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

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Message-ID: <Y/ZTHEACqwYUYGFP@x1n>
Date:   Wed, 22 Feb 2023 12:38:36 -0500
From:   Peter Xu <peterx@...hat.com>
To:     Johannes Weiner <hannes@...xchg.org>
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

On Wed, Feb 22, 2023 at 12:06:20PM -0500, Johannes Weiner wrote:
> 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.

Indeed, I didn't really notice there's zero side effect of calling that,
sorry.  In that case both "Fixes" and "Cc: stable" do not apply.

> 
> 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.

Yes, the benefit is having a clear interface for alloc_charge_hpage() with
no prone to leaking huge page.

The patch comes from a review for David's other patch here:

https://lore.kernel.org/all/Y%2FU9fBxVJdhxiZ1v@x1n/

I've attached a new version just to reword and remove the inproper tags.
Do you think that's acceptable?

Thanks,

-- 
Peter Xu

View attachment "0001-mm-khugepaged-alloc_charge_hpage-take-care-of-mem-ch.patch" of type "text/plain" (1730 bytes)