linux-kernel - Re: [RFC][PATCH] kexec: Teach indirect pages how to live in high memory

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Date:   Tue, 05 May 2020 13:39:16 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Hari Bathini <hbathini@...ux.ibm.com>
Cc:     Joonsoo Kim <js1304@...il.com>, kernel-team@....com,
        Michal Hocko <mhocko@...e.com>,
        Minchan Kim <minchan@...nel.org>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.ibm.com>,
        Rik van Riel <riel@...riel.com>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        LKML <linux-kernel@...r.kernel.org>,
        Christian Koenig <christian.koenig@....com>,
        Christoph Hellwig <hch@...radead.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        Huang Rui <ray.huang@....com>,
        Kexec Mailing List <kexec@...ts.infradead.org>,
        Pavel Machek <pavel@....cz>,
        Johannes Weiner <hannes@...xchg.org>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Laura Abbott <labbott@...hat.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Roman Gushchin <guro@...com>, Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [RFC][PATCH] kexec: Teach indirect pages how to live in high memory

Hari Bathini <hbathini@...ux.ibm.com> writes:

> On 05/05/20 3:29 am, Eric W. Biederman wrote:
>> 
>> Recently a patch was proposed to kimage_alloc_page to slightly alter
>> the logic of how pages allocated with incompatible flags were
>> detected.  The logic was being altered because the semantics of the
>> page alloctor were changing yet again.
>> 
>> Looking at that case I realized that there is no reason for it to even
>> exist.  Either the indirect page allocations and the source page
>> allocations could be separated out, or I could do as I am doing now
>> and simply teach the indirect pages to live in high memory.
>> 
>> This patch replaced pointers of type kimage_entry_t * with a new type
>> kimage_entry_pos_t.  This new type holds the physical address of the
>> indirect page and the offset within that page of the next indirect
>> entry to write.  A special constant KIMAGE_ENTRY_POS_INVALID is added
>> that kimage_image_pos_t variables that don't currently have a valid
>> may be set to.
>> 
>> Two new functions kimage_read_entry and kimage_write_entry have been
>> provided to write entries in way that works if they live in high
>> memory.
>> 
>> The now unnecessary checks to see if a destination entry is non-zero
>> and to increment it if so have been removed.  For safety new indrect
>> pages are now cleared so we have a guarantee everything that has not
>> been used yet is zero.  Along with this writing an extra trailing 0
>> entry has been removed, as it is known all trailing entries are now 0.
>> 
>> With highmem support implemented for indirect pages
>> kimage_image_alloc_page has been updated to always allocate
>> GFP_HIGHUSER pages, and handling of pages with different
>> gfp flags has been removed.
>> 
>> Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>
>
> Eric, the patch failed with data access exception on ppc64. Using the below patch on top
> got me going...

Doh!  Somehow I thought I had put that logic or something equivalent
into kimage_write_entry and it appears I did not.  I will see if I can
respin the patch.

Thank you very much for testing.

Eric


> diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
> index 45862fd..bef52f1 100644
> --- a/kernel/kexec_core.c
> +++ b/kernel/kexec_core.c
> @@ -570,7 +570,12 @@ static int kimage_add_entry(struct kimage *image, kimage_entry_t entry)
>  			return -ENOMEM;
>  
>  		ind_addr = page_to_boot_pfn(page) << PAGE_SHIFT;
> -		kimage_write_entry(image->entry_pos, ind_addr | IND_INDIRECTION);
> +
> +		/* If it is the first entry, handle it here */
> +		if (!image->head)
> +			image->head = ind_addr | IND_INDIRECTION;
> +		else
> +			kimage_write_entry(image->entry_pos, ind_addr | IND_INDIRECTION);
>  
>  		clear_highpage(page);
>  
> @@ -623,7 +628,11 @@ int __weak machine_kexec_post_load(struct kimage *image)
>  
>  void kimage_terminate(struct kimage *image)
>  {
> -	kimage_write_entry(image->entry_pos, IND_DONE);
> +	/* This could be the only entry in case of kdump */
> +	if (!image->head)
> +		image->head = IND_DONE;
> +	else
> +		kimage_write_entry(image->entry_pos, IND_DONE);
>  }
>  
>  #define for_each_kimage_entry(image, pos, entry) 				\
>
>
> Thanks
> Hari