Determining the reason for reserved RAM

From: Mikhail Krylov <sqarert@gmail.com>
To: kernelnewbies@kernelnewbies.org
Subject: Determining the reason for reserved RAM
Date: Tue, 3 Jan 2023 21:51:08 +0300	[thread overview]
Message-ID: <Y7R5HAvVyoQr05NC@sqrt.uni.cx> (raw)

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Hello,

I was doing some research figuring out what kernel reserves memory for,
and found myself in a sort of dead-end.

I found out that /proc/kpageflags bit 32 is set for reserved physical
pages in include/linux/kernel-page-flags.c, wrote a simple decoder of
that file, and on an old 32-bit x86 machine (it's the simplest there,
since it has less memory and fewest amount of memory holes) those are
reserved:

0x00000000-0x0000ffff - I assume those addresses are used for interrupt
vectors and whatnot
0x0009b000-0x000fffff - Video RAM/ROM for VGA/EGA/CGA and whatnot
0x01c00000-0x01c1ffff - no idea, it also has KPF_UNCACHED flag as well
0x1d000000-0x1dd73fff - Kernel code, rodata, rwdata, bss
0x35b4a000-0x36b55fff - no idea.

I also found out that region #5 is getting bigger or smaller depending
on the machine's memory size. On this 1.5G machine it's a bit more than
16M. On a 32G machine it's more than 500M size, feels a bit wasteful
IMO. So my question is, could you tell me where to look for what those
regions are and why those regions exist?  I've checked /proc/iomem and
it doesn't say it is reserved or anything, just "System RAM".

Thanks in advance, Mikhail.

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