linux-kernel - Re: [PATCH v4 1/4] lazy tlb: introduce lazy mm refcount helper functions

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Message-Id: <20210607184805.eddf8eb26b80e8af85d5777e@linux-foundation.org>
Date:   Mon, 7 Jun 2021 18:48:05 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Nicholas Piggin <npiggin@...il.com>
Cc:     Anton Blanchard <anton@...abs.org>, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linuxppc-dev@...ts.ozlabs.org, Andy Lutomirski <luto@...nel.org>,
        Randy Dunlap <rdunlap@...radead.org>
Subject: Re: [PATCH v4 1/4] lazy tlb: introduce lazy mm refcount helper
 functions

On Tue, 08 Jun 2021 11:39:56 +1000 Nicholas Piggin <npiggin@...il.com> wrote:

> > Looks like a functional change.  What's happening here?
> 
> That's kthread_use_mm being clever about the lazy tlb mm. If it happened 
> that the kthread had inherited a the lazy tlb mm that happens to be the 
> one we want to use here, then we already have a refcount to it via the 
> lazy tlb ref.
> 
> So then it doesn't have to touch the refcount, but rather just converts
> it from the lazy tlb ref to the returned reference. If the lazy tlb mm
> doesn't get a reference, we can't do that.

Please cover this in the changelog and perhaps a code comment.