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

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Message-ID: <20180714183657.GK30522@ZenIV.linux.org.uk>
Date:   Sat, 14 Jul 2018 19:36:57 +0100
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Pavel Machek <pavel@....cz>, Jan Kara <jack@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        Waiman Long <longman@...hat.com>,
        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 07:34:45PM +0100, Al Viro wrote:
> On Sat, Jul 14, 2018 at 11:00:32AM -0700, Linus Torvalds wrote:
> > 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).
> 
> BTW, one point that might not be realized by everyone: negative dentries
> are *not* the hard case.
> mount -t tmpfs none /mnt
> touch /mnt/a
> for i in `seq 100000`; do ln /mnt/a /mnt/$i; done
> 
> and you've got 100000 *unevictable* dentries, with the time per iteration
> being not all that high (especially if you just call link(2) in a loop).
> They are all positive and all pinned.  And you've got only one inode
> there and no persistently opened files, so rlimit and quota won't help
> any.

OK, this
        /*   
         * No ordinary (disk based) filesystem counts links as inodes;
         * but each new link needs a new dentry, pinning lowmem, and
         * tmpfs dentries cannot be pruned until they are unlinked.
         */
        ret = shmem_reserve_inode(inode->i_sb);
        if (ret)
                goto out;
will probably help (on ramfs it won't, though).