linux-kernel - Re: [PATCH] kexec: allocate buffer in top-down, if specified, correctly

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Date:   Fri, 28 Apr 2017 13:19:50 +0800
From:   Dave Young <dyoung@...hat.com>
To:     AKASHI Takahiro <takahiro.akashi@...aro.org>,
        "vgoyal@...hat.com Thiago Jung Bauermann" 
        <bauerman@...ux.vnet.ibm.com>, bhe@...hat.com,
        kexec@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kexec: allocate buffer in top-down, if specified,
 correctly

Vivek, can you help to give some comments about the locate hole isssue
in kexec_file?

On 04/28/17 at 09:51am, AKASHI Takahiro wrote:
> Thiago,
> 
> Thank you for the comment.
> 
> On Thu, Apr 27, 2017 at 07:00:04PM -0300, Thiago Jung Bauermann wrote:
> > Hello,
> > 
> > Am Mittwoch, 26. April 2017, 17:22:09 BRT schrieb AKASHI Takahiro:
> > > The current kexec_locate_mem_hole(kbuf.top_down == 1) stops searching at
> > > the first memory region that has enough space for requested size even if
> > > some of higher regions may also have.
> > 
> > kexec_locate_mem_hole expects arch_kexec_walk_mem to walk memory from top to 
> > bottom if top_down is true. That is what powerpc's version does.
> 
> Ah, I haven't noticed that, but x86 doesn't have arch_kexec_walk_mem and
> how can it work for x86?
> 
> > Isn't it possible to walk resources from top to bottom?
> 
> Yes, it will be, but it seems to me that such a behavior is not intuitive
> and even confusing if it doesn't come with explicit explanation.

Thing need to make clear is why do we need the change, it might be a
problem for crashkernel=xM,low since that is for softiotlb in case
crashkernel=xM,high being used, otherwise seems current code is fine.
 
Need seeking for old memory from Vivek to confirm.
> 
> > > This behavior is not consistent with locate_hole(hole_end == -1) function
> > > of kexec-tools.
> > > 
> > > This patch fixes the bug, going though all the memory regions anyway.
> > 
> > This patch would break powerpc, because at the end of the memory walk kbuf 
> > would have the lowest memory hole.
> > 
> > If it's not possible to walk resources in reverse order, then this patch needs 
> > to change powerpc to always walk memory from bottom to top.
> 
> So I would like to hear from x86 guys.
> 
> Thanks
> -Takahiro AKASHI
> 
> > -- 
> > Thiago Jung Bauermann
> > IBM Linux Technology Center
> >