Re: [PATCH v2] x86/vmemmap: Handle unpopulated sub-pmd ranges

[David Hildenbrand <david@redhat.com>]

On 29.01.21 07:40, Oscar Salvador wrote:
> When the size of a struct page is not multiple of 2MB, sections do
> not span a PMD anymore and so when populating them some parts of the
> PMD will remain unused.
> Because of this, PMDs will be left behind when depopulating sections
> since remove_pmd_table() thinks that those unused parts are still in
> use.
> 
> Fix this by marking the unused parts with PAGE_INUSE, so memchr_inv() will
> do the right thing and will let us free the PMD when the last user of it
> is gone.
> 
> This patch is based on a similar patch by David Hildenbrand:
> 
> https://lore.kernel.org/linux-mm/20200722094558.9828-9-david@redhat.com/
> https://lore.kernel.org/linux-mm/20200722094558.9828-10-david@redhat.com/
> 
> Signed-off-by: Oscar Salvador <osalvador@suse.de>
> ---
> 
>   v1 -> v2:
>   - Rename PAGE_INUSE to PAGE_UNUSED as it better describes what we do
> 
> ---
>   arch/x86/mm/init_64.c | 91 +++++++++++++++++++++++++++++++++++++------
>   1 file changed, 79 insertions(+), 12 deletions(-)
> 
> diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
> index b5a3fa4033d3..dbb76160ed52 100644
> --- a/arch/x86/mm/init_64.c
> +++ b/arch/x86/mm/init_64.c
> @@ -871,7 +871,72 @@ int arch_add_memory(int nid, u64 start, u64 size,
>   	return add_pages(nid, start_pfn, nr_pages, params);
>   }
>   
> -#define PAGE_INUSE 0xFD
> +#define PAGE_UNUSED 0xFD
> +
> +/*
> + * The unused vmemmap range, which was not yet memset(PAGE_UNUSED) ranges
> + * from unused_pmd_start to next PMD_SIZE boundary.
> + */
> +static unsigned long unused_pmd_start __meminitdata;
> +
> +static void __meminit vmemmap_flush_unused_pmd(void)
> +{
> +	if (!unused_pmd_start)
> +		return;
> +	/*
> +	 * Clears (unused_pmd_start, PMD_END]
> +	 */
> +	memset((void *)unused_pmd_start, PAGE_UNUSED,
> +	       ALIGN(unused_pmd_start, PMD_SIZE) - unused_pmd_start);
> +	unused_pmd_start = 0;
> +}
> +
> +/* Returns true if the PMD is completely unused and thus it can be freed */
> +static bool __meminit vmemmap_unuse_sub_pmd(unsigned long addr, unsigned long end)
> +{
> +	unsigned long start = ALIGN_DOWN(addr, PMD_SIZE);
> +
> +	vmemmap_flush_unused_pmd();
> +	memset((void *)addr, PAGE_UNUSED, end - addr);
> +
> +	return !memchr_inv((void *)start, PAGE_UNUSED, PMD_SIZE);
> +}
> +
> +static void __meminit vmemmap_use_sub_pmd(unsigned long start, unsigned long end)
> +{
> +	/*
> +	 * We only optimize if the new used range directly follows the
> +	 * previously unused range (esp., when populating consecutive sections).
> +	 */
> +	if (unused_pmd_start == start) {
> +		if (likely(IS_ALIGNED(end, PMD_SIZE)))
> +			unused_pmd_start = 0;
> +		else
> +			unused_pmd_start = end;
> +		return;
> +	}
> +
> +	vmemmap_flush_unused_pmd();
> +}
> +
> +static void __meminit vmemmap_use_new_sub_pmd(unsigned long start, unsigned long end)
> +{
> +	vmemmap_flush_unused_pmd();
> +
> +	/*
> +	 * Mark the unused parts of the new memmap range
> +	 */
> +	if (!IS_ALIGNED(start, PMD_SIZE))
> +		memset((void *)start, PAGE_UNUSED,
> +		       start - ALIGN_DOWN(start, PMD_SIZE));
> +	/*
> +	 * We want to avoid memset(PAGE_UNUSED) when populating the vmemmap of
> +	 * consecutive sections. Remember for the last added PMD the last
> +	 * unused range in the populated PMD.
> +	 */
> +	if (!IS_ALIGNED(end, PMD_SIZE))
> +		unused_pmd_start = end;
> +}
>   
>   static void __meminit free_pagetable(struct page *page, int order)
>   {
> @@ -1008,10 +1073,10 @@ remove_pte_table(pte_t *pte_start, unsigned long addr, unsigned long end,
>   			 * with 0xFD, and remove the page when it is wholly
>   			 * filled with 0xFD.
>   			 */
> -			memset((void *)addr, PAGE_INUSE, next - addr);
> +			memset((void *)addr, PAGE_UNUSED, next - addr);
>   
>   			page_addr = page_address(pte_page(*pte));
> -			if (!memchr_inv(page_addr, PAGE_INUSE, PAGE_SIZE)) {
> +			if (!memchr_inv(page_addr, PAGE_UNUSED, PAGE_SIZE)) {
>   				free_pagetable(pte_page(*pte), 0);
>   

I remember already raising this, in the context of other cleanups, but 
let's start anew:

How could we ever even end up in "!PAGE_ALIGNED(addr) && 
PAGE_ALIGNED(next)"? As the comment correctly indicates, it would only 
make sense for "freeing vmemmap pages".

This would mean we are removing parts of a vmemmap page (4k), calling 
vmemmap_free()->remove_pagetable() on sub-page granularity.

Even sub-sections (2MB - 512 pages) have a memmap size with base pages:
- 56 bytes: 7 pages
- 64 bytes: 8 pages
- 72 bytes: 9 pages

sizeof(struct page) is always multiples of 8 bytes, so that will hold.

E.g., in __populate_section_memmap(), we already enforce proper 
subsection alignment.

IMHO, we should rip out that code here and enforce page alignment in 
vmemmap_populate()/vmemmap_free().

Am I missing something?

-- 
Thanks,

David / dhildenb