 Call hugetlb_reserve_pages with from = 1, to = 2. So css reference
> count is 2 associated with 1 file_region.
>   1.2 Call hugetlb_reserve_pages with from = 2, to = 3. So css reference
> count is 3 associated with 2 file_region.
>   1.3 coalesce_file_region will coalesce these two file_regions into one.
> So css reference count is 3 associated with 1 file_region now.
> 
> 2.Assume initial css reference count of hugetlb_cgroup is 1 again:
>   2.1 Call hugetlb_reserve_pages with from = 1, to = 3. So css reference
> count is 2 associated with 1 file_region.
> 
> Therefore, we might have one file_region while holding one or more css
> reference counts. This inconsistency could lead to unbalanced css_put().
> If we do css_put one by one (i.g. hole punch case), scenario 2 would put
> one more css reference. If we do css_put all together (i.g. truncate case),
> scenario 1 will leak one css reference.

Sorry for the delay in replying.  This is tricky code and I needed some quiet
time to study it.

I agree that the issue described exists.  Can you describe what a user would
see in the above imbalance scenarios?  What happens if we do one too many
css_put calls?  What happens if we leak the reference and do not do the
required number of css_puts?

The code changes look correct.

I just wish this code was not so complicated.  I think the private mapping
case could be simplified to only take a single css_ref per reserve map.
However, for shared mappings we need to track each individual reservation
which adds the complexity.  I can not think of a better way to do things.

Please update commit message with an explanation of what users might see
because of this issue and resubmit as a patch.

Thanks,
-- 
Mike Kravetz

> 
> In order to fix this, we have to make sure that one file_region may hold
> and must hold one css reference. So in coalesce_file_region case, we should
> release one css reference before coalescence. Also only put css reference
> when the entire file_region is removed.
> 
> Fixes: 075a61d07a8e ("hugetlb_cgroup: add accounting for shared mappings")
> Signed-off-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxx
> ---
>  include/linux/hugetlb_cgroup.h |  6 ++++--
>  mm/hugetlb.c                   | 18 ++++++++++++++----
>  mm/hugetlb_cgroup.c            | 10 ++++++++--
>  3 files changed, 26 insertions(+), 8 deletions(-)
> 
> diff --git a/include/linux/hugetlb_cgroup.h b/include/linux/hugetlb_cgroup.h
> index 2ad6e92f124a..7610efcd96bd 100644
> --- a/include/linux/hugetlb_cgroup.h
> +++ b/include/linux/hugetlb_cgroup.h
> @@ -138,7 +138,8 @@ extern void hugetlb_cgroup_uncharge_counter(struct resv_map *resv,
>  
>  extern void hugetlb_cgroup_uncharge_file_region(struct resv_map *resv,
>  						struct file_region *rg,
> -						unsigned long nr_pages);
> +						unsigned long nr_pages,
> +						bool region_del);
>  
>  extern void hugetlb_cgroup_file_init(void) __init;
>  extern void hugetlb_cgroup_migrate(struct page *oldhpage,
> @@ -147,7 +148,8 @@ extern void hugetlb_cgroup_migrate(struct page *oldhpage,
>  #else
>  static inline void hugetlb_cgroup_uncharge_file_region(struct resv_map *resv,
>  						       struct file_region *rg,
> -						       unsigned long nr_pages)
> +						       unsigned long nr_pages,
> +						       bool region_del)
>  {
>  }
>  
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index a6bad1f686c5..777bc0e45bf3 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -298,6 +298,14 @@ static void record_hugetlb_cgroup_uncharge_info(struct hugetlb_cgroup *h_cg,
>  #endif
>  }
>  
> +static void put_uncharge_info(struct file_region *rg)
> +{
> +#ifdef CONFIG_CGROUP_HUGETLB
> +	if (rg->css)
> +		css_put(rg->css);
> +#endif
> +}
> +
>  static bool has_same_uncharge_info(struct file_region *rg,
>  				   struct file_region *org)
>  {
> @@ -321,6 +329,7 @@ static void coalesce_file_region(struct resv_map *resv, struct file_region *rg)
>  		prg->to = rg->to;
>  
>  		list_del(&rg->link);
> +		put_uncharge_info(rg);
>  		kfree(rg);
>  
>  		rg = prg;
> @@ -332,6 +341,7 @@ static void coalesce_file_region(struct resv_map *resv, struct file_region *rg)
>  		nrg->from = rg->from;
>  
>  		list_del(&rg->link);
> +		put_uncharge_info(rg);
>  		kfree(rg);
>  	}
>  }
> @@ -664,7 +674,7 @@ static long region_del(struct resv_map *resv, long f, long t)
>  
>  			del += t - f;
>  			hugetlb_cgroup_uncharge_file_region(
> -				resv, rg, t - f);
> +				resv, rg, t - f, false);
>  
>  			/* New entry for end of split region */
>  			nrg->from = t;
> @@ -685,7 +695,7 @@ static long region_del(struct resv_map *resv, long f, long t)
>  		if (f <= rg->from && t >= rg->to) { /* Remove entire region */
>  			del += rg->to - rg->from;
>  			hugetlb_cgroup_uncharge_file_region(resv, rg,
> -							    rg->to - rg->from);
> +							    rg->to - rg->from, true);
>  			list_del(&rg->link);
>  			kfree(rg);
>  			continue;
> @@ -693,13 +703,13 @@ static long region_del(struct resv_map *resv, long f, long t)
>  
>  		if (f <= rg->from) {	/* Trim beginning of region */
>  			hugetlb_cgroup_uncharge_file_region(resv, rg,
> -							    t - rg->from);
> +							    t - rg->from, false);
>  
>  			del += t - rg->from;
>  			rg->from = t;
>  		} else {		/* Trim end of region */
>  			hugetlb_cgroup_uncharge_file_region(resv, rg,
> -							    rg->to - f);
> +							    rg->to - f, false);
>  
>  			del += rg->to - f;
>  			rg->to = f;
> diff --git a/mm/hugetlb_cgroup.c b/mm/hugetlb_cgroup.c
> index 9182848dda3e..8909e075d441 100644
> --- a/mm/hugetlb_cgroup.c
> +++ b/mm/hugetlb_cgroup.c
> @@ -391,7 +391,8 @@ void hugetlb_cgroup_uncharge_counter(struct resv_map *resv, unsigned long start,
>  
>  void hugetlb_cgroup_uncharge_file_region(struct resv_map *resv,
>  					 struct file_region *rg,
> -					 unsigned long nr_pages)
> +					 unsigned long nr_pages,
> +					 bool region_del)
>  {
>  	if (hugetlb_cgroup_disabled() || !resv || !rg || !nr_pages)
>  		return;
> @@ -400,7 +401,12 @@ void hugetlb_cgroup_uncharge_file_region(struct resv_map *resv,
>  	    !resv->reservation_counter) {
>  		page_counter_uncharge(rg->reservation_counter,
>  				      nr_pages * resv->pages_per_hpage);
> -		css_put(rg->css);
> +		/*
> +		 * Only do css_put(rg->css) when we delete the entire region
> +		 * because one file_region only holds one rg->css reference.
> +		 */
> +		if (region_del)
> +			css_put(rg->css);
>  	}
>  }
>  
>