Re: [PATCH v2] hugetlb_cgroup: fix imbalanced css_get and css_put pair for shared mappings

[Mike Kravetz]

On 3/1/21 4:05 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 imbalanced css_get()
> and css_put() pair. 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.
> 
> The imbalanced css_get() and css_put() pair would result in a non-zero
> reference when we try to destroy the hugetlb cgroup. The hugetlb cgroup
> directory is removed __but__ associated resource is not freed. This might
> result in OOM or can not create a new hugetlb cgroup in a busy workload
> ultimately.
> 
> In order to fix this, we have to make sure that one file_region must hold
> and only hold one css reference. So in coalesce_file_region case, we should

I think this would sound/read better if stated as:
... one file_region must hold exactly one css reference ...

This phrase is repeated in comments throughout the patch.

> release one css reference before coalescence. Also only put css reference
> when the entire file_region is removed.
> 
> The last thing to note is that the caller of region_add() will only hold
> one reference to h_cg->css for the whole contiguous reservation region.
> But this area might be scattered when there are already some file_regions
> reside in it. As a result, many file_regions may share only one h_cg->css
> reference. In order to ensure that one file_region must hold and only hold
> one h_cg->css reference, we should do css_get() for each file_region and
> release the reference held by caller when they are done.

Thanks for adding this to the commit message.

> 
> Fixes: 075a61d07a8e ("hugetlb_cgroup: add accounting for shared mappings")
> Reported-by: kernel test robot <lkp@xxxxxxxxx> (auto build test ERROR)
> Signed-off-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxx
> ---
> v1->v2:
> 	Fix auto build test ERROR when CGROUP_HUGETLB is disabled.
> ---
>  include/linux/hugetlb_cgroup.h | 15 ++++++++++--
>  mm/hugetlb.c                   | 42 ++++++++++++++++++++++++++++++----
>  mm/hugetlb_cgroup.c            | 11 +++++++--
>  3 files changed, 60 insertions(+), 8 deletions(-)

Just a few minor nits below, all in comments.  It is not required, but
would be nice to update these.  Code looks good.

Reviewed-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>

> 
> diff --git a/include/linux/hugetlb_cgroup.h b/include/linux/hugetlb_cgroup.h
> index 2ad6e92f124a..0bff345c4bc6 100644
> --- a/include/linux/hugetlb_cgroup.h
> +++ b/include/linux/hugetlb_cgroup.h
> @@ -113,6 +113,11 @@ static inline bool hugetlb_cgroup_disabled(void)
>  	return !cgroup_subsys_enabled(hugetlb_cgrp_subsys);
>  }
>  
> +static inline void hugetlb_cgroup_put_rsvd_cgroup(struct hugetlb_cgroup *h_cg)
> +{
> +	css_put(&h_cg->css);
> +}
> +
>  extern int hugetlb_cgroup_charge_cgroup(int idx, unsigned long nr_pages,
>  					struct hugetlb_cgroup **ptr);
>  extern int hugetlb_cgroup_charge_cgroup_rsvd(int idx, unsigned long nr_pages,
> @@ -138,7 +143,8 @@ 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 +153,8 @@ 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)
>  {
>  }
>  
> @@ -185,6 +192,10 @@ static inline bool hugetlb_cgroup_disabled(void)
>  	return true;
>  }
>  
> +static inline void hugetlb_cgroup_put_rsvd_cgroup(struct hugetlb_cgroup *h_cg)
> +{
> +}
> +
>  static inline int hugetlb_cgroup_charge_cgroup(int idx, unsigned long nr_pages,
>  					       struct hugetlb_cgroup **ptr)
>  {
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 8fb42c6dd74b..87db91dff47f 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -280,6 +280,18 @@ static void record_hugetlb_cgroup_uncharge_info(struct hugetlb_cgroup *h_cg,
>  		nrg->reservation_counter =
>  			&h_cg->rsvd_hugepage[hstate_index(h)];
>  		nrg->css = &h_cg->css;
> +		/*
> +		 * The caller (hugetlb_reserve_pages now) will only hold one

This can be called from other places such as alloc_huge_page.  Correct?
If so, we should drop the mention of hugetlb_reserve_pages.

> +		 * h_cg->css reference for the whole contiguous reservation
> +		 * region. But this area might be scattered when there are
> +		 * already some file_regions reside in it. As a result, many
> +		 * file_regions may share only one h_cg->css reference. In
> +		 * order to ensure that one file_region must hold and only
> +		 * hold one h_cg->css reference, we should do css_get for

... must hold exactly one ...

> +		 * each file_region and leave the reference held by caller
> +		 * untouched.
> +		 */
> +		css_get(&h_cg->css);
>  		if (!resv->pages_per_hpage)
>  			resv->pages_per_hpage = pages_per_huge_page(h);
>  		/* pages_per_hpage should be the same for all entries in
> @@ -293,6 +305,14 @@ 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)
>  {
> @@ -316,6 +336,7 @@ 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;
> @@ -327,6 +348,7 @@ 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);
>  	}
>  }
> @@ -659,7 +681,7 @@ 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;
> @@ -680,7 +702,7 @@ 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;
> @@ -688,13 +710,13 @@ 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;
> @@ -5128,6 +5150,10 @@ bool hugetlb_reserve_pages(struct inode *inode,
>  			 */
>  			long rsv_adjust;
>  
> +			/*
> +			 * hugetlb_cgroup_uncharge_cgroup_rsvd() will put the
> +			 * reference to h_cg->css. See comment below for detail.
> +			 */
>  			hugetlb_cgroup_uncharge_cgroup_rsvd(
>  				hstate_index(h),
>  				(chg - add) * pages_per_huge_page(h), h_cg);
> @@ -5135,6 +5161,14 @@ bool hugetlb_reserve_pages(struct inode *inode,
>  			rsv_adjust = hugepage_subpool_put_pages(spool,
>  								chg - add);
>  			hugetlb_acct_memory(h, -rsv_adjust);
> +		} else if (h_cg) {
> +			/*
> +			 * The file_regions will hold their own reference to
> +			 * h_cg->css. So we should release the reference held
> +			 * via hugetlb_cgroup_charge_cgroup_rsvd() when we are
> +			 * done.
> +			 */
> +			hugetlb_cgroup_put_rsvd_cgroup(h_cg);
>  		}
>  	}
>  	return true;
> diff --git a/mm/hugetlb_cgroup.c b/mm/hugetlb_cgroup.c
> index f68b51fcda3d..8668ba87cfe4 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 +401,13 @@ 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 must hold and only hold one rg->css

... must hold exactly one ...

-- 
Mike Kravetz

> +		 * reference.
> +		 */
> +		if (region_del)
> +			css_put(rg->css);
>  	}
>  }
>  
>