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SubjectRe: [PATCH RFC] hugetlb_cgroup: fix unbalanced css_put for shared mappings
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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@huawei.com>
> Cc: stable@kernel.org
> ---
> 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);
> }
> }
>
>

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