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

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Message-ID: <Y/ZLjF9Xe1F6Mu76@cmpxchg.org>
Date:   Wed, 22 Feb 2023 12:06:20 -0500
From:   Johannes Weiner <hannes@...xchg.org>
To:     Peter Xu <peterx@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Yang Shi <shy828301@...il.com>,
        David Stevens <stevensd@...omium.org>, stable@...r.kernel.org
Subject: 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.