The patch titled Subject: hugetlb: use same fault hash key for shared and private mappings has been added to the -mm tree. Its filename is hugetlb-use-same-fault-hash-key-for-shared-and-private-mappings.patch

This patch should soon appear at http://ozlabs.org/~akpm/mmots/broken-out/hugetlb-use-same-fault-hash-key-for... and later at http://ozlabs.org/~akpm/mmotm/broken-out/hugetlb-use-same-fault-hash-key-for...

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------------------------------------------------------ From: Mike Kravetz mike.kravetz@oracle.com Subject: hugetlb: use same fault hash key for shared and private mappings

hugetlb uses a fault mutex hash table to prevent page faults of the same pages concurrently. The key for shared and private mappings is different. Shared keys off address_space and file index. Private keys off mm and virtual address. Consider a private mappings of a populated hugetlbfs file. A fault will map the page from the file and if needed do a COW to map a writable page.

Hugetlbfs hole punch uses the fault mutex to prevent mappings of file pages. It uses the address_space file index key. However, private mappings will use a different key and could race with this code to map the file page. This causes problems (BUG) for the page cache remove code as it expects the page to be unmapped. A sample stack is:

page dumped because: VM_BUG_ON_PAGE(page_mapped(page)) kernel BUG at mm/filemap.c:169! ... RIP: 0010:unaccount_page_cache_page+0x1b8/0x200 ... Call Trace: __delete_from_page_cache+0x39/0x220 delete_from_page_cache+0x45/0x70 remove_inode_hugepages+0x13c/0x380 ? __add_to_page_cache_locked+0x162/0x380 hugetlbfs_fallocate+0x403/0x540 ? _cond_resched+0x15/0x30 ? __inode_security_revalidate+0x5d/0x70 ? selinux_file_permission+0x100/0x130 vfs_fallocate+0x13f/0x270 ksys_fallocate+0x3c/0x80 __x64_sys_fallocate+0x1a/0x20 do_syscall_64+0x5b/0x180 entry_SYSCALL_64_after_hwframe+0x44/0xa9

There seems to be another potential COW issue/race with this approach of different private and shared keys as noted in commit 8382d914ebf7 ("mm, hugetlb: improve page-fault scalability").

Since every hugetlb mapping (even anon and private) is actually a file mapping, just use the address_space index key for all mappings. This results in potentially more hash collisions. However, this should not be the common case.

Link: http://lkml.kernel.org/r/20190328234704.27083-3-mike.kravetz@oracle.com Link: http://lkml.kernel.org/r/20190412165235.t4sscoujczfhuiyt@linux-r8p5 Fixes: b5cec28d36f5 ("hugetlbfs: truncate_hugepages() takes a range of pages") Signed-off-by: Mike Kravetz mike.kravetz@oracle.com Reviewed-by: Naoya Horiguchi n-horiguchi@ah.jp.nec.com Reviewed-by: Davidlohr Bueso dbueso@suse.de Cc: Joonsoo Kim iamjoonsoo.kim@lge.com Cc: "Kirill A . Shutemov" kirill.shutemov@linux.intel.com Cc: Michal Hocko mhocko@kernel.org Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

fs/hugetlbfs/inode.c | 7 ++----- include/linux/hugetlb.h | 4 +--- mm/hugetlb.c | 22 ++++++---------------- mm/userfaultfd.c | 3 +-- 4 files changed, 10 insertions(+), 26 deletions(-)

--- a/fs/hugetlbfs/inode.c~hugetlb-use-same-fault-hash-key-for-shared-and-private-mappings +++ a/fs/hugetlbfs/inode.c @@ -440,9 +440,7 @@ static void remove_inode_hugepages(struc u32 hash;

index = page->index; - hash = hugetlb_fault_mutex_hash(h, current->mm, - &pseudo_vma, - mapping, index, 0); + hash = hugetlb_fault_mutex_hash(h, mapping, index, 0); mutex_lock(&hugetlb_fault_mutex_table[hash]);

/* @@ -639,8 +637,7 @@ static long hugetlbfs_fallocate(struct f addr = index * hpage_size;

/* mutex taken here, fault path and hole punch */ - hash = hugetlb_fault_mutex_hash(h, mm, &pseudo_vma, mapping, - index, addr); + hash = hugetlb_fault_mutex_hash(h, mapping, index, addr); mutex_lock(&hugetlb_fault_mutex_table[hash]);

/* See if already present in mapping to avoid alloc/free */ --- a/include/linux/hugetlb.h~hugetlb-use-same-fault-hash-key-for-shared-and-private-mappings +++ a/include/linux/hugetlb.h @@ -123,9 +123,7 @@ void move_hugetlb_state(struct page *old void free_huge_page(struct page *page); void hugetlb_fix_reserve_counts(struct inode *inode); extern struct mutex *hugetlb_fault_mutex_table; -u32 hugetlb_fault_mutex_hash(struct hstate *h, struct mm_struct *mm, - struct vm_area_struct *vma, - struct address_space *mapping, +u32 hugetlb_fault_mutex_hash(struct hstate *h, struct address_space *mapping, pgoff_t idx, unsigned long address);

pte_t *huge_pmd_share(struct mm_struct *mm, unsigned long addr, pud_t *pud); --- a/mm/hugetlb.c~hugetlb-use-same-fault-hash-key-for-shared-and-private-mappings +++ a/mm/hugetlb.c @@ -3823,8 +3823,7 @@ retry: * handling userfault. Reacquire after handling * fault to make calling code simpler. */ - hash = hugetlb_fault_mutex_hash(h, mm, vma, mapping, - idx, haddr); + hash = hugetlb_fault_mutex_hash(h, mapping, idx, haddr); mutex_unlock(&hugetlb_fault_mutex_table[hash]); ret = handle_userfault(&vmf, VM_UFFD_MISSING); mutex_lock(&hugetlb_fault_mutex_table[hash]); @@ -3932,21 +3931,14 @@ backout_unlocked: }

#ifdef CONFIG_SMP -u32 hugetlb_fault_mutex_hash(struct hstate *h, struct mm_struct *mm, - struct vm_area_struct *vma, - struct address_space *mapping, +u32 hugetlb_fault_mutex_hash(struct hstate *h, struct address_space *mapping, pgoff_t idx, unsigned long address) { unsigned long key[2]; u32 hash;

- if (vma->vm_flags & VM_SHARED) { - key[0] = (unsigned long) mapping; - key[1] = idx; - } else { - key[0] = (unsigned long) mm; - key[1] = address >> huge_page_shift(h); - } + key[0] = (unsigned long) mapping; + key[1] = idx;

hash = jhash2((u32 *)&key, sizeof(key)/sizeof(u32), 0);

@@ -3957,9 +3949,7 @@ u32 hugetlb_fault_mutex_hash(struct hsta * For uniprocesor systems we always use a single mutex, so just * return 0 and avoid the hashing overhead. */ -u32 hugetlb_fault_mutex_hash(struct hstate *h, struct mm_struct *mm, - struct vm_area_struct *vma, - struct address_space *mapping, +u32 hugetlb_fault_mutex_hash(struct hstate *h, struct address_space *mapping, pgoff_t idx, unsigned long address) { return 0; @@ -4004,7 +3994,7 @@ vm_fault_t hugetlb_fault(struct mm_struc * get spurious allocation failures if two CPUs race to instantiate * the same page in the page cache. */ - hash = hugetlb_fault_mutex_hash(h, mm, vma, mapping, idx, haddr); + hash = hugetlb_fault_mutex_hash(h, mapping, idx, haddr); mutex_lock(&hugetlb_fault_mutex_table[hash]);

entry = huge_ptep_get(ptep); --- a/mm/userfaultfd.c~hugetlb-use-same-fault-hash-key-for-shared-and-private-mappings +++ a/mm/userfaultfd.c @@ -271,8 +271,7 @@ retry: */ idx = linear_page_index(dst_vma, dst_addr); mapping = dst_vma->vm_file->f_mapping; - hash = hugetlb_fault_mutex_hash(h, dst_mm, dst_vma, mapping, - idx, dst_addr); + hash = hugetlb_fault_mutex_hash(h, mapping, idx, dst_addr); mutex_lock(&hugetlb_fault_mutex_table[hash]);

err = -ENOMEM; _

Patches currently in -mm which might be from mike.kravetz@oracle.com are

hugetlbfs-fix-potential-over-underflow-setting-node-specific-nr_hugepages.patch huegtlbfs-on-restore-reserve-error-path-retain-subpool-reservation.patch hugetlb-use-same-fault-hash-key-for-shared-and-private-mappings.patch