vmcore: reference e_phoff member explicitly to get position of program header table

vmcore: reference e_phoff member explicitly to get position of program header table

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.

In particular, the current main user of this mmap() is makedumpfile, which
not only reads memory from /proc/vmcore but also does other processing
like filtering, compression and IO work.  Update of page table and the
following TLB flush makes such processing much slow; though I have yet to
make patch for makedumpfile and yet to confirm how it's improved.

To address the issue, this patch implements mmap() on /proc/vmcore to
improve read performance.  My simple benchmark shows the improvement from
200 [MiB/sec] to over 50.0 [GiB/sec].

This patch:

Currently, the code assumes that position of program header table is next
to ELF header.  But future change can break the assumption on kexec-tools
and the 1st kernel.  To avoid worst case, reference e_phoff member
explicitly to get position of program header table in file-offset.

Signed-off-by: Zhang Yanfei <zhangyanfei@cn.fujitsu.com>
Signed-off-by: HATAYAMA Daisuke <d.hatayama@jp.fujitsu.com>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>