Re: [PATCH] cgroup/dmem: return -ENOMEM on failed pool preallocation

["Michal Koutný" <mkoutny@suse.com>]

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On Mon, May 11, 2026 at 09:31:50AM +0800, Guopeng Zhang <zhangguopeng@kylinos.cn> wrote:
> get_cg_pool_unlocked() handles allocation failures under dmemcg_lock by
> dropping the lock, preallocating a pool with GFP_KERNEL, and retrying the
> locked lookup and creation path.
> 
> If the fallback allocation fails too, pool remains NULL. Since the loop
> condition is while (!pool), the function can keep retrying instead of
> propagating the allocation failure to the caller.

This implies that it's OK when the function keeps retrying with
allocpool != NULL (and repeated WARN_ON()s)?

> Set pool to ERR_PTR(-ENOMEM) when the fallback allocation fails so the
> loop exits through the existing common return path. The callers already
> handle ERR_PTR() from get_cg_pool_unlocked(), so this restores the
> expected error path.

If the callers can handle it, maybe there's no need to retry at all.
Perhpas dmem fellows can step in here.

> 
> Fixes: b168ed458dde ("kernel/cgroup: Add "dmem" memory accounting cgroup")
> Signed-off-by: Guopeng Zhang <zhangguopeng@kylinos.cn>
> ---
>  kernel/cgroup/dmem.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/kernel/cgroup/dmem.c b/kernel/cgroup/dmem.c
> index 1ab1fb47f271..4753a67d0f0f 100644
> --- a/kernel/cgroup/dmem.c
> +++ b/kernel/cgroup/dmem.c
> @@ -602,6 +602,7 @@ get_cg_pool_unlocked(struct dmemcg_state *cg, struct dmem_cgroup_region *region)
>  				pool = NULL;

This 2nd pool zeroing seems pointless.

>  				continue;
>  			}
> +			pool = ERR_PTR(-ENOMEM);
>  		}
>  	}


HTH,
Michal

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