MADV_USERFAULT is a new madvise flag that will set VM_USERFAULT in the
vma flags. Whenever VM_USERFAULT is set in an anonymous vma, if
userland touches a still unmapped virtual address, a sigbus signal is
sent instead of allocating a new page. The sigbus signal handler will
then resolve the page fault in userland by calling the
remap_anon_pages syscall.
This functionality is needed to reliably implement postcopy live
migration in KVM (without having to use a special chardevice that
would disable all advanced Linux VM features, like swapping, KSM, THP,
automatic NUMA balancing, etc...).
MADV_USERFAULT could also be used to offload parts of anonymous memory
regions to remote nodes or to implement network distributed shared
memory.
Here I enlarged the vm_flags to 64bit as we run out of bits (noop on
64bit kernels). An alternative is to find some combination of flags
that are mutually exclusive if set.
Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
---
arch/alpha/include/uapi/asm/mman.h | 3 ++
arch/mips/include/uapi/asm/mman.h | 3 ++
arch/parisc/include/uapi/asm/mman.h | 3 ++
arch/xtensa/include/uapi/asm/mman.h | 3 ++
fs/proc/task_mmu.c | 1 +
include/linux/mm.h | 1 +
include/uapi/asm-generic/mman-common.h | 3 ++
mm/huge_memory.c | 60 +++++++++++++++++++++-------------
mm/madvise.c | 17 ++++++++++
mm/memory.c | 13 ++++++++
10 files changed, 85 insertions(+), 22 deletions(-)
diff --git a/arch/alpha/include/uapi/asm/mman.h b/arch/alpha/include/uapi/asm/mman.h
index 0086b47..a10313c 100644
--- a/arch/alpha/include/uapi/asm/mman.h
+++ b/arch/alpha/include/uapi/asm/mman.h
@@ -60,6 +60,9 @@
overrides the coredump filter bits */
#define MADV_DODUMP 17 /* Clear the MADV_NODUMP flag */
+#define MADV_USERFAULT 18 /* Trigger user faults if not mapped */
+#define MADV_NOUSERFAULT 19 /* Don't trigger user faults */
+
/* compatibility flags */
#define MAP_FILE 0
diff --git a/arch/mips/include/uapi/asm/mman.h b/arch/mips/include/uapi/asm/mman.h
index cfcb876..d9d11a4 100644
--- a/arch/mips/include/uapi/asm/mman.h
+++ b/arch/mips/include/uapi/asm/mman.h
@@ -84,6 +84,9 @@
overrides the coredump filter bits */
#define MADV_DODUMP 17 /* Clear the MADV_NODUMP flag */
+#define MADV_USERFAULT 18 /* Trigger user faults if not mapped */
+#define MADV_NOUSERFAULT 19 /* Don't trigger user faults */
+
/* compatibility flags */
#define MAP_FILE 0
diff --git a/arch/parisc/include/uapi/asm/mman.h b/arch/parisc/include/uapi/asm/mman.h
index 294d251..7bc7b7b 100644
--- a/arch/parisc/include/uapi/asm/mman.h
+++ b/arch/parisc/include/uapi/asm/mman.h
@@ -66,6 +66,9 @@
overrides the coredump filter bits */
#define MADV_DODUMP 70 /* Clear the MADV_NODUMP flag */
+#define MADV_USERFAULT 71 /* Trigger user faults if not mapped */
+#define MADV_NOUSERFAULT 72 /* Don't trigger user faults */
+
/* compatibility flags */
#define MAP_FILE 0
#define MAP_VARIABLE 0
diff --git a/arch/xtensa/include/uapi/asm/mman.h b/arch/xtensa/include/uapi/asm/mman.h
index 00eed67..5448d88 100644
--- a/arch/xtensa/include/uapi/asm/mman.h
+++ b/arch/xtensa/include/uapi/asm/mman.h
@@ -90,6 +90,9 @@
overrides the coredump filter bits */
#define MADV_DODUMP 17 /* Clear the MADV_NODUMP flag */
+#define MADV_USERFAULT 18 /* Trigger user faults if not mapped */
+#define MADV_NOUSERFAULT 19 /* Don't trigger user faults */
+
/* compatibility flags */
#define MAP_FILE 0
diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index ee1c3a2..6033cb8 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -568,6 +568,7 @@ static void show_smap_vma_flags(struct seq_file *m, struct vm_area_struct *vma)
[ilog2(VM_HUGEPAGE)] = "hg",
[ilog2(VM_NOHUGEPAGE)] = "nh",
[ilog2(VM_MERGEABLE)] = "mg",
+ [ilog2(VM_USERFAULT)] = "uf",
};
size_t i;
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 8900ba9..bf3df07 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -139,6 +139,7 @@ extern unsigned int kobjsize(const void *objp);
#define VM_HUGEPAGE 0x20000000 /* MADV_HUGEPAGE marked this vma */
#define VM_NOHUGEPAGE 0x40000000 /* MADV_NOHUGEPAGE marked this vma */
#define VM_MERGEABLE 0x80000000 /* KSM may merge identical pages */
+#define VM_USERFAULT 0x100000000ULL /* Trigger user faults if not mapped */
#if defined(CONFIG_X86)
# define VM_PAT VM_ARCH_1 /* PAT reserves whole VMA at once (x86) */
diff --git a/include/uapi/asm-generic/mman-common.h b/include/uapi/asm-generic/mman-common.h
index ddc3b36..dbf1e70 100644
--- a/include/uapi/asm-generic/mman-common.h
+++ b/include/uapi/asm-generic/mman-common.h
@@ -52,6 +52,9 @@
overrides the coredump filter bits */
#define MADV_DODUMP 17 /* Clear the MADV_DONTDUMP flag */
+#define MADV_USERFAULT 18 /* Trigger user faults if not mapped */
+#define MADV_NOUSERFAULT 19 /* Don't trigger user faults */
+
/* compatibility flags */
#define MAP_FILE 0
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index e913a19..b402d60 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -721,12 +721,16 @@ static int __do_huge_pmd_anonymous_page(struct mm_struct *mm,
VM_BUG_ON_PAGE(!PageCompound(page), page);
- if (mem_cgroup_try_charge(page, mm, GFP_TRANSHUGE, &memcg))
- return VM_FAULT_OOM;
+ if (mem_cgroup_try_charge(page, mm, GFP_TRANSHUGE, &memcg)) {
+ put_page(page);
+ count_vm_event(THP_FAULT_FALLBACK);
+ return VM_FAULT_FALLBACK;
+ }
pgtable = pte_alloc_one(mm, haddr);
if (unlikely(!pgtable)) {
mem_cgroup_cancel_charge(page, memcg);
+ put_page(page);
return VM_FAULT_OOM;
}
@@ -746,6 +750,16 @@ static int __do_huge_pmd_anonymous_page(struct mm_struct *mm,
pte_free(mm, pgtable);
} else {
pmd_t entry;
+
+ /* Deliver the page fault to userland */
+ if (vma->vm_flags & VM_USERFAULT) {
+ spin_unlock(ptl);
+ mem_cgroup_cancel_charge(page, memcg);
+ put_page(page);
+ pte_free(mm, pgtable);
+ return VM_FAULT_SIGBUS;
+ }
+
entry = mk_huge_pmd(page, vma->vm_page_prot);
entry = maybe_pmd_mkwrite(pmd_mkdirty(entry), vma);
page_add_new_anon_rmap(page, vma, haddr);
@@ -756,6 +770,7 @@ static int __do_huge_pmd_anonymous_page(struct mm_struct *mm,
add_mm_counter(mm, MM_ANONPAGES, HPAGE_PMD_NR);
atomic_long_inc(&mm->nr_ptes);
spin_unlock(ptl);
+ count_vm_event(THP_FAULT_ALLOC);
}
return 0;
@@ -776,20 +791,17 @@ static inline struct page *alloc_hugepage_vma(int defrag,
}
/* Caller must hold page table lock. */
-static bool set_huge_zero_page(pgtable_t pgtable, struct mm_struct *mm,
+static void set_huge_zero_page(pgtable_t pgtable, struct mm_struct *mm,
struct vm_area_struct *vma, unsigned long haddr, pmd_t *pmd,
struct page *zero_page)
{
pmd_t entry;
- if (!pmd_none(*pmd))
- return false;
entry = mk_pmd(zero_page, vma->vm_page_prot);
entry = pmd_wrprotect(entry);
entry = pmd_mkhuge(entry);
pgtable_trans_huge_deposit(mm, pmd, pgtable);
set_pmd_at(mm, haddr, pmd, entry);
atomic_long_inc(&mm->nr_ptes);
- return true;
}
int do_huge_pmd_anonymous_page(struct mm_struct *mm, struct vm_area_struct *vma,
@@ -811,6 +823,7 @@ int do_huge_pmd_anonymous_page(struct mm_struct *mm, struct vm_area_struct *vma,
pgtable_t pgtable;
struct page *zero_page;
bool set;
+ int ret;
pgtable = pte_alloc_one(mm, haddr);
if (unlikely(!pgtable))
return VM_FAULT_OOM;
@@ -821,14 +834,24 @@ int do_huge_pmd_anonymous_page(struct mm_struct *mm, struct vm_area_struct *vma,
return VM_FAULT_FALLBACK;
}
ptl = pmd_lock(mm, pmd);
- set = set_huge_zero_page(pgtable, mm, vma, haddr, pmd,
- zero_page);
+ ret = 0;
+ set = false;
+ if (pmd_none(*pmd)) {
+ if (vma->vm_flags & VM_USERFAULT)
+ ret = VM_FAULT_SIGBUS;
+ else {
+ set_huge_zero_page(pgtable, mm, vma,
+ haddr, pmd,
+ zero_page);
+ set = true;
+ }
+ }
spin_unlock(ptl);
if (!set) {
pte_free(mm, pgtable);
put_huge_zero_page();
}
- return 0;
+ return ret;
}
page = alloc_hugepage_vma(transparent_hugepage_defrag(vma),
vma, haddr, numa_node_id(), 0);
@@ -836,14 +859,7 @@ int do_huge_pmd_anonymous_page(struct mm_struct *mm, struct vm_area_struct *vma,
count_vm_event(THP_FAULT_FALLBACK);
return VM_FAULT_FALLBACK;
}
- if (unlikely(__do_huge_pmd_anonymous_page(mm, vma, haddr, pmd, page))) {
- put_page(page);
- count_vm_event(THP_FAULT_FALLBACK);
- return VM_FAULT_FALLBACK;
- }
-
- count_vm_event(THP_FAULT_ALLOC);
- return 0;
+ return __do_huge_pmd_anonymous_page(mm, vma, haddr, pmd, page);
}
int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm,
@@ -878,16 +894,14 @@ int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm,
*/
if (is_huge_zero_pmd(pmd)) {
struct page *zero_page;
- bool set;
/*
* get_huge_zero_page() will never allocate a new page here,
* since we already have a zero page to copy. It just takes a
* reference.
*/
zero_page = get_huge_zero_page();
- set = set_huge_zero_page(pgtable, dst_mm, vma, addr, dst_pmd,
+ set_huge_zero_page(pgtable, dst_mm, vma, addr, dst_pmd,
zero_page);
- BUG_ON(!set); /* unexpected !pmd_none(dst_pmd) */
ret = 0;
goto out_unlock;
}
@@ -2148,7 +2162,8 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
_pte++, address += PAGE_SIZE) {
pte_t pteval = *_pte;
if (pte_none(pteval)) {
- if (++none <= khugepaged_max_ptes_none)
+ if (!(vma->vm_flags & VM_USERFAULT) &&
+ ++none <= khugepaged_max_ptes_none)
continue;
else
goto out;
@@ -2569,7 +2584,8 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
_pte++, _address += PAGE_SIZE) {
pte_t pteval = *_pte;
if (pte_none(pteval)) {
- if (++none <= khugepaged_max_ptes_none)
+ if (!(vma->vm_flags & VM_USERFAULT) &&
+ ++none <= khugepaged_max_ptes_none)
continue;
else
goto out_unmap;
diff --git a/mm/madvise.c b/mm/madvise.c
index d5aee71..24620c0 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -93,6 +93,21 @@ static long madvise_behavior(struct vm_area_struct *vma,
if (error)
goto out;
break;
+ case MADV_USERFAULT:
+ if (vma->vm_ops) {
+ error = -EINVAL;
+ goto out;
+ }
+ new_flags |= VM_USERFAULT;
+ break;
+ case MADV_NOUSERFAULT:
+ if (vma->vm_ops) {
+ WARN_ON(new_flags & VM_USERFAULT);
+ error = -EINVAL;
+ goto out;
+ }
+ new_flags &= ~VM_USERFAULT;
+ break;
}
if (new_flags == vma->vm_flags) {
@@ -408,6 +423,8 @@ madvise_behavior_valid(int behavior)
case MADV_HUGEPAGE:
case MADV_NOHUGEPAGE:
#endif
+ case MADV_USERFAULT:
+ case MADV_NOUSERFAULT:
case MADV_DONTDUMP:
case MADV_DODUMP:
return 1;
diff --git a/mm/memory.c b/mm/memory.c
index e229970..16e4c8a 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2645,6 +2645,11 @@ static int do_anonymous_page(struct mm_struct *mm, struct vm_area_struct *vma,
page_table = pte_offset_map_lock(mm, pmd, address, &ptl);
if (!pte_none(*page_table))
goto unlock;
+ /* Deliver the page fault to userland, check inside PT lock */
+ if (vma->vm_flags & VM_USERFAULT) {
+ pte_unmap_unlock(page_table, ptl);
+ return VM_FAULT_SIGBUS;
+ }
goto setpte;
}
@@ -2672,6 +2677,14 @@ static int do_anonymous_page(struct mm_struct *mm, struct vm_area_struct *vma,
if (!pte_none(*page_table))
goto release;
+ /* Deliver the page fault to userland, check inside PT lock */
+ if (vma->vm_flags & VM_USERFAULT) {
+ pte_unmap_unlock(page_table, ptl);
+ mem_cgroup_cancel_charge(page, memcg);
+ page_cache_release(page);
+ return VM_FAULT_SIGBUS;
+ }
+
inc_mm_counter_fast(mm, MM_ANONPAGES);
page_add_new_anon_rmap(page, vma, address);
mem_cgroup_commit_charge(page, memcg, false);
--
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