Re: [PATCH v8 06/12] mm/hugetlb: Allocate the vmemmap pages associated with each HugeTLB page

Oscar Salvador
2020-12-11T09:37:03Z

On Thu, Dec 10, 2020 at 11:55:20AM +0800, Muchun Song wrote:
> When we free a HugeTLB page to the buddy allocator, we should allocate the
> vmemmap pages associated with it. We can do that in the __free_hugepage()
"vmemmap pages that describe the range" would look better to me, but it is ok.

> +#define GFP_VMEMMAP_PAGE \
> + (GFP_KERNEL | __GFP_RETRY_MAYFAIL | __GFP_HIGH | __GFP_NOWARN)
>
> #ifndef VMEMMAP_HPAGE_SHIFT
> #define VMEMMAP_HPAGE_SHIFT HPAGE_SHIFT
> @@ -197,6 +200,11 @@
> (__boundary - 1 < (end) - 1) ? __boundary : (end); \
> })
>
> +typedef void (*vmemmap_remap_pte_func_t)(struct page *reuse, pte_t *pte,
> + unsigned long start, unsigned long end,
> + void *priv);

Any reason to not have defined GFP_VMEMMAP_PAGE and the new typedef into
hugetlb_vmemmap.h?

> +static void vmemmap_restore_pte_range(struct page *reuse, pte_t *pte,
> + unsigned long start, unsigned long end,
> + void *priv)
> +{
> + pgprot_t pgprot = PAGE_KERNEL;
> + void *from = page_to_virt(reuse);
> + unsigned long addr;
> + struct list_head *pages = priv;
[...]
> +
> + /*
> + * Make sure that any data that writes to the @to is made
> + * visible to the physical page.
> + */
> + flush_kernel_vmap_range(to, PAGE_SIZE);

Correct me if I am wrong, but flush_kernel_vmap_range is a NOOP under arches which
do not have ARCH_HAS_FLUSH_KERNEL_DCACHE_PAGE.
Since we only enable support for x86_64, and x86_64 is one of those arches,
could we remove this, and introduced later on in case we enable this feature
on an arch that needs it?

I am not sure if you need to flush the range somehow, as you did in
vmemmap_remap_range.

> +retry:
> + page = alloc_page(GFP_VMEMMAP_PAGE);
> + if (unlikely(!page)) {
> + msleep(100);
> + /*
> + * We should retry infinitely, because we cannot
> + * handle allocation failures. Once we allocate
> + * vmemmap pages successfully, then we can free
> + * a HugeTLB page.
> + */
> + goto retry;

I think this is the trickiest part.
With 2MB HugeTLB pages we only need 6 pages, but with 1GB, the number of pages
we need to allocate increases significantly (4088 pages IIRC).
And you are using __GFP_HIGH, which will allow us to use more memory (by
cutting down the watermark), but it might lead to putting the system
on its knees wrt. memory.
And yes, I know that once we allocate the 4088 pages, 1GB gets freed, but
still.

I would like to hear Michal's thoughts on this one, but I wonder if it makes
sense to not let 1GB-HugeTLB pages be freed.

--
Oscar Salvador
SUSE L3