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

Hari Bathini
2020-05-05T17:45:02Z

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