From: Joao Martins joao.m.martins@oracle.com

mainline inclusion from mainline-v6.2-rc1 commit 11aad2631bf74b3c811dee76154702aab855a323 category: feature bugzilla: https://gitee.com/openeuler/kernel/issues/I6SROX CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?i...

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Today with `hugetlb_free_vmemmap=on` the struct page memory that is freed back to page allocator is as following: for a 2M hugetlb page it will reuse the first 4K vmemmap page to remap the remaining 7 vmemmap pages, and for a 1G hugetlb it will remap the remaining 4095 vmemmap pages. Essentially, that means that it breaks the first 4K of a potentially contiguous chunk of memory of 32K (for 2M hugetlb pages) or 16M (for 1G hugetlb pages). For this reason the memory that it's free back to page allocator cannot be used for hugetlb to allocate huge pages of the same size, but rather only of a smaller huge page size:

Trying to assign a 64G node to hugetlb (on a 128G 2node guest, each node having 64G):

* Before allocation: Free pages count per migrate type at order 0 1 2 3 4 5 6 7 8 9 10 ... Node 0, zone Normal, type Movable 340 100 32 15 1 2 0 0 0 1 15558

$ echo 32768 > /sys/devices/system/node/node0/hugepages/hugepages-2048kB/nr_hugepages $ cat /sys/devices/system/node/node0/hugepages/hugepages-2048kB/nr_hugepages 31987

* After:

Node 0, zone Normal, type Movable 30893 32006 31515 7 0 0 0 0 0 0 0

Notice how the memory freed back are put back into 4K / 8K / 16K page pools. And it allocates a total of 31987 pages (63974M).

To fix this behaviour rather than remapping second vmemmap page (thus breaking the contiguous block of memory backing the struct pages) repopulate the first vmemmap page with a new one. We allocate and copy from the currently mapped vmemmap page, and then remap it later on. The same algorithm works if there's a pre initialized walk::reuse_page and the head page doesn't need to be skipped and instead we remap it when the @addr being changed is the @reuse_addr.

The new head page is allocated in vmemmap_remap_free() given that on restore there's no need for functional change. Note that, because right now one hugepage is remapped at a time, thus only one free 4K page at a time is needed to remap the head page. Should it fail to allocate said new page, it reuses the one that's already mapped just like before. As a result, for every 64G of contiguous hugepages it can give back 1G more of contiguous memory per 64G, while needing in total 128M new 4K pages (for 2M hugetlb) or 256k (for 1G hugetlb).

After the changes, try to assign a 64G node to hugetlb (on a 128G 2node guest, each node with 64G):

* Before allocation Free pages count per migrate type at order 0 1 2 3 4 5 6 7 8 9 10 ... Node 0, zone Normal, type Movable 1 1 1 0 0 1 0 0 1 1 15564

$ echo 32768 > /sys/devices/system/node/node0/hugepages/hugepages-2048kB/nr_hugepages $ cat /sys/devices/system/node/node0/hugepages/hugepages-2048kB/nr_hugepages 32394

* After:

Node 0, zone Normal, type Movable 0 50 97 108 96 81 70 46 18 0 0

In the example above, 407 more hugeltb 2M pages are allocated i.e. 814M out of the 32394 (64788M) allocated. So the memory freed back is indeed being used back in hugetlb and there's no massive order-0..order-2 pages accumulated unused.

[joao.m.martins@oracle.com: v3] Link: https://lkml.kernel.org/r/20221109200623.96867-1-joao.m.martins@oracle.com [joao.m.martins@oracle.com: add smp_wmb() to ensure page contents are visible prior to PTE write] Link: https://lkml.kernel.org/r/20221110121214.6297-1-joao.m.martins@oracle.com Link: https://lkml.kernel.org/r/20221107153922.77094-1-joao.m.martins@oracle.com Signed-off-by: Joao Martins joao.m.martins@oracle.com Reviewed-by: Muchun Song songmuchun@bytedance.com Cc: Mike Kravetz mike.kravetz@oracle.com Signed-off-by: Andrew Morton akpm@linux-foundation.org Conflicts: mm/hugetlb_vmemmap.c Signed-off-by: Liu Shixin liushixin2@huawei.com --- mm/sparse-vmemmap.c | 41 ++++++++++++++++++++++++++++++++++------- 1 file changed, 34 insertions(+), 7 deletions(-)

diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c index a5c2efcf59e2..6803c89c5d21 100644 --- a/mm/sparse-vmemmap.c +++ b/mm/sparse-vmemmap.c @@ -221,12 +221,7 @@ static int vmemmap_remap_range(unsigned long start, unsigned long end, return ret; } while (pgd++, addr = next, addr != end);

- /* - * We only change the mapping of the vmemmap virtual address range - * [@start + PAGE_SIZE, end), so we only need to flush the TLB which - * belongs to the range. - */ - flush_tlb_kernel_range(start + PAGE_SIZE, end); + flush_tlb_kernel_range(start, end);

return 0; } @@ -264,9 +259,23 @@ static void vmemmap_remap_pte(pte_t *pte, unsigned long addr, * to the tail pages. */ pgprot_t pgprot = PAGE_KERNEL_RO; - pte_t entry = mk_pte(walk->reuse_page, pgprot); struct page *page = pte_page(*pte); + pte_t entry;

+ /* Remapping the head page requires r/w */ + if (unlikely(addr == walk->reuse_addr)) { + pgprot = PAGE_KERNEL; + list_del(&walk->reuse_page->lru); + + /* + * Makes sure that preceding stores to the page contents from + * vmemmap_remap_free() become visible before the set_pte_at() + * write. + */ + smp_wmb(); + } + + entry = mk_pte(walk->reuse_page, pgprot); list_add_tail(&page->lru, walk->vmemmap_pages); set_pte_at(&init_mm, addr, pte, entry); } @@ -331,6 +340,24 @@ int vmemmap_remap_free(unsigned long start, unsigned long end, .reuse_addr = reuse, .vmemmap_pages = &vmemmap_pages, }; + int nid = page_to_nid((struct page *)start); + gfp_t gfp_mask = GFP_KERNEL | __GFP_THISNODE | __GFP_NORETRY | + __GFP_NOWARN; + + /* + * Allocate a new head vmemmap page to avoid breaking a contiguous + * block of struct page memory when freeing it back to page allocator + * in free_vmemmap_page_list(). This will allow the likely contiguous + * struct page backing memory to be kept contiguous and allowing for + * more allocations of hugepages. Fallback to the currently + * mapped head page in case should it fail to allocate. + */ + walk.reuse_page = alloc_pages_node(nid, gfp_mask, 0); + if (walk.reuse_page) { + copy_page(page_to_virt(walk.reuse_page), + (void *)walk.reuse_addr); + list_add(&walk.reuse_page->lru, &vmemmap_pages); + }

/* * In order to make remapping routine most efficient for the huge pages,