Re: [PATCH v4 3/3] x86/vmemmap: Handle unpopulated sub-pmd ranges

Zi Yan
2021-03-09T19:39:55Z

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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

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