Re: [PATCH] mm/khugepaged: alloc_charge_hpage() take care of mem charge errors

Hello,

On Tue, Feb 21, 2023 at 04:43:44PM -0500, Peter Xu wrote:

If memory charge failed, the caller shouldn't call mem_cgroup_uncharge(). Let alloc_charge_hpage() handle the error itself and clear hpage properly if mem charge fails.

I'm a bit confused by this patch.

There isn't anything wrong with calling mem_cgroup_uncharge() on an uncharged page, functionally. It checks and bails out.

It's an unnecessary call of course, but since it's an error path it's also not a cost issue, either.

I could see an argument for improving the code, but this is actually more code, and the caller still has the uncharge-and-put branch anyway for when the collapse fails later on.

So I'm not sure I understand the benefit of this change.