Re: [PATCH v5 2/2] tmpfs: Support 64-bit inums per-sb

[Chris Down]

Hi Hugh,

Sorry this response took so long, I had some non-work issues that took a lot of 
time last week.

Hugh Dickins writes:
> On Fri, 10 Jan 2020, Chris Down wrote:
> > Hugh Dickins writes:
> > > Dave, Amir, Chris, many thanks for the info you've filled in -
> > > and absolutely no need to run any scan on your fleet for this,
> > > I think we can be confident that even if fb had some 15-year-old tool
> > > in use on its fleet of 2GB-file filesystems, it would not be the one
> > > to insist on a kernel revert of 64-bit tmpfs inos.
> > >
> > > The picture looks clear now: while ChrisD does need to hold on to his
> > > config option and inode32/inode64 mount option patch, it is much better
> > > left out of the kernel until (very unlikely) proved necessary.
> >
> > Based on Mikael's comment above about Steam binaries, and the lack of
> > likelihood that they can be rebuilt, I'm inclined to still keep inode{64,32},
> > but make legacy behaviour require explicit opt-in. That is:
> >
> > - Default it to inode64
> > - Remove the Kconfig option
> > - Only print it as an option if tmpfs was explicitly mounted with inode32
> >
> > The reason I suggest keeping this is that I'm mildly concerned that the kind
> > of users who might be impacted by this change due to 32-bit _FILE_OFFSET_BITS
> > -- like the not-too-uncommon case that Mikael brings up -- seem unlikely to
> > be the kind of people that would find it in an rc.
>
> Okay.  None of us are thrilled with it, but I agree that
> Mikael's observation should override our developer's preference.
>
> So the "inode64" option will be accepted but redundant on mounting,
> but exists for use as a remount option after mounting or remounting
> with "inode32": allowing the admin to switch temporarily to mask off
> the high ino bits with "inode32" when needing to run a limited binary.
>
> Documentation and commit message to alert Andrew and Linus and distros
> that we are risking some breakage with this, but supplying the antidote
> (not breakage of any distros themselves, no doubt they're all good;
> but breakage of what some users might run on them).

Sounds good.

> > Other than that, the first patch could be similar to how it is now,
> > incorporating Hugh's improvements to the first patch to put everything under
> > the same stat_lock in shmem_reserve_inode.
>
> So, I persuaded Amir to the other aspects my version, but did not
> persuade you?  Well, I can live with that (or if not, can send mods
> on top of yours): but please read again why I was uncomfortable with
> yours, to check that you still prefer it (I agree that your patch is
> simpler, and none of my discomfort decisive).

Hmm, which bit were you thinking of? The lack of batching, shmem_encode_fh(), 
or the fact that nr_inodes can now be 0 on non-internal mounts?

For batching, I'm neutral. I'm happy to use the approach from your patch and 
integrate it (and credit you, of course).

For shmem_encode_fh, I'm not totally sure I understand the concern, if that's 
what you mean.

For nr_inodes, I agree that intentional or unintentional, we should at least 
handle this case for now and can adjust later if the behaviour changes.

Thanks again,

Chris