linux-kernel - Re: [PATCH v5 0/6] fs/dcache: Track & limit # of negative dentries

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Message-ID: <CA+55aFw1vrsTjJyoq4Q3jBwv1nXaTkkmSbHO6vozWZuTc7_6Kg@mail.gmail.com>
Date:   Sat, 14 Jul 2018 11:00:32 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Pavel Machek <pavel@....cz>
Cc:     Jan Kara <jack@...e.cz>, Andrew Morton <akpm@...ux-foundation.org>,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        Waiman Long <longman@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        Ingo Molnar <mingo@...nel.org>,
        Miklos Szeredi <mszeredi@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Larry Woodman <lwoodman@...hat.com>,
        "Wangkai (Kevin,C)" <wangkai86@...wei.com>,
        linux-mm <linux-mm@...ck.org>, Michal Hocko <mhocko@...nel.org>
Subject: Re: [PATCH v5 0/6] fs/dcache: Track & limit # of negative dentries

On Sat, Jul 14, 2018 at 10:35 AM Pavel Machek <pavel@....cz> wrote:
>
> Could we allocate -ve entries from separate slab?

No, because negative dentrires don't stay negative.

Every single positive dentry starts out as a negative dentry that is
passed in to "lookup()" to maybe be made positive.

And most of the time they <i>do</i> turn positive, because most of the
time people actually open files that exist.

But then occasionally you don't, because you're just blindly opening a
filename whether it exists or not (to _check_ whether it's there).

              Linus