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Date: Fri, 12 Mar 2021 11:09:13 -0800
From: Mike Kravetz <mike.kravetz@...cle.com>
To: Miaohe Lin <linmiaohe@...wei.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 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.
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 ...
--
Mike Kravetz
> + * reference.
> + */
> + if (region_del)
> + css_put(rg->css);
> }
> }
>
>
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