[PATCH v2 0/2] tmpfs: Reduce risk of inum overflow

[Chris Down]

In Facebook production we are seeing heavy i_ino wraparounds on tmpfs.
On affected tiers, in excess of 10% of hosts show multiple files with
different content and the same inode number, with some servers even
having as many as 150 duplicated inode numbers with differing file
content.

This causes actual, tangible problems in production. For example, we
have complaints from those working on remote caches that their
application is reporting cache corruptions because it uses (device,
inodenum) to establish the identity of a particular cache object, but
because it's not unique any more, the application refuses to continue
and reports cache corruption. Even worse, sometimes applications may not
even detect the corruption but may continue anyway, causing phantom and
hard to debug behaviour.

In general, userspace applications expect that (device, inodenum) should
be enough to be uniquely point to one inode, which seems fair enough.
One might also need to check the generation, but in this case:

1. That's not currently exposed to userspace
   (ioctl(...FS_IOC_GETVERSION...) returns ENOTTY on tmpfs);
2. Even with generation, there shouldn't be two live inodes with the
   same inode number on one device.

In order to mitigate this, we take a two-pronged approach:

1. Moving inum generation from being global to per-sb for tmpfs. This
   itself allows some reduction in i_ino churn. This works on both 64-
   and 32- bit machines.
2. Adding inode{64,32} for tmpfs. This fix is supported on machines with
   64-bit ino_t only: we allow users to mount tmpfs with a new inode64
   option that uses the full width of ino_t, or CONFIG_TMPFS_INODE64.

Chris Down (2):
  tmpfs: Add per-superblock i_ino support
  tmpfs: Support 64-bit inums per-sb

 Documentation/filesystems/tmpfs.txt |  11 +++
 fs/Kconfig                          |  18 +++++
 include/linux/shmem_fs.h            |   2 +
 mm/shmem.c                          | 110 ++++++++++++++++++++++++++--
 4 files changed, 133 insertions(+), 8 deletions(-)

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2.24.1