Re: [PATCH v2] writeback: fix obtain a reference to a freeing memcg css

[Michal Hocko]

On Thu 01-04-21 17:33:43, Muchun Song wrote:
> The caller of wb_get_create() should pin the memcg, because
> wb_get_create() relies on this guarantee. The rcu read lock
> only can guarantee that the memcg css returned by css_from_id()
> cannot be released, but the reference of the memcg can be zero.
> Fix it by holding a reference to the css before calling
> wb_get_create(). This is not a problem I encountered in the
> real world. Just the result of a code review.
> 
> And it is unnecessary to use GFP_ATOMIC, so replace it with
> GFP_NOIO.

This should go into it's own patch. With more explanation why NOIO is
required.

> Fixes: 682aa8e1a6a1 ("writeback: implement unlocked_inode_to_wb transaction and use it for stat updates")
> Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>

For the css part feel free to add
Acked-by: Michal Hocko <mhocko@xxxxxxxx>

Even if the css ref count is not really necessary it shouldn't cause any
harm and it makes the code easier to understand. At least a comment
explaining why that is not necessary would be required without it.

Thanks!

> ---
> Changelog in v2:
>  1. Replace GFP_ATOMIC with GFP_NOIO suggested by Matthew.
> 
>  fs/fs-writeback.c | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> index e91980f49388..df7f89f8f771 100644
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -501,16 +501,21 @@ static void inode_switch_wbs(struct inode *inode, int new_wb_id)
>  	if (atomic_read(&isw_nr_in_flight) > WB_FRN_MAX_IN_FLIGHT)
>  		return;
>  
> -	isw = kzalloc(sizeof(*isw), GFP_ATOMIC);
> +	isw = kzalloc(sizeof(*isw), GFP_NOIO);
>  	if (!isw)
>  		return;
>  
>  	/* find and pin the new wb */
>  	rcu_read_lock();
>  	memcg_css = css_from_id(new_wb_id, &memory_cgrp_subsys);
> -	if (memcg_css)
> -		isw->new_wb = wb_get_create(bdi, memcg_css, GFP_ATOMIC);
> +	if (memcg_css && !css_tryget(memcg_css))
> +		memcg_css = NULL;
>  	rcu_read_unlock();
> +	if (!memcg_css)
> +		goto out_free;
> +
> +	isw->new_wb = wb_get_create(bdi, memcg_css, GFP_NOIO);
> +	css_put(memcg_css);
>  	if (!isw->new_wb)
>  		goto out_free;
>  
> -- 
> 2.11.0

-- 
Michal Hocko
SUSE Labs