Re: [PATCH v4] mm: Add MREMAP_DONTUNMAP to mremap().

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Fri, Feb 07, 2020 at 12:18:56PM -0800, Brian Geffon wrote:
> When remapping an anonymous, private mapping, if MREMAP_DONTUNMAP is
> set, the source mapping will not be removed. Instead it will be
> cleared as if a brand new anonymous, private mapping had been created
> atomically as part of the mremap() call.  If a userfaultfd was watching
> the source, it will continue to watch the new mapping.  For a mapping
> that is shared or not anonymous, MREMAP_DONTUNMAP will cause the
> mremap() call to fail. Because MREMAP_DONTUNMAP always results in moving
> a VMA you MUST use the MREMAP_MAYMOVE flag. The final result is two
> equally sized VMAs where the destination contains the PTEs of the source.
> 
> We hope to use this in Chrome OS where with userfaultfd we could write
> an anonymous mapping to disk without having to STOP the process or worry
> about VMA permission changes.
> 
> This feature also has a use case in Android, Lokesh Gidra has said
> that "As part of using userfaultfd for GC, We'll have to move the physical
> pages of the java heap to a separate location. For this purpose mremap
> will be used. Without the MREMAP_DONTUNMAP flag, when I mremap the java
> heap, its virtual mapping will be removed as well. Therefore, we'll
> require performing mmap immediately after. This is not only time consuming
> but also opens a time window where a native thread may call mmap and
> reserve the java heap's address range for its own usage. This flag
> solves the problem."
>            
> Signed-off-by: Brian Geffon <bgeffon@google.com>
> ---
>  include/uapi/linux/mman.h |  5 +-
>  mm/mremap.c               | 98 ++++++++++++++++++++++++++++++---------
>  2 files changed, 80 insertions(+), 23 deletions(-)
> 
> diff --git a/include/uapi/linux/mman.h b/include/uapi/linux/mman.h
> index fc1a64c3447b..923cc162609c 100644
> --- a/include/uapi/linux/mman.h
> +++ b/include/uapi/linux/mman.h
> @@ -5,8 +5,9 @@
>  #include <asm/mman.h>
>  #include <asm-generic/hugetlb_encode.h>
>  
> -#define MREMAP_MAYMOVE	1
> -#define MREMAP_FIXED	2
> +#define MREMAP_MAYMOVE		1
> +#define MREMAP_FIXED		2
> +#define MREMAP_DONTUNMAP	4
>  
>  #define OVERCOMMIT_GUESS		0
>  #define OVERCOMMIT_ALWAYS		1
> diff --git a/mm/mremap.c b/mm/mremap.c
> index 122938dcec15..9f4aa17f178b 100644
> --- a/mm/mremap.c
> +++ b/mm/mremap.c
> @@ -318,8 +318,8 @@ unsigned long move_page_tables(struct vm_area_struct *vma,
>  static unsigned long move_vma(struct vm_area_struct *vma,
>  		unsigned long old_addr, unsigned long old_len,
>  		unsigned long new_len, unsigned long new_addr,
> -		bool *locked, struct vm_userfaultfd_ctx *uf,
> -		struct list_head *uf_unmap)
> +		bool *locked, unsigned long flags,
> +		struct vm_userfaultfd_ctx *uf, struct list_head *uf_unmap)
>  {
>  	struct mm_struct *mm = vma->vm_mm;
>  	struct vm_area_struct *new_vma;
> @@ -408,11 +408,41 @@ static unsigned long move_vma(struct vm_area_struct *vma,
>  	if (unlikely(vma->vm_flags & VM_PFNMAP))
>  		untrack_pfn_moved(vma);
>  
> +	if (unlikely(!err && (flags & MREMAP_DONTUNMAP))) {
> +		if (vm_flags & VM_ACCOUNT) {
> +			/* Always put back VM_ACCOUNT since we won't unmap */
> +			vma->vm_flags |= VM_ACCOUNT;
> +
> +			vm_acct_memory(vma_pages(new_vma));
> +		}
> +
> +		/*
> +		 * locked_vm accounting: if the mapping remained the same size
> +		 * it will have just moved and we don't need to touch locked_vm
> +		 * because we skip the do_unmap. If the mapping shrunk before
> +		 * being moved then the do_unmap on that portion will have
> +		 * adjusted vm_locked. Only if the mapping grows do we need to
> +		 * do something special; the reason is locked_vm only accounts
> +		 * for old_len, but we're now adding new_len - old_len locked
> +		 * bytes to the new mapping.
> +		 */
> +		if (new_len > old_len)
> +			mm->locked_vm += (new_len - old_len) >> PAGE_SHIFT;

Hm. How do you enforce that we're not over RLIMIT_MEMLOCK?


-- 
 Kirill A. Shutemov