Re: [PATCH v3 00/21] kdump, vmcore: support mmap() on /proc/vmcore

From: HATAYAMA Daisuke
Date: Wed Mar 20 2013 - 23:53:00 EST

From: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Subject: Re: [PATCH v3 00/21] kdump, vmcore: support mmap() on /proc/vmcore
Date: Tue, 19 Mar 2013 12:30:05 -0700

> On Sat, 16 Mar 2013 13:00:47 +0900 HATAYAMA Daisuke <d.hatayama@xxxxxxxxxxxxxx> 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.
>> 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].
> There are quite a lot of userspace-visible vmcore changes here. Is it
> all fully back-compatible? Will all known userspace continue to work
> OK on newer kernels?

I designed it to keep backward-compatibility at least for gdb and
binutils but not less for makedumpfile since it should follow kernel
changes; old makedumpfile cannot use newer kernels, and this is within
the range of this review.


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