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Date: Sat, 14 Jul 2018 19:36:57 +0100
From: Al Viro <viro@ZenIV.linux.org.uk>
Subject: Re: [PATCH v5 0/6] fs/dcache: Track & limit # of negative dentries
Message-ID: <20180714183657.GK30522@ZenIV.linux.org.uk>
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To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Pavel Machek <pavel@ucw.cz>, Jan Kara <jack@suse.cz>, Andrew Morton <akpm@linux-foundation.org>, James Bottomley <James.Bottomley@hansenpartnership.com>, Waiman Long <longman@redhat.com>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, linux-fsdevel <linux-fsdevel@vger.kernel.org>, Paul McKenney <paulmck@linux.vnet.ibm.com>, Ingo Molnar <mingo@kernel.org>, Miklos Szeredi <mszeredi@redhat.com>, Matthew Wilcox <willy@infradead.org>, Larry Woodman <lwoodman@redhat.com>, "Wangkai (Kevin,C)" <wangkai86@huawei.com>, linux-mm <linux-mm@kvack.org>, Michal Hocko <mhocko@kernel.org>

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@ucw.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).