I think I (partly) found the answer myself. The biggest part that is in 0x35b4a000-0x36b55fff area is memory map. Its size corresponds really well with sizeof(struct page)*number of pages (it should be exactly 15M instead of 16). And looking on it with gdb I was able to see 40-byte patterns that closely resemble array of these structs. The only problem I had is that kernel's symbol mem_map doesn't point to this structure, it is set to 0, but I guess it's probably not used anymore. On Tue, Jan 03, 2023 at 09:51:08PM +0300, Mikhail Krylov wrote:

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.