On Wed, Jul 31, 2019 at 8:26 AM Darrick J. Wong <darrick.wong@xxxxxxxxxx> wrote:
>
> On Mon, Jul 29, 2019 at 06:49:13PM -0700, Deepa Dinamani wrote:
> > ext4 has different overflow limits for max filesystem
> > timestamps based on the extra bytes available.
> >
> > The timestamp limits are calculated according to the
> > encoding table in
> > a4dad1ae24f85i(ext4: Fix handling of extended tv_sec):
> >
> > * extra msb of adjust for signed
> > * epoch 32-bit 32-bit tv_sec to
> > * bits time decoded 64-bit tv_sec 64-bit tv_sec valid time range
> > * 0 0 1 -0x80000000..-0x00000001 0x000000000 1901-12-13..1969-12-31
> > * 0 0 0 0x000000000..0x07fffffff 0x000000000 1970-01-01..2038-01-19
> > * 0 1 1 0x080000000..0x0ffffffff 0x100000000 2038-01-19..2106-02-07
> > * 0 1 0 0x100000000..0x17fffffff 0x100000000 2106-02-07..2174-02-25
> > * 1 0 1 0x180000000..0x1ffffffff 0x200000000 2174-02-25..2242-03-16
> > * 1 0 0 0x200000000..0x27fffffff 0x200000000 2242-03-16..2310-04-04
> > * 1 1 1 0x280000000..0x2ffffffff 0x300000000 2310-04-04..2378-04-22
> > * 1 1 0 0x300000000..0x37fffffff 0x300000000 2378-04-22..2446-05-10
>
> My recollection of ext4 has gotten rusty, so this could be a bogus
> question:
>
> Say you have a filesystem with s_inode_size > 128 where not all of the
> ondisk inodes have been upgraded to i_extra_isize > 0 and therefore
> don't support nanoseconds or times beyond 2038. I think this happens on
> ext3 filesystems that reserved extra space for inode attrs that are
> subsequently converted to ext4?
>
> In any case, that means that you have some inodes that support 34-bit
> tv_sec and some inodes that only support 32-bit tv_sec. For the inodes
> with 32-bit tv_sec, I think all that happens is that a large timestamp
> will be truncated further, right?
>
> And no mount time warning because at least /some/ of the inodes are
> ready to go for the next 30 years?
I'm confused about ext3 being converted to ext4. If the converted
inodes have extra space, then ext4_iget() will start using the extra
space when it modifies the on disk inode, won't it?
But, if there are 32 bit tv_sec and 34 bit tv_sec in a superblock then
from this macro below, if an inode has space for extra bits in
timestamps, it uses it. Otherwise, only the first 32 bits are copied
to the on disk timestamp. This matches the behavior today for 32 bit
tv_sec. But, the 34 bit tv_sec has a better behavior after the series
because of the clamping and warning.
#define EXT4_INODE_SET_XTIME(xtime, inode, raw_inode) \
do { \
(raw_inode)->xtime = cpu_to_le32((inode)->xtime.tv_sec); \
if (EXT4_FITS_IN_INODE(raw_inode, EXT4_I(inode), xtime ## _extra)) {\
(raw_inode)->xtime ## _extra = \
ext4_encode_extra_time(&(inode)->xtime); \
} \
} while (0)
I'm not sure if this corner case if important. Maybe the maintainers
can help me here. If this is important, then the inode time updates
for an ext4 inode should always be through ext4_setattr() and we can
clamp the timestamps there as a special case. And, this patch can be
added separately?
Thanks,
Deepa
