Perhaps not the best example, but virtually free considering
a bitmap of 64K bits is 8KiB no matter what.
Kernel will allocate:
1 page for internal metadata (rounded up to a full page)
N pages rounded up from array size, which is:
number of elements (65536 bits / 64 bits per u64) = 1024
* element size (u64 rounded up to a multiple of 8) = 8
= 8192
This rounding up is unique to *mmapable* arrays.
thus on 4KiB page size: 12 KiB - waste is at most a few (<4) KiB
and on 16KiB page size: 32 KiB - which isn't great, but well that's 16KiB...
(presumably 16KiB is being used on devices with a lot of ram,
where performance matters more than a little ram waste)
Note: AFAICT a kernel bpf program R/W map cannot be mapped R/O
into userspace, as the permissions on the page mapping would conflict.
(and obviously you cannot map R/W if you have only R/O access)
I'm not sure if this is architecture specific,
or simply an implementation limitation, but it is what I observed.
I think it's likely a missing kernel feature (even on 6.12)...
Test: TreeHugger
Flag: EXEMPT mainline
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I609ae61e17917ff441cac10a9b29af3fcacb53c5
12f752aae9