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SubjectRe: [PATCH v14 4/8] mm: hugetlb: alloc the vmemmap pages associated with each HugeTLB page
On Thu, Feb 04, 2021 at 11:50:39AM +0800, Muchun Song wrote:
> When we free a HugeTLB page to the buddy allocator, we should allocate the
> vmemmap pages associated with it. But we may cannot allocate vmemmap pages
> when the system is under memory pressure, in this case, we just refuse to
> free the HugeTLB page instead of looping forever trying to allocate the
> pages.
>
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>

[...]

> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 4cfca27c6d32..5518283aa667 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1397,16 +1397,26 @@ static void __free_huge_page(struct page *page)
> h->resv_huge_pages++;
>
> if (HPageTemporary(page)) {
> - list_del(&page->lru);
> ClearHPageTemporary(page);
> +
> + if (alloc_huge_page_vmemmap(h, page, GFP_ATOMIC)) {
> + h->surplus_huge_pages++;
> + h->surplus_huge_pages_node[nid]++;
> + goto enqueue;
> + }
> + list_del(&page->lru);
> update_and_free_page(h, page);
> } else if (h->surplus_huge_pages_node[nid]) {
> + if (alloc_huge_page_vmemmap(h, page, GFP_ATOMIC))
> + goto enqueue;
> +
> /* remove the page from active list */
> list_del(&page->lru);
> update_and_free_page(h, page);
> h->surplus_huge_pages--;
> h->surplus_huge_pages_node[nid]--;
> } else {
> +enqueue:
> arch_clear_hugepage_flags(page);
> enqueue_huge_page(h, page);

Ok, we just keep them in the pool in case we fail to allocate.


> diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
> index ddd872ab6180..0bd6b8d7282d 100644
> --- a/mm/hugetlb_vmemmap.c
> +++ b/mm/hugetlb_vmemmap.c
> @@ -169,6 +169,8 @@
> * (last) level. So this type of HugeTLB page can be optimized only when its
> * size of the struct page structs is greater than 2 pages.

[...]

> +int alloc_huge_page_vmemmap(struct hstate *h, struct page *head, gfp_t gfp_mask)
> +{
> + int ret;
> + unsigned long vmemmap_addr = (unsigned long)head;
> + unsigned long vmemmap_end, vmemmap_reuse;
> +
> + if (!free_vmemmap_pages_per_hpage(h))
> + return 0;
> +
> + vmemmap_addr += RESERVE_VMEMMAP_SIZE;
> + vmemmap_end = vmemmap_addr + free_vmemmap_pages_size_per_hpage(h);
> + vmemmap_reuse = vmemmap_addr - PAGE_SIZE;
> +
> + /*
> + * The pages which the vmemmap virtual address range [@vmemmap_addr,
> + * @vmemmap_end) are mapped to are freed to the buddy allocator, and
> + * the range is mapped to the page which @vmemmap_reuse is mapped to.
> + * When a HugeTLB page is freed to the buddy allocator, previously
> + * discarded vmemmap pages must be allocated and remapping.
> + */
> + ret = vmemmap_remap_alloc(vmemmap_addr, vmemmap_end, vmemmap_reuse,
> + gfp_mask | __GFP_NOWARN | __GFP_THISNODE);

Why don't you set all the GFP flags here?

vmemmap_remap_alloc(vmemmap_addr, vmemmap_end, vmemmap_reuse, GFP_ATOMIC|
__GFP_NOWARN | __GFP_THISNODE) ?

> diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
> index 50c1dc00b686..277eb43aebd5 100644
> --- a/mm/sparse-vmemmap.c
> +++ b/mm/sparse-vmemmap.c

[...]

> +static int alloc_vmemmap_page_list(unsigned long start, unsigned long end,
> + gfp_t gfp_mask, struct list_head *list)

I think it would make more sense for this function to get the nid and the
nr_pages to allocate directly.

> +{
> + unsigned long addr;
> + int nid = page_to_nid((const void *)start);

Uh, that void is a bit ugly. page_to_nid(struct page *)start).
Do not need the const either.

> + struct page *page, *next;
> +
> + for (addr = start; addr < end; addr += PAGE_SIZE) {
> + page = alloc_pages_node(nid, gfp_mask, 0);
> + if (!page)
> + goto out;
> + list_add_tail(&page->lru, list);
> + }

and replace this by while(--nr_pages) etc.

I did not really go in depth, but looks good to me, and much more simply
overall.

The only thing I am not sure about is the use of GFP_ATOMIC.
It has been raised before than when we are close to OOM, the user might want
to try to free up some memory by dissolving free_huge_pages, and so we might
want to dip in the reserves.

Given the fact that we are prepared to fail, and that we do not retry, I would
rather use GFP_KERNEL than to have X pages atomically allocated and then realize
we need to drop them on the ground because we cannot go further at some point.
I think those reserves would be better off used by someone else in that
situation.

But this is just my thoughs, and given the fact that there seems to be a consensus
of susing GFP_ATOMIC.

--
Oscar Salvador
SUSE L3

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