Re: [PATCH v5 1/2] mm: Add MREMAP_DONTUNMAP to mremap().

Kirill A. Shutemov
2020-02-14T14:28:40Z

On Thu, Feb 13, 2020 at 08:09:51PM -0800, Brian Geffon wrote:
> When remapping an anonymous, private mapping, if MREMAP_DONTUNMAP is
> set, the source mapping will not be removed. The remap operation
> will be performed as it would have been normally by moving over the
> page tables to the new mapping. The old vma will have any locked
> flags cleared, have no pagetables, and any userfaultfds that were
> watching that range will continue watching it.
>
> 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."
>
> v4 -> v5:
> - Correct commit message to more accurately reflect the behavior.
> - Clear VM_LOCKED and VM_LOCKEDONFAULT on the old vma.
>
> Signed-off-by: Brian Geffon <bgeffon@google.com>
>
> ---
> include/uapi/linux/mman.h | 5 +-
> mm/mremap.c | 106 ++++++++++++++++++++++++++++++--------
> 2 files changed, 88 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 1fc8a29fbe3f..a2a792fdbc64 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 +408,49 @@ 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 (vm_flags & VM_LOCKED) {
> + /* We always clear VM_LOCKED[ONFAULT] on the old vma */
> + vma->vm_flags &= VM_LOCKED_CLEAR_MASK;
> +
> + if (new_len > old_len) {
> + mm->locked_vm +=
> + (new_len - old_len) >> PAGE_SHIFT;
> + *locked = true;

This level of code indentation suggests that code restructuring is
required.

> + }
> + }
> +
> + goto out;
> + }
> +
> if (do_munmap(mm, old_addr, old_len, uf_unmap) < 0) {
> /* OOM: unable to split vma, just get accounts right */
> vm_unacct_memory(excess >> PAGE_SHIFT);
> excess = 0;
> }
> +
> + if (vm_flags & VM_LOCKED) {
> + mm->locked_vm += new_len >> PAGE_SHIFT;
> + *locked = true;
> + }

I don't follow why this is required.

> +out:
> mm->hiwater_vm = hiwater_vm;
>
> /* Restore VM_ACCOUNT if one or two pieces of vma left */
> @@ -422,16 +460,12 @@ static unsigned long move_vma(struct vm_area_struct *vma,
> vma->vm_next->vm_flags |= VM_ACCOUNT;
> }
>
> - if (vm_flags & VM_LOCKED) {
> - mm->locked_vm += new_len >> PAGE_SHIFT;
> - *locked = true;
> - }
> -

Ah. You moved this piece. Why?

> return new_addr;
> }
>
> static struct vm_area_struct *vma_to_resize(unsigned long addr,
> - unsigned long old_len, unsigned long new_len, unsigned long *p)
> + unsigned long old_len, unsigned long new_len, unsigned long flags,
> + unsigned long *p)
> {
> struct mm_struct *mm = current->mm;
> struct vm_area_struct *vma = find_vma(mm, addr);
> @@ -453,6 +487,10 @@ static struct vm_area_struct *vma_to_resize(unsigned long addr,
> return ERR_PTR(-EINVAL);
> }
>
> + if (flags & MREMAP_DONTUNMAP && (!vma_is_anonymous(vma) ||
> + vma->vm_flags & VM_SHARED))
> + return ERR_PTR(-EINVAL);
> +
> if (is_vm_hugetlb_page(vma))
> return ERR_PTR(-EINVAL);
>
> @@ -497,7 +535,7 @@ static struct vm_area_struct *vma_to_resize(unsigned long addr,
>
> static unsigned long mremap_to(unsigned long addr, unsigned long old_len,
> unsigned long new_addr, unsigned long new_len, bool *locked,
> - struct vm_userfaultfd_ctx *uf,
> + unsigned long flags, struct vm_userfaultfd_ctx *uf,
> struct list_head *uf_unmap_early,
> struct list_head *uf_unmap)
> {
> @@ -505,7 +543,7 @@ static unsigned long mremap_to(unsigned long addr, unsigned long old_len,
> struct vm_area_struct *vma;
> unsigned long ret = -EINVAL;
> unsigned long charged = 0;
> - unsigned long map_flags;
> + unsigned long map_flags = 0;
>
> if (offset_in_page(new_addr))
> goto out;
> @@ -534,9 +572,11 @@ static unsigned long mremap_to(unsigned long addr, unsigned long old_len,
> if ((mm->map_count + 2) >= sysctl_max_map_count - 3)
> return -ENOMEM;
>
> - ret = do_munmap(mm, new_addr, new_len, uf_unmap_early);
> - if (ret)
> - goto out;
> + if (flags & MREMAP_FIXED) {

I think it has to be

if (!(flags & MREMAP_DONTUNMAP)) {

No?

> + ret = do_munmap(mm, new_addr, new_len, uf_unmap_early);
> + if (ret)
> + goto out;
> + }
>
> if (old_len >= new_len) {
> ret = do_munmap(mm, addr+new_len, old_len - new_len, uf_unmap);
> @@ -545,13 +585,26 @@ static unsigned long mremap_to(unsigned long addr, unsigned long old_len,
> old_len = new_len;
> }
>
> - vma = vma_to_resize(addr, old_len, new_len, &charged);
> + vma = vma_to_resize(addr, old_len, new_len, flags, &charged);
> if (IS_ERR(vma)) {
> ret = PTR_ERR(vma);
> goto out;
> }
>
> - map_flags = MAP_FIXED;
> + /*
> + * MREMAP_DONTUNMAP expands by new_len - (new_len - old_len), we will
> + * check that we can expand by new_len and vma_to_resize will handle
> + * the vma growing which is (new_len - old_len).
> + */
> + if (flags & MREMAP_DONTUNMAP &&
> + !may_expand_vm(mm, vma->vm_flags, new_len >> PAGE_SHIFT)) {
> + ret = -ENOMEM;
> + goto out;
> + }
> +
> + if (flags & MREMAP_FIXED)
> + map_flags |= MAP_FIXED;
> +
> if (vma->vm_flags & VM_MAYSHARE)
> map_flags |= MAP_SHARED;
>
> @@ -561,10 +614,16 @@ static unsigned long mremap_to(unsigned long addr, unsigned long old_len,
> if (offset_in_page(ret))
> goto out1;
>
> - ret = move_vma(vma, addr, old_len, new_len, new_addr, locked, uf,
> + /* We got a new mapping */
> + if (!(flags & MREMAP_FIXED))
> + new_addr = ret;
> +
> + ret = move_vma(vma, addr, old_len, new_len, new_addr, locked, flags, uf,
> uf_unmap);
> +
> if (!(offset_in_page(ret)))
> goto out;
> +
> out1:
> vm_unacct_memory(charged);
>
> @@ -609,12 +668,16 @@ SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len,
> addr = untagged_addr(addr);
> new_addr = untagged_addr(new_addr);
>
> - if (flags & ~(MREMAP_FIXED | MREMAP_MAYMOVE))
> + if (flags & ~(MREMAP_FIXED | MREMAP_MAYMOVE | MREMAP_DONTUNMAP))
> return ret;
>
> if (flags & MREMAP_FIXED && !(flags & MREMAP_MAYMOVE))
> return ret;
>
> + /* MREMAP_DONTUNMAP is always a move */
> + if (flags & MREMAP_DONTUNMAP && !(flags & MREMAP_MAYMOVE))
> + return ret;
> +
> if (offset_in_page(addr))
> return ret;
>
> @@ -632,9 +695,10 @@ SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len,
> if (down_write_killable(&current->mm->mmap_sem))
> return -EINTR;
>
> - if (flags & MREMAP_FIXED) {
> + if (flags & MREMAP_FIXED || flags & MREMAP_DONTUNMAP) {

if (flags & (MREMAP_FIXED | MREMAP_DONTUNMAP)) {

> ret = mremap_to(addr, old_len, new_addr, new_len,
> - &locked, &uf, &uf_unmap_early, &uf_unmap);
> + &locked, flags, &uf, &uf_unmap_early,
> + &uf_unmap);
> goto out;
> }
>
> @@ -662,7 +726,7 @@ SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len,
> /*
> * Ok, we need to grow..
> */
> - vma = vma_to_resize(addr, old_len, new_len, &charged);
> + vma = vma_to_resize(addr, old_len, new_len, flags, &charged);
> if (IS_ERR(vma)) {
> ret = PTR_ERR(vma);
> goto out;
> @@ -712,7 +776,7 @@ SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len,
> }
>
> ret = move_vma(vma, addr, old_len, new_len, new_addr,
> - &locked, &uf, &uf_unmap);
> + &locked, flags, &uf, &uf_unmap);
> }
> out:
> if (offset_in_page(ret)) {
> --
> 2.25.0.265.gbab2e86ba0-goog
>

--
Kirill A. Shutemov