On 1 Mar 2021, at 3:32, 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_UNUSED, 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>
> Reviewed-by: David Hildenbrand <david@redhat.com>
> ---
>  arch/x86/mm/init_64.c | 106 ++++++++++++++++++++++++++++++++++++++++++++++----
>  1 file changed, 98 insertions(+), 8 deletions(-)
>
> diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
> index 9ecb3c488ac8..7e8de63f02b3 100644
> --- a/arch/x86/mm/init_64.c
> +++ b/arch/x86/mm/init_64.c
> @@ -871,7 +871,93 @@ int arch_add_memory(int nid, u64 start, u64 size,
>  	return add_pages(nid, start_pfn, nr_pages, params);
>  }
>
> -#define PAGE_INUSE 0xFD
> +#ifdef CONFIG_SPARSEMEM_VMEMMAP
> +#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)
> +{
> +	/*
> +	 * As we expect to add in the same granularity as we remove, it's
> +	 * sufficient to mark only some piece used to block the memmap page from
> +	 * getting removed when removing some other adjacent memmap (just in
> +	 * case the first memmap never gets initialized e.g., because the memory
> +	 * block never gets onlined).
> +	 */
> +	memset((void *)start, 0, sizeof(struct page));
> +}
> +
> +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();
> +	__vmemmap_use_sub_pmd(start);
> +}
> +
> +static void __meminit vmemmap_use_new_sub_pmd(unsigned long start, unsigned long end)
> +{
> +	vmemmap_flush_unused_pmd();
> +
> +	/*
> +	 * Could be our memmap page is filled with PAGE_UNUSED already from a
> +	 * previous remove.
> +	 */
> +	__vmemmap_use_sub_pmd(start);
> +
> +	/*
> +	 * 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;
> +}
> +#endif
>
>  static void __meminit free_pagetable(struct page *page, int order)
>  {
> @@ -1006,7 +1092,6 @@ remove_pmd_table(pmd_t *pmd_start, unsigned long addr, unsigned long end,
>  	unsigned long next, pages = 0;
>  	pte_t *pte_base;
>  	pmd_t *pmd;
> -	void *page_addr;
>
>  	pmd = pmd_start + pmd_index(addr);
>  	for (; addr < end; addr = next, pmd++) {
> @@ -1027,12 +1112,11 @@ remove_pmd_table(pmd_t *pmd_start, unsigned long addr, unsigned long end,
>  				spin_unlock(&init_mm.page_table_lock);
>  				pages++;
>  			} else {
> -				/* If here, we are freeing vmemmap pages. */
> -				memset((void *)addr, PAGE_INUSE, next - addr);
> -
> -				page_addr = page_address(pmd_page(*pmd));
> -				if (!memchr_inv(page_addr, PAGE_INUSE,
> -						PMD_SIZE)) {
> +#ifdef CONFIG_SPARSEMEM_VMEMMAP
> +				/*
> +				 * Free the PMD if the whole range is unused.
> +				 */
> +				if (vmemmap_unuse_sub_pmd(addr, next)) {
>  					free_hugepage_table(pmd_page(*pmd),
>  							    altmap);
>
> @@ -1040,6 +1124,7 @@ remove_pmd_table(pmd_t *pmd_start, unsigned long addr, unsigned long end,
>  					pmd_clear(pmd);
>  					spin_unlock(&init_mm.page_table_lock);
>  				}
> +#endif
>  			}
>
>  			continue;
> @@ -1492,11 +1577,16 @@ static int __meminit vmemmap_populate_hugepages(unsigned long start,
>
>  				addr_end = addr + PMD_SIZE;
>  				p_end = p + PMD_SIZE;
> +
> +				if (!IS_ALIGNED(addr, PMD_SIZE) ||
> +				    !IS_ALIGNED(next, PMD_SIZE))
> +					vmemmap_use_new_sub_pmd(addr, next);
>  				continue;
>  			} else if (altmap)
>  				return -ENOMEM; /* no fallback */
>  		} else if (pmd_large(*pmd)) {
>  			vmemmap_verify((pte_t *)pmd, node, addr, next);
> +			vmemmap_use_sub_pmd(addr, next);

Hi Oscar,

vmemmap_use_new_sub_pmd and vmemmap_use_sub_pmd are not defined when CONFIG_SPARSEMEM_VMEMMAP
and !CONFIG_MEMORY_HOTPLUG. It leads to compilation errors.


—
Best Regards,
Yan Zi