Re: [PATCH v3 3/9] kexec_file: Factor out kexec_locate_mem_hole from kexec_add_buffer.

[Dave Young <dyoung@redhat.com>]

On 06/28/16 at 07:18pm, Thiago Jung Bauermann wrote:
> Am Dienstag, 28 Juni 2016, 15:20:55 schrieb Dave Young:
> > On 06/27/16 at 04:21pm, Dave Young wrote:
> > > Please ignore previous reply, I mistakenly send a broken mail without
> > > subject, sorry about it. Resend the reply here.
> > > 
> > > On 06/27/16 at 01:37pm, Thiago Jung Bauermann wrote:
> > > > Am Dienstag, 28 Juni 2016, 00:19:48 schrieb Dave Young:
> > > > > On 06/23/16 at 12:37pm, Thiago Jung Bauermann wrote:
> > > > > > Am Donnerstag, 23 Juni 2016, 01:44:07 schrieb Dave Young:
> > > > > > What is bad about the description of top_down?
> > > > > 
> > > > > It is not clear enough to me, I personally think the original one in
> > > > > source code is better:
> > > > > /* allocate from top of memory hole */
> > > > 
> > > > Actually I realized there's some discrepancy in how the x86 code uses
> > > > top_down and how I need it to work in powerpc. This may be what is
> > > > confusing about my comment and the existing comment.
> > > > 
> > > > x86 always walks memory from bottom to top but if top_down is true, in
> > > > each memory region it will allocate the memory hole in the highest
> > > > address within that region. I don't know why it is done that way,
> > > > though.
> > > 
> > > I think we did not meaning to do this, considering kdump we have only
> > > one crashkernel region for searching (crashk_res) so it is fine.
> > > For kexec maybe changing the walking function to accept top_down is
> > > reasonable.
> > > 
> > > Ccing Vivek see if he can remember something..
> > > 
> > > > On powerpc, the memory walk itself should be from top to bottom, as
> > > > well as the memory hole allocation within each memory region.
> > 
> > What is the particular reason in powerpc for a mandatory top to bottom
> > walking?
> 
> I'm walking unreserved memory ranges, so reservations made low in memory 
> (such as the reservation for the initrd) may create a memory hole that is a 
> lot lower than the true memory limit where I want to allocate from (768 MB). 
> In this situation, allocating at the highest address in the lowest free 
> memory range will allocate the buffer very low in memory, and in that case 
> top_down doesn't mean much.
> 
> Walking memory from lowest to highest address but then allocating memory at 
> the highest address inside the memory range is peculiar and surprising. Is 
> there a particular reason for it?

I do not know if there's some historic reason, personally I think it
should be an accident.

> 
> If it's an accident and doesn't affect x86, I'd suggest that top_down should
> have its expected behavior, which (at least for me) is: allocate from the
> highest available memory address within the desired range.

I tend to agree, but we need test it first to see if it breaks something.

> 
> In any case, my patch series allows each architecture to define what
> top_down should mean. It doesn't change the behavior in x86, since
> the default implementation of arch_kexec_walk_mem ignores
> kexec_buf.top_down, and allows powerpc to take top_down into account
> when walking memory.
> 
> > > > Should I add a separate top_down argument to kexec_locate_mem_hole to
> > > > control if the memory walk should be from top to bottom, and then the
> > > > bottom_up member of struct kexec_buf controls where inside each memory
> > > > region the memory hole will be allocated?
> > 
> > Using one argument for both sounds more reasonable than using a separate
> > argument for memory walk..
> 
> I agree. This patch doesn't use a separate top_down argument, it's the same
> patch I sent earlier except that the comments to struct kexec_buf are in
> patch 2/9. What do you think?

It looks good except one nitpick inline..

> 
> -- 
> []'s
> Thiago Jung Bauermann
> IBM Linux Technology Center
> 
> 
> Subject: [PATCH 3/9] kexec_file: Factor out kexec_locate_mem_hole from
>  kexec_add_buffer.
> 
> kexec_locate_mem_hole will be used by the PowerPC kexec_file_load
> implementation to find free memory for the purgatory stack.
> 
> Signed-off-by: Thiago Jung Bauermann <bauerman@linux.vnet.ibm.com>
> Cc: Eric Biederman <ebiederm@xmission.com>
> Cc: Dave Young <dyoung@redhat.com>
> Cc: kexec@lists.infradead.org
> Cc: linux-kernel@vger.kernel.org
> ---
>  include/linux/kexec.h |  1 +
>  kernel/kexec_file.c   | 25 ++++++++++++++++++++-----
>  2 files changed, 21 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/kexec.h b/include/linux/kexec.h
> index e16d845d587f..2b34e69db679 100644
> --- a/include/linux/kexec.h
> +++ b/include/linux/kexec.h
> @@ -169,6 +169,7 @@ struct kexec_buf {
>  
>  int __weak arch_kexec_walk_mem(struct kexec_buf *kbuf,
>  			       int (*func)(u64, u64, void *));
> +int kexec_locate_mem_hole(struct kexec_buf *kbuf);
>  #endif /* CONFIG_KEXEC_FILE */
>  
>  struct kimage {
> diff --git a/kernel/kexec_file.c b/kernel/kexec_file.c
> index b1f1f6402518..445d66add8ca 100644
> --- a/kernel/kexec_file.c
> +++ b/kernel/kexec_file.c
> @@ -449,6 +449,23 @@ int __weak arch_kexec_walk_mem(struct kexec_buf *kbuf,
>  		return walk_system_ram_res(0, ULONG_MAX, kbuf, func);
>  }
>  
> +/**
> + * kexec_locate_mem_hole - find free memory to load segment or use in purgatory

It is not necessary to use only for purgatory load..

> + * @kbuf:	Parameters for the memory search.
> + *
> + * On success, kbuf->mem will have the start address of the memory region found.
> + *
> + * Return: 0 on success, negative errno on error.
> + */
> +int kexec_locate_mem_hole(struct kexec_buf *kbuf)
> +{
> +	int ret;
> +
> +	ret = arch_kexec_walk_mem(kbuf, locate_mem_hole_callback);
> +
> +	return ret == 1 ? 0 : -EADDRNOTAVAIL;
> +}
> +
>  /*
>   * Helper function for placing a buffer in a kexec segment. This assumes
>   * that kexec_mutex is held.
> @@ -493,11 +510,9 @@ int kexec_add_buffer(struct kimage *image, char *buffer, unsigned long bufsz,
>  	kbuf->top_down = top_down;
>  
>  	/* Walk the RAM ranges and allocate a suitable range for the buffer */
> -	ret = arch_kexec_walk_mem(kbuf, locate_mem_hole_callback);
> -	if (ret != 1) {
> -		/* A suitable memory range could not be found for buffer */
> -		return -EADDRNOTAVAIL;
> -	}
> +	ret = kexec_locate_mem_hole(kbuf);
> +	if (ret)
> +		return ret;
>  
>  	/* Found a suitable memory range */
>  	ksegment = &image->segment[image->nr_segments];
> -- 
> 1.9.1
> 
> 

Thanks
Dave