linux-kernel - Re: [PATCH 2/2] fs/dcache: Make negative dentries easier to be reclaimed

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Date:   Tue, 28 Aug 2018 16:22:07 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Waiman Long <longman@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Jonathan Corbet <corbet@....net>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        "Luis R. Rodriguez" <mcgrof@...nel.org>,
        Kees Cook <keescook@...omium.org>, Jan Kara <jack@...e.cz>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        Ingo Molnar <mingo@...nel.org>,
        Miklos Szeredi <mszeredi@...hat.com>,
        Larry Woodman <lwoodman@...hat.com>,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        "Wangkai (Kevin,C)" <wangkai86@...wei.com>,
        Michal Hocko <mhocko@...nel.org>
Subject: Re: [PATCH 2/2] fs/dcache: Make negative dentries easier to be
 reclaimed

On Tue, 28 Aug 2018 16:10:24 -0700 Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Tue, Aug 28, 2018 at 3:29 PM Waiman Long <longman@...hat.com> wrote:
> >
> > Yes, I can rewrite it. What is the problem with the abbreviated form?
> 
> Either gcc rewrites it for you, or you end up _actually_ using a
> function pointer and calling through it.
> 
> The latter would be absolutely horribly bad for something like
> "list_add()", which should expand to just a couple of instructions.
> 
> And the former would be ok, except for the "you wrote code the garbage
> way, and then depended on the compiler fixing it up". Which we
> generally try to avoid in the kernel.
> 
> (Don't get me wrong - we definitely depend on the compiler doing a
> good job at CSE and dead code elimination etc, but generally we try to
> avoid the whole "compiler has to rewrite code to be good" model).
> 

And the "abbreviated form" will surely explode if one or both of those
"functions" happens to be implemented (or later reimplemented) as a macro.
It's best not to unnecessarily make such assumptions.