[PATCH v6 0/8] kdump, vmcore: support mmap() on /proc/vmcore

[Eric W. Biederman]

"H. Peter Anvin" <hpa at zytor.com> writes:

> On 05/15/2013 02:05 AM, HATAYAMA Daisuke wrote:
>> Currently, read to /proc/vmcore is done by read_oldmem() that uses
>> ioremap/iounmap per a single page. For example, if memory is 1GB,
>> ioremap/iounmap is called (1GB / 4KB)-times, that is, 262144
>> times. This causes big performance degradation.
>
> read_oldmem() is fundamentally broken and unsafe.  It needs to be
> unified with the plain /dev/mem code and any missing functionality fixed
> instead of "let's just do a whole new driver".

That is completely and totally orthogonal to this change.

read_oldmem may have problems but in practice on a large systems those
problems are totally dwarfed by real life performance issues that come
from playing too much with the page tables.

I really don't find bringing up whatever foundational issues you have
with read_oldmem() appropriate or relevant here.

Eric