linux-ext4 - A bug in ext4 with big directories (was: NVFS XFS metadata)

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Date:   Wed, 23 Sep 2020 05:20:55 -0400 (EDT)
From:   Mikulas Patocka <mpatocka@...hat.com>
To:     "Theodore Ts'o" <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        linux-ext4@...r.kernel.org
cc:     Dan Williams <dan.j.williams@...el.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vishal Verma <vishal.l.verma@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>,
        Eric Sandeen <esandeen@...hat.com>,
        Dave Chinner <david@...morbit.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: A bug in ext4 with big directories (was: NVFS XFS metadata)

Hi

There seems to be a bug in ext4 - when I create very large directory, ext4 
fails with -ENOSPC despite the fact that there is plenty of free space and 
free inodes on the filesystem.

How to reproduce:
download the program dir-test: 
http://people.redhat.com/~mpatocka/benchmarks/dir-test.c

# modprobe brd rd_size=67108864
# mkfs.ext4 /dev/ram0
# mount -t ext4 /dev/ram0 /mnt/test
# dir-test /mnt/test/ 8000000 8000000
deleting: 7999000
2540000
file 2515327 can't be created: No space left on device
# df /mnt/test
/dev/ram0        65531436 633752 61525860   2% /mnt/test
# df -i /mnt/test
/dev/ram0        4194304 1881547 2312757   45% /mnt/test

(I tried to increase journal size, but it has no effect on this bug)

Mikulas