[PATCH v4] mm: add mremap flag for preserving the old mapping

From: Daniel Micay
Date: Thu Oct 02 2014 - 23:41:50 EST

This introduces the MREMAP_RETAIN flag for preserving the source mapping
when MREMAP_MAYMOVE moves the pages to a new destination. Accesses to
the source mapping will fault and map in fresh zeroed pages.

It is currently limited to writable MAP_PRIVATE|MAP_ANONYMOUS mappings
and will return EFAULT when used on anything else. This covers the
intended use case in general purpose allocators.

For consistency, the old_len >= new_len case could decommit the pages
instead of unmapping. However, userspace can accomplish the same thing
via madvise and the flag is coherent without the additional complexity.

Motivation:

TCMalloc and jemalloc avoid releasing virtual memory in order to reduce
virtual memory fragmentation. A call to munmap or mremap would leave a
hole in the address space. Instead, unused pages are lazily returned to
the operating system via MADV_DONTNEED.

Since mremap cannot be used to elide copies, TCMalloc and jemalloc end
up being significantly slower for patterns like repeated vector / hash
table reallocations. Consider the typical vector building pattern:

#include <string.h>
#include <stdlib.h>

int main(void) {
for (size_t i = 0; i < 100; i++) {
void *ptr = NULL;
size_t old_size = 0;
for (size_t size = 4; size < (1 << 30); size *= 2) {
ptr = realloc(ptr, size);
if (!ptr) return 1;
memset(ptr + old_size, 0xff, size - old_size);
old_size = size;
}
free(ptr);
}
}

Transparent huge pages disabled:

glibc (baseline, uses mremap already): 15.051s
jemalloc without MREMAP_RETAIN: 38.540s
jemalloc with MREMAP_RETAIN: 15.086s

Transparent huge pages enabled:

glibc (baseline, uses mremap already): 8.464s
jemalloc without MREMAP_RETAIN: 18.230s
jemalloc with MREMAP_RETAIN: 6.696s

In practice, in-place growth never occurs for huge allocations because
the heap grows in the downwards direction for all 3 allocators. TCMalloc
and jemalloc pay for enormous copies while glibc is only spending time
writing new elements to the vector. Even if it was grown in the other
direction, real-world applications would end up blocking in-place growth
with new allocations.

The allocators could attempt to map the source location again after an
mremap call, but there is no guarantee of success in a multi-threaded
program and fragmentating memory over time is considered unacceptable.

Signed-off-by: Daniel Micay <danielmicay@xxxxxxxxx>
---
include/linux/huge_mm.h | 2 +-
include/linux/mm.h | 6 ++++++
include/uapi/linux/mman.h | 1 +
mm/huge_memory.c | 4 ++--
mm/memory.c | 2 +-
mm/mmap.c | 12 +++++++++++
mm/mremap.c | 52 +++++++++++++++++++++++++++++++----------------
7 files changed, 57 insertions(+), 22 deletions(-)

diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index 63579cb..3c13b20 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -143,7 +143,7 @@ static inline void vma_adjust_trans_huge(struct vm_area_struct *vma,
unsigned long end,
long adjust_next)
{
- if (!vma->anon_vma || vma->vm_ops)
+ if (!vma->anon_vma || (vma->vm_ops && !vma->vm_ops->allow_huge_pages))
return;
__vma_adjust_trans_huge(vma, start, end, adjust_next);
}
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 8981cc8..1e61036 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -273,6 +273,12 @@ struct vm_operations_struct {
/* called by sys_remap_file_pages() to populate non-linear mapping */
int (*remap_pages)(struct vm_area_struct *vma, unsigned long addr,
unsigned long size, pgoff_t pgoff);
+
+ /* Check if the mapping may be duplicated by MREMAP_RETAIN */
+ bool (*may_duplicate)(struct vm_area_struct *vma);
+
+ /* if there is no vm_ops table, this is considered true */
+ bool allow_huge_pages;
};

struct mmu_gather;
diff --git a/include/uapi/linux/mman.h b/include/uapi/linux/mman.h
index ade4acd..4e9a546 100644
--- a/include/uapi/linux/mman.h
+++ b/include/uapi/linux/mman.h
@@ -5,6 +5,7 @@

#define MREMAP_MAYMOVE 1
#define MREMAP_FIXED 2
+#define MREMAP_RETAIN 4

#define OVERCOMMIT_GUESS 0
#define OVERCOMMIT_ALWAYS 1
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index d9a21d06..350b478 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2077,7 +2077,7 @@ int khugepaged_enter_vma_merge(struct vm_area_struct *vma)
* page fault if needed.
*/
return 0;
- if (vma->vm_ops)
+ if ((vma->vm_ops && !vma->vm_ops->allow_huge_pages))
/* khugepaged not yet working on file or special mappings */
return 0;
VM_BUG_ON(vma->vm_flags & VM_NO_THP);
@@ -2405,7 +2405,7 @@ static bool hugepage_vma_check(struct vm_area_struct *vma)
(vma->vm_flags & VM_NOHUGEPAGE))
return false;

- if (!vma->anon_vma || vma->vm_ops)
+ if (!vma->anon_vma || (vma->vm_ops && !vma->vm_ops->allow_huge_pages))
return false;
if (is_vma_temporary_stack(vma))
return false;
diff --git a/mm/memory.c b/mm/memory.c
index e229970..c181401 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3275,7 +3275,7 @@ static int __handle_mm_fault(struct mm_struct *mm, struct vm_area_struct *vma,
return VM_FAULT_OOM;
if (pmd_none(*pmd) && transparent_hugepage_enabled(vma)) {
int ret = VM_FAULT_FALLBACK;
- if (!vma->vm_ops)
+ if (!vma->vm_ops || vma->vm_ops->allow_huge_pages)
ret = do_huge_pmd_anonymous_page(mm, vma, address,
pmd, flags);
if (!(ret & VM_FAULT_FALLBACK))
diff --git a/mm/mmap.c b/mm/mmap.c
index c0a3637..6b644fe 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1500,6 +1500,16 @@ static inline int accountable_mapping(struct file *file, vm_flags_t vm_flags)
return (vm_flags & (VM_NORESERVE | VM_SHARED | VM_WRITE)) == VM_WRITE;
}

+static bool anon_may_duplicate(struct vm_area_struct *vma)
+{
+ return vma->vm_flags & VM_WRITE && !(vma->vm_flags & VM_SHARED);
+}
+
+static const struct vm_operations_struct anon_vmops = {
+ .may_duplicate = anon_may_duplicate,
+ .allow_huge_pages = true
+};
+
unsigned long mmap_region(struct file *file, unsigned long addr,
unsigned long len, vm_flags_t vm_flags, unsigned long pgoff)
{
@@ -1569,6 +1579,8 @@ munmap_back:
vma->vm_flags = vm_flags;
vma->vm_page_prot = vm_get_page_prot(vm_flags);
vma->vm_pgoff = pgoff;
+ if (!file)
+ vma->vm_ops = &anon_vmops;
INIT_LIST_HEAD(&vma->anon_vma_chain);

if (file) {
diff --git a/mm/mremap.c b/mm/mremap.c
index 05f1180..ca7a662 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -235,7 +235,8 @@ 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)
+ unsigned long new_len, unsigned long new_addr, bool retain,
+ bool *locked)
{
struct mm_struct *mm = vma->vm_mm;
struct vm_area_struct *new_vma;
@@ -287,15 +288,7 @@ static unsigned long move_vma(struct vm_area_struct *vma,
old_len = new_len;
old_addr = new_addr;
new_addr = -ENOMEM;
- }
-
- /* Conceal VM_ACCOUNT so old reservation is not undone */
- if (vm_flags & VM_ACCOUNT) {
- vma->vm_flags &= ~VM_ACCOUNT;
- excess = vma->vm_end - vma->vm_start - old_len;
- if (old_addr > vma->vm_start &&
- old_addr + old_len < vma->vm_end)
- split = 1;
+ retain = false;
}

/*
@@ -310,6 +303,19 @@ static unsigned long move_vma(struct vm_area_struct *vma,
hiwater_vm = mm->hiwater_vm;
vm_stat_account(mm, vma->vm_flags, vma->vm_file, new_len>>PAGE_SHIFT);

+ /* Leave the old mapping in place for MREMAP_RETAIN */
+ if (retain)
+ goto out;
+
+ /* Conceal VM_ACCOUNT so old reservation is not undone */
+ if (vm_flags & VM_ACCOUNT) {
+ vma->vm_flags &= ~VM_ACCOUNT;
+ excess = vma->vm_end - vma->vm_start - old_len;
+ if (old_addr > vma->vm_start &&
+ old_addr + old_len < vma->vm_end)
+ split = 1;
+ }
+
if (do_munmap(mm, old_addr, old_len) < 0) {
/* OOM: unable to split vma, just get accounts right */
vm_unacct_memory(excess >> PAGE_SHIFT);
@@ -324,6 +330,7 @@ static unsigned long move_vma(struct vm_area_struct *vma,
vma->vm_next->vm_flags |= VM_ACCOUNT;
}

+out:
if (vm_flags & VM_LOCKED) {
mm->locked_vm += new_len >> PAGE_SHIFT;
*locked = true;
@@ -333,7 +340,8 @@ static unsigned long move_vma(struct vm_area_struct *vma,
}

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, bool retain,
+ unsigned long *p)
{
struct mm_struct *mm = current->mm;
struct vm_area_struct *vma = find_vma(mm, addr);
@@ -348,6 +356,11 @@ static struct vm_area_struct *vma_to_resize(unsigned long addr,
if (old_len > vma->vm_end - addr)
goto Efault;

+ /* Forbid MREMAP_RETAIN if not explicitly permitted by the mapping */
+ if (retain && !(vma->vm_ops && vma->vm_ops->may_duplicate &&
+ vma->vm_ops->may_duplicate(vma)))
+ goto Efault;
+
/* Need to be careful about a growing mapping */
if (new_len > old_len) {
unsigned long pgoff;
@@ -392,7 +405,8 @@ Eagain:
}

static unsigned long mremap_to(unsigned long addr, unsigned long old_len,
- unsigned long new_addr, unsigned long new_len, bool *locked)
+ unsigned long new_addr, unsigned long new_len, bool retain,
+ bool *locked)
{
struct mm_struct *mm = current->mm;
struct vm_area_struct *vma;
@@ -426,7 +440,7 @@ 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, retain, &charged);
if (IS_ERR(vma)) {
ret = PTR_ERR(vma);
goto out;
@@ -442,7 +456,7 @@ static unsigned long mremap_to(unsigned long addr, unsigned long old_len,
if (ret & ~PAGE_MASK)
goto out1;

- ret = move_vma(vma, addr, old_len, new_len, new_addr, locked);
+ ret = move_vma(vma, addr, old_len, new_len, new_addr, retain, locked);
if (!(ret & ~PAGE_MASK))
goto out;
out1:
@@ -482,7 +496,7 @@ SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len,
unsigned long charged = 0;
bool locked = false;

- if (flags & ~(MREMAP_FIXED | MREMAP_MAYMOVE))
+ if (flags & ~(MREMAP_FIXED | MREMAP_MAYMOVE | MREMAP_RETAIN))
return ret;

if (flags & MREMAP_FIXED && !(flags & MREMAP_MAYMOVE))
@@ -506,7 +520,7 @@ SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len,

if (flags & MREMAP_FIXED) {
ret = mremap_to(addr, old_len, new_addr, new_len,
- &locked);
+ flags & MREMAP_RETAIN, &locked);
goto out;
}

@@ -526,7 +540,8 @@ 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 & MREMAP_RETAIN,
+ &charged);
if (IS_ERR(vma)) {
ret = PTR_ERR(vma);
goto out;
@@ -575,7 +590,8 @@ SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len,
goto out;
}

- ret = move_vma(vma, addr, old_len, new_len, new_addr, &locked);
+ ret = move_vma(vma, addr, old_len, new_len, new_addr,
+ flags & MREMAP_RETAIN, &locked);
}
out:
if (ret & ~PAGE_MASK)
--
2.1.2

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