Re: [PATCH v4 02/14] mm: Switch mm->get_unmapped_area() to a flag

[Dan Williams <dan.j.williams@intel.com>]

Rick Edgecombe wrote:
> The mm_struct contains a function pointer *get_unmapped_area(), which
> is set to either arch_get_unmapped_area() or
> arch_get_unmapped_area_topdown() during the initialization of the mm.
> 
> Since the function pointer only ever points to two functions that are named
> the same across all arch's, a function pointer is not really required. In
> addition future changes will want to add versions of the functions that
> take additional arguments. So to save a pointers worth of bytes in
> mm_struct, and prevent adding additional function pointers to mm_struct in
> future changes, remove it and keep the information about which
> get_unmapped_area() to use in a flag.
> 
> Add the new flag to MMF_INIT_MASK so it doesn't get clobbered on fork by
> mmf_init_flags(). Most MM flags get clobbered on fork. In the pre-existing
> behavior mm->get_unmapped_area() would get copied to the new mm in
> dup_mm(), so not clobbering the flag preserves the existing behavior
> around inheriting the topdown-ness.
> 
> Introduce a helper, mm_get_unmapped_area(), to easily convert code that
> refers to the old function pointer to instead select and call either
> arch_get_unmapped_area() or arch_get_unmapped_area_topdown() based on the
> flag. Then drop the mm->get_unmapped_area() function pointer. Leave the
> get_unmapped_area() pointer in struct file_operations alone. The main
> purpose of this change is to reorganize in preparation for future changes,
> but it also converts the calls of mm->get_unmapped_area() from indirect
> branches into a direct ones.
> 
> The stress-ng bigheap benchmark calls realloc a lot, which calls through
> get_unmapped_area() in the kernel. On x86, the change yielded a ~1%
> improvement there on a retpoline config.
> 
> In testing a few x86 configs, removing the pointer unfortunately didn't
> result in any actual size reductions in the compiled layout of mm_struct.
> But depending on compiler or arch alignment requirements, the change could
> shrink the size of mm_struct.
> 
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
> Acked-by: Dave Hansen <dave.hansen@linux.intel.com>
> Acked-by: Liam R. Howlett <Liam.Howlett@oracle.com>
> Reviewed-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Cc: linux-s390@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> Cc: sparclinux@vger.kernel.org
> Cc: linux-sgx@vger.kernel.org
> Cc: nvdimm@lists.linux.dev
> Cc: linux-cxl@vger.kernel.org
> Cc: linux-mm@kvack.org
> Cc: linux-fsdevel@vger.kernel.org
> Cc: io-uring@vger.kernel.org
> Cc: bpf@vger.kernel.org
> ---
> v4:
>  - Split out pde_get_unmapped_area() refactor into separate patch
>    (Christophe Leroy)
> 
> v3:
>  - Fix comment that still referred to mm->get_unmapped_area()
>  - Resolve trivial rebase conflicts with "mm: thp_get_unmapped_area must
>    honour topdown preference"
>  - Spelling fix in log
> 
> v2:
>  - Fix comment on MMF_TOPDOWN (Kirill, rppt)
>  - Move MMF_TOPDOWN to actually unused bit
>  - Add MMF_TOPDOWN to MMF_INIT_MASK so it doesn't get clobbered on fork,
>    and result in the children using the search up path.
>  - New lower performance results after above bug fix
>  - Add Reviews and Acks
> ---
>  arch/s390/mm/hugetlbpage.c       |  2 +-
>  arch/s390/mm/mmap.c              |  4 ++--
>  arch/sparc/kernel/sys_sparc_64.c | 15 ++++++---------
>  arch/sparc/mm/hugetlbpage.c      |  2 +-
>  arch/x86/kernel/cpu/sgx/driver.c |  2 +-
>  arch/x86/mm/hugetlbpage.c        |  2 +-
>  arch/x86/mm/mmap.c               |  4 ++--
>  drivers/char/mem.c               |  2 +-
>  drivers/dax/device.c             |  6 +++---
>  fs/hugetlbfs/inode.c             |  4 ++--
>  fs/proc/inode.c                  |  3 ++-
>  fs/ramfs/file-mmu.c              |  2 +-
>  include/linux/mm_types.h         |  6 +-----
>  include/linux/sched/coredump.h   |  5 ++++-
>  include/linux/sched/mm.h         |  5 +++++
>  io_uring/io_uring.c              |  2 +-
>  kernel/bpf/arena.c               |  2 +-
>  kernel/bpf/syscall.c             |  2 +-
>  mm/debug.c                       |  6 ------
>  mm/huge_memory.c                 |  9 ++++-----
>  mm/mmap.c                        | 21 ++++++++++++++++++---
>  mm/shmem.c                       | 11 +++++------
>  mm/util.c                        |  6 +++---
>  23 files changed, 66 insertions(+), 57 deletions(-)
> 
> diff --git a/arch/s390/mm/hugetlbpage.c b/arch/s390/mm/hugetlbpage.c
> index c2e8242bd15d..219d906fe830 100644
> --- a/arch/s390/mm/hugetlbpage.c
> +++ b/arch/s390/mm/hugetlbpage.c
> @@ -328,7 +328,7 @@ unsigned long hugetlb_get_unmapped_area(struct file *file, unsigned long addr,
>  			goto check_asce_limit;
>  	}
>  
> -	if (mm->get_unmapped_area == arch_get_unmapped_area)
> +	if (!test_bit(MMF_TOPDOWN, &mm->flags))
>  		addr = hugetlb_get_unmapped_area_bottomup(file, addr, len,
>  				pgoff, flags);
>  	else
> diff --git a/arch/s390/mm/mmap.c b/arch/s390/mm/mmap.c
> index b14fc0887654..6b2e4436ad4a 100644
> --- a/arch/s390/mm/mmap.c
> +++ b/arch/s390/mm/mmap.c
> @@ -185,10 +185,10 @@ void arch_pick_mmap_layout(struct mm_struct *mm, struct rlimit *rlim_stack)
>  	 */
>  	if (mmap_is_legacy(rlim_stack)) {
>  		mm->mmap_base = mmap_base_legacy(random_factor);
> -		mm->get_unmapped_area = arch_get_unmapped_area;
> +		clear_bit(MMF_TOPDOWN, &mm->flags);
>  	} else {
>  		mm->mmap_base = mmap_base(random_factor, rlim_stack);
> -		mm->get_unmapped_area = arch_get_unmapped_area_topdown;
> +		set_bit(MMF_TOPDOWN, &mm->flags);
>  	}
>  }
>  
> diff --git a/arch/sparc/kernel/sys_sparc_64.c b/arch/sparc/kernel/sys_sparc_64.c
> index 1e9a9e016237..1dbf7211666e 100644
> --- a/arch/sparc/kernel/sys_sparc_64.c
> +++ b/arch/sparc/kernel/sys_sparc_64.c
> @@ -218,14 +218,10 @@ arch_get_unmapped_area_topdown(struct file *filp, const unsigned long addr0,
>  unsigned long get_fb_unmapped_area(struct file *filp, unsigned long orig_addr, unsigned long len, unsigned long pgoff, unsigned long flags)
>  {
>  	unsigned long align_goal, addr = -ENOMEM;
> -	unsigned long (*get_area)(struct file *, unsigned long,
> -				  unsigned long, unsigned long, unsigned long);
> -
> -	get_area = current->mm->get_unmapped_area;
>  
>  	if (flags & MAP_FIXED) {
>  		/* Ok, don't mess with it. */
> -		return get_area(NULL, orig_addr, len, pgoff, flags);
> +		return mm_get_unmapped_area(current->mm, NULL, orig_addr, len, pgoff, flags);
>  	}
>  	flags &= ~MAP_SHARED;
>  
> @@ -238,7 +234,8 @@ unsigned long get_fb_unmapped_area(struct file *filp, unsigned long orig_addr, u
>  		align_goal = (64UL * 1024);
>  
>  	do {
> -		addr = get_area(NULL, orig_addr, len + (align_goal - PAGE_SIZE), pgoff, flags);
> +		addr = mm_get_unmapped_area(current->mm, NULL, orig_addr,
> +					    len + (align_goal - PAGE_SIZE), pgoff, flags);
>  		if (!(addr & ~PAGE_MASK)) {
>  			addr = (addr + (align_goal - 1UL)) & ~(align_goal - 1UL);
>  			break;
> @@ -256,7 +253,7 @@ unsigned long get_fb_unmapped_area(struct file *filp, unsigned long orig_addr, u
>  	 * be obtained.
>  	 */
>  	if (addr & ~PAGE_MASK)
> -		addr = get_area(NULL, orig_addr, len, pgoff, flags);
> +		addr = mm_get_unmapped_area(current->mm, NULL, orig_addr, len, pgoff, flags);
>  
>  	return addr;
>  }
> @@ -292,7 +289,7 @@ void arch_pick_mmap_layout(struct mm_struct *mm, struct rlimit *rlim_stack)
>  	    gap == RLIM_INFINITY ||
>  	    sysctl_legacy_va_layout) {
>  		mm->mmap_base = TASK_UNMAPPED_BASE + random_factor;
> -		mm->get_unmapped_area = arch_get_unmapped_area;
> +		clear_bit(MMF_TOPDOWN, &mm->flags);
>  	} else {
>  		/* We know it's 32-bit */
>  		unsigned long task_size = STACK_TOP32;
> @@ -303,7 +300,7 @@ void arch_pick_mmap_layout(struct mm_struct *mm, struct rlimit *rlim_stack)
>  			gap = (task_size / 6 * 5);
>  
>  		mm->mmap_base = PAGE_ALIGN(task_size - gap - random_factor);
> -		mm->get_unmapped_area = arch_get_unmapped_area_topdown;
> +		set_bit(MMF_TOPDOWN, &mm->flags);
>  	}
>  }
>  
> diff --git a/arch/sparc/mm/hugetlbpage.c b/arch/sparc/mm/hugetlbpage.c
> index b432500c13a5..38a1bef47efb 100644
> --- a/arch/sparc/mm/hugetlbpage.c
> +++ b/arch/sparc/mm/hugetlbpage.c
> @@ -123,7 +123,7 @@ hugetlb_get_unmapped_area(struct file *file, unsigned long addr,
>  		    (!vma || addr + len <= vm_start_gap(vma)))
>  			return addr;
>  	}
> -	if (mm->get_unmapped_area == arch_get_unmapped_area)
> +	if (!test_bit(MMF_TOPDOWN, &mm->flags))
>  		return hugetlb_get_unmapped_area_bottomup(file, addr, len,
>  				pgoff, flags);
>  	else
> diff --git a/arch/x86/kernel/cpu/sgx/driver.c b/arch/x86/kernel/cpu/sgx/driver.c
> index 262f5fb18d74..22b65a5f5ec6 100644
> --- a/arch/x86/kernel/cpu/sgx/driver.c
> +++ b/arch/x86/kernel/cpu/sgx/driver.c
> @@ -113,7 +113,7 @@ static unsigned long sgx_get_unmapped_area(struct file *file,
>  	if (flags & MAP_FIXED)
>  		return addr;
>  
> -	return current->mm->get_unmapped_area(file, addr, len, pgoff, flags);
> +	return mm_get_unmapped_area(current->mm, file, addr, len, pgoff, flags);
>  }
>  
>  #ifdef CONFIG_COMPAT
> diff --git a/arch/x86/mm/hugetlbpage.c b/arch/x86/mm/hugetlbpage.c
> index 5804bbae4f01..6d77c0039617 100644
> --- a/arch/x86/mm/hugetlbpage.c
> +++ b/arch/x86/mm/hugetlbpage.c
> @@ -141,7 +141,7 @@ hugetlb_get_unmapped_area(struct file *file, unsigned long addr,
>  	}
>  
>  get_unmapped_area:
> -	if (mm->get_unmapped_area == arch_get_unmapped_area)
> +	if (!test_bit(MMF_TOPDOWN, &mm->flags))
>  		return hugetlb_get_unmapped_area_bottomup(file, addr, len,
>  				pgoff, flags);
>  	else
> diff --git a/arch/x86/mm/mmap.c b/arch/x86/mm/mmap.c
> index c90c20904a60..a2cabb1c81e1 100644
> --- a/arch/x86/mm/mmap.c
> +++ b/arch/x86/mm/mmap.c
> @@ -129,9 +129,9 @@ static void arch_pick_mmap_base(unsigned long *base, unsigned long *legacy_base,
>  void arch_pick_mmap_layout(struct mm_struct *mm, struct rlimit *rlim_stack)
>  {
>  	if (mmap_is_legacy())
> -		mm->get_unmapped_area = arch_get_unmapped_area;
> +		clear_bit(MMF_TOPDOWN, &mm->flags);
>  	else
> -		mm->get_unmapped_area = arch_get_unmapped_area_topdown;
> +		set_bit(MMF_TOPDOWN, &mm->flags);
>  
>  	arch_pick_mmap_base(&mm->mmap_base, &mm->mmap_legacy_base,
>  			arch_rnd(mmap64_rnd_bits), task_size_64bit(0),
> diff --git a/drivers/char/mem.c b/drivers/char/mem.c
> index 3c6670cf905f..9b80e622ae80 100644
> --- a/drivers/char/mem.c
> +++ b/drivers/char/mem.c
> @@ -544,7 +544,7 @@ static unsigned long get_unmapped_area_zero(struct file *file,
>  	}
>  
>  	/* Otherwise flags & MAP_PRIVATE: with no shmem object beneath it */
> -	return current->mm->get_unmapped_area(file, addr, len, pgoff, flags);
> +	return mm_get_unmapped_area(current->mm, file, addr, len, pgoff, flags);
>  #else
>  	return -ENOSYS;
>  #endif
> diff --git a/drivers/dax/device.c b/drivers/dax/device.c
> index 93ebedc5ec8c..47c126d37b59 100644
> --- a/drivers/dax/device.c
> +++ b/drivers/dax/device.c
> @@ -329,14 +329,14 @@ static unsigned long dax_get_unmapped_area(struct file *filp,
>  	if ((off + len_align) < off)
>  		goto out;
>  
> -	addr_align = current->mm->get_unmapped_area(filp, addr, len_align,
> -			pgoff, flags);
> +	addr_align = mm_get_unmapped_area(current->mm, filp, addr, len_align,
> +					  pgoff, flags);
>  	if (!IS_ERR_VALUE(addr_align)) {
>  		addr_align += (off - addr_align) & (align - 1);
>  		return addr_align;
>  	}
>   out:
> -	return current->mm->get_unmapped_area(filp, addr, len, pgoff, flags);
> +	return mm_get_unmapped_area(current->mm, filp, addr, len, pgoff, flags);
>  }
>  
>  static const struct address_space_operations dev_dax_aops = {
> diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
> index 6502c7e776d1..3dee18bf47ed 100644
> --- a/fs/hugetlbfs/inode.c
> +++ b/fs/hugetlbfs/inode.c
> @@ -249,11 +249,11 @@ generic_hugetlb_get_unmapped_area(struct file *file, unsigned long addr,
>  	}
>  
>  	/*
> -	 * Use mm->get_unmapped_area value as a hint to use topdown routine.
> +	 * Use MMF_TOPDOWN flag as a hint to use topdown routine.
>  	 * If architectures have special needs, they should define their own
>  	 * version of hugetlb_get_unmapped_area.
>  	 */
> -	if (mm->get_unmapped_area == arch_get_unmapped_area_topdown)
> +	if (test_bit(MMF_TOPDOWN, &mm->flags))
>  		return hugetlb_get_unmapped_area_topdown(file, addr, len,
>  				pgoff, flags);
>  	return hugetlb_get_unmapped_area_bottomup(file, addr, len,
> diff --git a/fs/proc/inode.c b/fs/proc/inode.c
> index 75396a24fd8c..d19434e2a58e 100644
> --- a/fs/proc/inode.c
> +++ b/fs/proc/inode.c
> @@ -455,8 +455,9 @@ pde_get_unmapped_area(struct proc_dir_entry *pde, struct file *file, unsigned lo
>  		return pde->proc_ops->proc_get_unmapped_area(file, orig_addr, len, pgoff, flags);
>  
>  #ifdef CONFIG_MMU
> -	return current->mm->get_unmapped_area(file, orig_addr, len, pgoff, flags);
> +	return mm_get_unmapped_area(current->mm, file, orig_addr, len, pgoff, flags);
>  #endif
> +
>  	return orig_addr;
>  }
>  
> diff --git a/fs/ramfs/file-mmu.c b/fs/ramfs/file-mmu.c
> index c7a1aa3c882b..b45c7edc3225 100644
> --- a/fs/ramfs/file-mmu.c
> +++ b/fs/ramfs/file-mmu.c
> @@ -35,7 +35,7 @@ static unsigned long ramfs_mmu_get_unmapped_area(struct file *file,
>  		unsigned long addr, unsigned long len, unsigned long pgoff,
>  		unsigned long flags)
>  {
> -	return current->mm->get_unmapped_area(file, addr, len, pgoff, flags);
> +	return mm_get_unmapped_area(current->mm, file, addr, len, pgoff, flags);
>  }
>  
>  const struct file_operations ramfs_file_operations = {
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 5240bd7bca33..9313e43123d4 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -777,11 +777,7 @@ struct mm_struct {
>  		} ____cacheline_aligned_in_smp;
>  
>  		struct maple_tree mm_mt;
> -#ifdef CONFIG_MMU
> -		unsigned long (*get_unmapped_area) (struct file *filp,
> -				unsigned long addr, unsigned long len,
> -				unsigned long pgoff, unsigned long flags);
> -#endif
> +
>  		unsigned long mmap_base;	/* base of mmap area */
>  		unsigned long mmap_legacy_base;	/* base of mmap area in bottom-up allocations */
>  #ifdef CONFIG_HAVE_ARCH_COMPAT_MMAP_BASES
> diff --git a/include/linux/sched/coredump.h b/include/linux/sched/coredump.h
> index 02f5090ffea2..e62ff805cfc9 100644
> --- a/include/linux/sched/coredump.h
> +++ b/include/linux/sched/coredump.h
> @@ -92,9 +92,12 @@ static inline int get_dumpable(struct mm_struct *mm)
>  #define MMF_VM_MERGE_ANY	30
>  #define MMF_VM_MERGE_ANY_MASK	(1 << MMF_VM_MERGE_ANY)
>  
> +#define MMF_TOPDOWN		31	/* mm searches top down by default */
> +#define MMF_TOPDOWN_MASK	(1 << MMF_TOPDOWN)
> +
>  #define MMF_INIT_MASK		(MMF_DUMPABLE_MASK | MMF_DUMP_FILTER_MASK |\
>  				 MMF_DISABLE_THP_MASK | MMF_HAS_MDWE_MASK |\
> -				 MMF_VM_MERGE_ANY_MASK)
> +				 MMF_VM_MERGE_ANY_MASK | MMF_TOPDOWN_MASK)
>  
>  static inline unsigned long mmf_init_flags(unsigned long flags)
>  {
> diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
> index b6543f9d78d6..ed1caa26c8be 100644
> --- a/include/linux/sched/mm.h
> +++ b/include/linux/sched/mm.h
> @@ -8,6 +8,7 @@
>  #include <linux/mm_types.h>
>  #include <linux/gfp.h>
>  #include <linux/sync_core.h>
> +#include <linux/sched/coredump.h>
>  
>  /*
>   * Routines for handling mm_structs
> @@ -186,6 +187,10 @@ arch_get_unmapped_area_topdown(struct file *filp, unsigned long addr,
>  			  unsigned long len, unsigned long pgoff,
>  			  unsigned long flags);
>  
> +unsigned long mm_get_unmapped_area(struct mm_struct *mm, struct file *filp,
> +				   unsigned long addr, unsigned long len,
> +				   unsigned long pgoff, unsigned long flags);
> +
>  unsigned long
>  generic_get_unmapped_area(struct file *filp, unsigned long addr,
>  			  unsigned long len, unsigned long pgoff,
> diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
> index 5d4b448fdc50..405bab0a560c 100644
> --- a/io_uring/io_uring.c
> +++ b/io_uring/io_uring.c
> @@ -3520,7 +3520,7 @@ static unsigned long io_uring_mmu_get_unmapped_area(struct file *filp,
>  #else
>  	addr = 0UL;
>  #endif
> -	return current->mm->get_unmapped_area(filp, addr, len, pgoff, flags);
> +	return mm_get_unmapped_area(current->mm, filp, addr, len, pgoff, flags);
>  }
>  
>  #else /* !CONFIG_MMU */
> diff --git a/kernel/bpf/arena.c b/kernel/bpf/arena.c
> index 86571e760dd6..74d566dcd2cb 100644
> --- a/kernel/bpf/arena.c
> +++ b/kernel/bpf/arena.c
> @@ -314,7 +314,7 @@ static unsigned long arena_get_unmapped_area(struct file *filp, unsigned long ad
>  			return -EINVAL;
>  	}
>  
> -	ret = current->mm->get_unmapped_area(filp, addr, len * 2, 0, flags);
> +	ret = mm_get_unmapped_area(current->mm, filp, addr, len * 2, 0, flags);
>  	if (IS_ERR_VALUE(ret))
>  		return ret;
>  	if ((ret >> 32) == ((ret + len - 1) >> 32))
> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> index ae2ff73bde7e..dead5e1977d8 100644
> --- a/kernel/bpf/syscall.c
> +++ b/kernel/bpf/syscall.c
> @@ -980,7 +980,7 @@ static unsigned long bpf_get_unmapped_area(struct file *filp, unsigned long addr
>  	if (map->ops->map_get_unmapped_area)
>  		return map->ops->map_get_unmapped_area(filp, addr, len, pgoff, flags);
>  #ifdef CONFIG_MMU
> -	return current->mm->get_unmapped_area(filp, addr, len, pgoff, flags);
> +	return mm_get_unmapped_area(current->mm, filp, addr, len, pgoff, flags);
>  #else
>  	return addr;
>  #endif
> diff --git a/mm/debug.c b/mm/debug.c
> index c1c1a6a484e4..37a17f77df9f 100644
> --- a/mm/debug.c
> +++ b/mm/debug.c
> @@ -180,9 +180,6 @@ EXPORT_SYMBOL(dump_vma);
>  void dump_mm(const struct mm_struct *mm)
>  {
>  	pr_emerg("mm %px task_size %lu\n"
> -#ifdef CONFIG_MMU
> -		"get_unmapped_area %px\n"
> -#endif
>  		"mmap_base %lu mmap_legacy_base %lu\n"
>  		"pgd %px mm_users %d mm_count %d pgtables_bytes %lu map_count %d\n"
>  		"hiwater_rss %lx hiwater_vm %lx total_vm %lx locked_vm %lx\n"
> @@ -208,9 +205,6 @@ void dump_mm(const struct mm_struct *mm)
>  		"def_flags: %#lx(%pGv)\n",
>  
>  		mm, mm->task_size,
> -#ifdef CONFIG_MMU
> -		mm->get_unmapped_area,
> -#endif
>  		mm->mmap_base, mm->mmap_legacy_base,
>  		mm->pgd, atomic_read(&mm->mm_users),
>  		atomic_read(&mm->mm_count),
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 9859aa4f7553..cede9ccb84dc 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -824,8 +824,8 @@ static unsigned long __thp_get_unmapped_area(struct file *filp,
>  	if (len_pad < len || (off + len_pad) < off)
>  		return 0;
>  
> -	ret = current->mm->get_unmapped_area(filp, addr, len_pad,
> -					      off >> PAGE_SHIFT, flags);
> +	ret = mm_get_unmapped_area(current->mm, filp, addr, len_pad,
> +				   off >> PAGE_SHIFT, flags);
>  
>  	/*
>  	 * The failure might be due to length padding. The caller will retry
> @@ -843,8 +843,7 @@ static unsigned long __thp_get_unmapped_area(struct file *filp,
>  
>  	off_sub = (off - ret) & (size - 1);
>  
> -	if (current->mm->get_unmapped_area == arch_get_unmapped_area_topdown &&
> -	    !off_sub)
> +	if (test_bit(MMF_TOPDOWN, &current->mm->flags) && !off_sub)
>  		return ret + size;
>  
>  	ret += off_sub;
> @@ -861,7 +860,7 @@ unsigned long thp_get_unmapped_area(struct file *filp, unsigned long addr,
>  	if (ret)
>  		return ret;
>  
> -	return current->mm->get_unmapped_area(filp, addr, len, pgoff, flags);
> +	return mm_get_unmapped_area(current->mm, filp, addr, len, pgoff, flags);
>  }
>  EXPORT_SYMBOL_GPL(thp_get_unmapped_area);
>  
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 6dbda99a47da..224e9ce1e2fd 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1813,7 +1813,8 @@ get_unmapped_area(struct file *file, unsigned long addr, unsigned long len,
>  		unsigned long pgoff, unsigned long flags)
>  {
>  	unsigned long (*get_area)(struct file *, unsigned long,
> -				  unsigned long, unsigned long, unsigned long);
> +				  unsigned long, unsigned long, unsigned long)
> +				  = NULL;
>  
>  	unsigned long error = arch_mmap_check(addr, len, flags);
>  	if (error)
> @@ -1823,7 +1824,6 @@ get_unmapped_area(struct file *file, unsigned long addr, unsigned long len,
>  	if (len > TASK_SIZE)
>  		return -ENOMEM;
>  
> -	get_area = current->mm->get_unmapped_area;
>  	if (file) {
>  		if (file->f_op->get_unmapped_area)
>  			get_area = file->f_op->get_unmapped_area;
> @@ -1842,7 +1842,11 @@ get_unmapped_area(struct file *file, unsigned long addr, unsigned long len,
>  	if (!file)
>  		pgoff = 0;
>  
> -	addr = get_area(file, addr, len, pgoff, flags);
> +	if (get_area)
> +		addr = get_area(file, addr, len, pgoff, flags);
> +	else
> +		addr = mm_get_unmapped_area(current->mm, file, addr, len,
> +					    pgoff, flags);
>  	if (IS_ERR_VALUE(addr))
>  		return addr;
>  
> @@ -1857,6 +1861,17 @@ get_unmapped_area(struct file *file, unsigned long addr, unsigned long len,
>  
>  EXPORT_SYMBOL(get_unmapped_area);
>  
> +unsigned long
> +mm_get_unmapped_area(struct mm_struct *mm, struct file *file,
> +		     unsigned long addr, unsigned long len,
> +		     unsigned long pgoff, unsigned long flags)
> +{

Seems like a small waste to have all call sites now need to push an
additional @mm argument onto the stack just to figure out what function
to call.

> +	if (test_bit(MMF_TOPDOWN, &mm->flags))
> +		return arch_get_unmapped_area_topdown(file, addr, len, pgoff, flags);
> +	return arch_get_unmapped_area(file, addr, len, pgoff, flags);

This seems small enough to be amenable to a static inline, but that
would require exporting the arch calls which might be painful.

Would it not be possible to drop the @mm argument and just reference
current->mm internal to this function? Just call this funcion
current_get_unmapped_area()?

Otherwise looks like a useful change to me.