about shared_buffer and kernel.shmmax

Hi everybody,
shared_buffer maximum limit is ?
I set shared_buffer more than 250G, I got error at startup pg.

------------------------------------------------------------------------
This error usually means that PostgreSQL's request for a shared memory
segment exceeded your kernel's SHMMAX parameter. You can either reduce
the request size or reconfigure the kernel with larger SHMMAX. To reduce
the request size (currently 439371702272 bytes), reduce PostgreSQL's
shared_buffers parameter (currently 52428800) and/or its max_connections
parameter (currently 104).
-------------------------------------------------------------------------

but now
kernel.shmmax = 966367641600
kernel.shmall = 235929600

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Yours sincerely,
Qi Wang
                         Edgesoft (China) Co.,Ltd
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