Re: [PATCH 2/5] mm,memory_hotplug: Allocate memmap from the added memory range

Oscar Salvador
2021-01-26T03:10:11Z

On Mon, Jan 25, 2021 at 11:39:55AM +0100, Oscar Salvador wrote:
> > Interresting, so we automatically support differeing sizeof(struct
> > page). I guess it will be problematic in case of sizeof(struct page) !=
> > 64, because then, we might not have multiples of 2MB for the memmap of a
> > memory block.
> >
> > IIRC, it could happen that if we add two consecutive memory blocks, that
> > the second one might reuse parts of the vmemmap residing on the first
> > memory block. If you remove the first one, you might be in trouble.
> >
> > E.g., on x86-64
> > vmemmap_populate()->vmemmap_populate_hugepages()->vmemmap_alloc_block_buf():
> > - Populate a huge page
> >
> > vmemmap_free()->remove_pagetable()...->remove_pmd_table():
> > - memchr_inv() will leave the hugepage populated.
> >
> > Do we want to fence that off, maybe in mhp_supports_memmap_on_memory()?
> > Or do we somehow want to fix that? We should never populate partial huge
> > pages from an altmap ...
> >
> > But maybe I am missing something.
>
> No, you are not missing anything.
>
> I think that remove_pmd_table() should be fixed.
> vmemmap_populate_hugepages() allocates PMD_SIZE chunk and marks the PMD as
> PAGE_KERNEL_LARGE, so when remove_pmd_table() sees that 1) the PMD
> is large and 2) the chunk is not aligned, the memset() should be writing
> PAGE_INUSE for a PMD chunk.
>
> I do not really a see a reason why this should not happen.
> Actually, we will be leaving pagetables behind as we never get to free
> the PMD chunk when sizeof(struct page) is not a multiple of 2MB.
>
> I plan to fix remove_pmd_table(), but for now let us not allow to use this feature
> if the size of a struct page is not 64.
> Let us keep it simply for the time being, shall we?

My first intention was:

diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
index b5a3fa4033d3..0c9756a2eb24 100644
--- a/arch/x86/mm/init_64.c
+++ b/arch/x86/mm/init_64.c
@@ -1044,32 +1044,14 @@ remove_pmd_table(pmd_t *pmd_start, unsigned long addr, unsigned long end,
continue;

if (pmd_large(*pmd)) {
- if (IS_ALIGNED(addr, PMD_SIZE) &&
- IS_ALIGNED(next, PMD_SIZE)) {
- if (!direct)
- free_hugepage_table(pmd_page(*pmd),
- altmap);
-
- spin_lock(&init_mm.page_table_lock);
- pmd_clear(pmd);
- 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)) {
- free_hugepage_table(pmd_page(*pmd),
- altmap);
-
- spin_lock(&init_mm.page_table_lock);
- pmd_clear(pmd);
- spin_unlock(&init_mm.page_table_lock);
- }
- }
+ if (!direct)
+ free_hugepage_table(pmd_page(*pmd),
+ altmap);

+ spin_lock(&init_mm.page_table_lock);
+ pmd_clear(pmd);
+ spin_unlock(&init_mm.page_table_lock);
+ pages++;
continue;
}

I did not try it out yet and this might explode badly, but AFAICS, a PMD size
chunk is always allocated even when the section does not spand 2MB.
E.g: sizeof(struct page) = 56.

PAGES_PER_SECTION * 64 = 2097152
PAGES_PER_SECTION * 56 = 1835008

Even in the latter case, vmemmap_populate_hugepages will allocate a PMD size chunk
to satisfy the unaligned range.
So, unless I am missing something, it should not be a problem to free that 2MB in
remove_pmd_table when 1) the PMD is large and 2) the range is not aligned.

--
Oscar Salvador
SUSE L3