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Date: Sat, 13 Mar 2021 11:11:06 +0800
From: Miaohe Lin <linmiaohe@...wei.com>
To: Mike Kravetz <mike.kravetz@...cle.com>, <akpm@...ux-foundation.org>
CC: <almasrymina@...gle.com>, <rientjes@...gle.com>,
<linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] hugetlb_cgroup: fix imbalanced css_get and css_put
pair for shared mappings
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@...el.com> (auto build test ERROR)
>> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
>> Cc: stable@...nel.org
>> ---
>> 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@...cle.com>
>
>>
>> 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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