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Subject: Re: [RFC][PATCH] kexec: Teach indirect pages how to live in high
 memory
To:     "Eric W. Biederman" <ebiederm@xmission.com>,
        Joonsoo Kim <js1304@gmail.com>
Cc:     kernel-team@lge.com, Michal Hocko <mhocko@suse.com>,
        Minchan Kim <minchan@kernel.org>,
        "Aneesh Kumar K . V" <aneesh.kumar@linux.ibm.com>,
        Rik van Riel <riel@surriel.com>,
        "Rafael J . Wysocki" <rjw@rjwysocki.net>,
        LKML <linux-kernel@vger.kernel.org>,
        Christian Koenig <christian.koenig@amd.com>,
        Christoph Hellwig <hch@infradead.org>,
        Linux Memory Management List <linux-mm@kvack.org>,
        Huang Rui <ray.huang@amd.com>,
        Kexec Mailing List <kexec@lists.infradead.org>,
        Pavel Machek <pavel@ucw.cz>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Laura Abbott <labbott@redhat.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Roman Gushchin <guro@fb.com>, Vlastimil Babka <vbabka@suse.cz>
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From:   Hari Bathini <hbathini@linux.ibm.com>
Message-ID: <54a53bfe-6929-2790-9b1d-943e9f47cd62@linux.ibm.com>
Date:   Tue, 5 May 2020 23:14:32 +0530
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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@xmission.com>

Eric, the patch failed with data access exception on ppc64. Using the below patch on top
got me going...


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