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

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On 2021/3/13 3:09, Mike Kravetz wrote:
> 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.
> 

Many thanks for review! I will fix all this nits. Should I resend this patch or send another
one to fix this? Just let me know which is the easiest one for you.

Thanks again. :)

> 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 ...
> 





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