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

Eric W. Biederman
2020-05-05T18:43:35Z

Hari Bathini <hbathini@linux.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@xmission.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