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

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Date:   Fri, 28 Apr 2017 13:23:26 +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

Correct Vivek's email address
On 04/28/17 at 01:19pm, Dave Young wrote:
> 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
> > >