Re: Server crash due to SIGBUS(Bus Error) when trying to access the memory created using dsm_create().

On Tue, Sep 26, 2017 at 7:56 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> I wrote:
>> Hmm, so I tested this patch on my RHEL6 box (kernel 2.6.32) and it
>> immediately fell over with
>> 2017-09-25 14:23:48.410 EDT [325] FATAL:  could not resize shared memory segment "/PostgreSQL.1682054886" to 6928 bytes: Operation not supported
>> during startup.  I wonder whether we need to round off the request.
>
> Nope, rounding off doesn't help.  What does help is using posix_fallocate
> instead.  I surmise that glibc knows something we don't about how to call
> fallocate(2) successfully on this kernel version.
>
> Rather than dig into the guts of glibc to find that out, though, I think
> we should just s/fallocate/posix_fallocate/g on this patch.  The argument
> for using the former seemed pretty thin to begin with.

I didn't dig into the glibc source but my first guess is that
posix_fallocate() sees ENOTSUPP (from tmpfs on that vintage kernel)
and falls back to a write loop.  I thought I was doing enough by
testing for ENOSYS (based on the man page for fallocate which said
that should be expected on kernels < 2.6.23), but I see now that
ENOTSUPP is possible per-filesystem implementation and tmpfs support
was added some time after the 2.6.32 era:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=e2d12e22c59ce714008aa5266d769f8568d74eac

I'm not sure why glibc would provide a fallback for posix_fallocate()
but let ENOTSUPP escape from fallocate().

-- 
Thomas Munro
http://www.enterprisedb.com