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SubjectRe: in_compat_syscall() returns from kernel thread for X86_32.
On Wed, Oct 24 2018, Theodore Y. Ts'o wrote:

> On Wed, Oct 24, 2018 at 12:47:57PM +1100, NeilBrown wrote:
>> I doubt it was copied - more likely independent evolution.
>> But on reflection, I see that it is probably reasonable that it
>> shouldn't be used this way - or at all in this context.
>> I'll try to understand what the issues really are and see if I can
>> find a solution that doesn't depend on this interface.
>> Thanks for your help.
> At least for ext4, the primary problem is that we want to use a 64-bit
> telldir/seekdir cookie if all 64-bits will make it to user space, and
> a 32-bit telldir cookie if only 32 bits will make it userspace. This
> impacts NFS as well because if there are people who are still using
> NFSv2, which has 32-bit directory offsets, we need to constrain the
> telldir/seekdir cookies we give to NFS to be a 32 has as opposed to a
> 64-bit hash.

NFSd uses FMODE_32BITHASH or FMODE64BITHASH to allow ext4 to do the
right thing. FMODE_32BITHASH is set for NFSv2 only.

Maybe sys_getdents needs to set FMODE_32BITHASH, and sys_getdent64 needs
to set FMODE_64BITHASH - or something like that.

For lustre it is a bit more complex. The internal "inode number" is 128
bits and we (sort of) hash it to 32 or 64 bits. cp_compat_stat() just
says -EOVERFLOW if we give a 64 bit number when 32 are expected, and
there is no flag to say "this is a 32-bit 'stat' request".

But I need to dig into exactly what that "sort-of" means - maybe there
is an answer in there.

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