[PATCH v2 4/5] mm/mprotect: use mmu_gather

Nadav Amit
2021-10-21T19:52:58Z

From: Nadav Amit <namit@vmware.com>

change_pXX_range() currently does not use mmu_gather, but instead
implements its own deferred TLB flushes scheme. This both complicates
the code, as developers need to be aware of different invalidation
schemes, and prevents opportunities to avoid TLB flushes or perform them
in finer granularity.

The use of mmu_gather for modified PTEs has benefits in various
scenarios even if pages are not released. For instance, if only a single
page needs to be flushed out of a range of many pages, only that page
would be flushed. If a THP page is flushed, on x86 a single TLB invlpg
instruction can be used instead of 512 instructions (or a full TLB
flush, which would Linux would actually use by default). mprotect() over
multiple VMAs requires a single flush.

Use mmu_gather in change_pXX_range(). As the pages are not released,
only record the flushed range using tlb_flush_pXX_range().

Handle THP similarly and get rid of flush_cache_range() which becomes
redundant since tlb_start_vma() calls it when needed.

Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Will Deacon <will@kernel.org>
Cc: Yu Zhao <yuzhao@google.com>
Cc: Nick Piggin <npiggin@gmail.com>
Cc: x86@kernel.org
Signed-off-by: Nadav Amit <namit@vmware.com>
---
fs/exec.c | 6 ++-
include/linux/huge_mm.h | 5 ++-
include/linux/mm.h | 5 ++-
mm/huge_memory.c | 10 ++++-
mm/mprotect.c | 93 ++++++++++++++++++++++-------------------
mm/userfaultfd.c | 6 ++-
6 files changed, 75 insertions(+), 50 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index 5a7a07dfdc81..7f8609bbc6b3 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -752,6 +752,7 @@ int setup_arg_pages(struct linux_binprm *bprm,
unsigned long stack_size;
unsigned long stack_expand;
unsigned long rlim_stack;
+ struct mmu_gather tlb;

#ifdef CONFIG_STACK_GROWSUP
/* Limit stack size */
@@ -806,8 +807,11 @@ int setup_arg_pages(struct linux_binprm *bprm,
vm_flags |= mm->def_flags;
vm_flags |= VM_STACK_INCOMPLETE_SETUP;

- ret = mprotect_fixup(vma, &prev, vma->vm_start, vma->vm_end,
+ tlb_gather_mmu(&tlb, mm);
+ ret = mprotect_fixup(&tlb, vma, &prev, vma->vm_start, vma->vm_end,
vm_flags);
+ tlb_finish_mmu(&tlb);
+
if (ret)
goto out_unlock;
BUG_ON(prev != vma);
diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index f280f33ff223..a9b6e03e9c4c 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -36,8 +36,9 @@ int zap_huge_pud(struct mmu_gather *tlb, struct vm_area_struct *vma, pud_t *pud,
unsigned long addr);
bool move_huge_pmd(struct vm_area_struct *vma, unsigned long old_addr,
unsigned long new_addr, pmd_t *old_pmd, pmd_t *new_pmd);
-int change_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd, unsigned long addr,
- pgprot_t newprot, unsigned long cp_flags);
+int change_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
+ pmd_t *pmd, unsigned long addr, pgprot_t newprot,
+ unsigned long cp_flags);
vm_fault_t vmf_insert_pfn_pmd_prot(struct vm_fault *vmf, pfn_t pfn,
pgprot_t pgprot, bool write);

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 00bb2d938df4..f46bab158560 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2001,10 +2001,11 @@ extern unsigned long move_page_tables(struct vm_area_struct *vma,
#define MM_CP_UFFD_WP_ALL (MM_CP_UFFD_WP | \
MM_CP_UFFD_WP_RESOLVE)

-extern unsigned long change_protection(struct vm_area_struct *vma, unsigned long start,
+extern unsigned long change_protection(struct mmu_gather *tlb,
+ struct vm_area_struct *vma, unsigned long start,
unsigned long end, pgprot_t newprot,
unsigned long cp_flags);
-extern int mprotect_fixup(struct vm_area_struct *vma,
+extern int mprotect_fixup(struct mmu_gather *tlb, struct vm_area_struct *vma,
struct vm_area_struct **pprev, unsigned long start,
unsigned long end, unsigned long newflags);

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 435da011b1a2..f5d0357a25ce 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1720,8 +1720,9 @@ bool move_huge_pmd(struct vm_area_struct *vma, unsigned long old_addr,
* or if prot_numa but THP migration is not supported
* - HPAGE_PMD_NR if protections changed and TLB flush necessary
*/
-int change_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
- unsigned long addr, pgprot_t newprot, unsigned long cp_flags)
+int change_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
+ pmd_t *pmd, unsigned long addr, pgprot_t newprot,
+ unsigned long cp_flags)
{
struct mm_struct *mm = vma->vm_mm;
spinlock_t *ptl;
@@ -1732,6 +1733,8 @@ int change_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
bool uffd_wp = cp_flags & MM_CP_UFFD_WP;
bool uffd_wp_resolve = cp_flags & MM_CP_UFFD_WP_RESOLVE;

+ tlb_change_page_size(tlb, HPAGE_PMD_SIZE);
+
if (prot_numa && !thp_migration_supported())
return 1;

@@ -1817,6 +1820,9 @@ int change_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
}
ret = HPAGE_PMD_NR;
set_pmd_at(mm, addr, pmd, entry);
+
+ tlb_flush_pmd_range(tlb, addr, HPAGE_PMD_SIZE);
+
BUG_ON(vma_is_anonymous(vma) && !preserve_write && pmd_write(entry));
unlock:
spin_unlock(ptl);
diff --git a/mm/mprotect.c b/mm/mprotect.c
index 883e2cc85cad..0f5c87af5c60 100644
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -32,12 +32,13 @@
#include <asm/cacheflush.h>
#include <asm/mmu_context.h>
#include <asm/tlbflush.h>
+#include <asm/tlb.h>

#include "internal.h"

-static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
- unsigned long addr, unsigned long end, pgprot_t newprot,
- unsigned long cp_flags)
+static unsigned long change_pte_range(struct mmu_gather *tlb,
+ struct vm_area_struct *vma, pmd_t *pmd, unsigned long addr,
+ unsigned long end, pgprot_t newprot, unsigned long cp_flags)
{
pte_t *pte, oldpte;
spinlock_t *ptl;
@@ -48,6 +49,8 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
bool uffd_wp = cp_flags & MM_CP_UFFD_WP;
bool uffd_wp_resolve = cp_flags & MM_CP_UFFD_WP_RESOLVE;

+ tlb_change_page_size(tlb, PAGE_SIZE);
+
/*
* Can be called with only the mmap_lock for reading by
* prot_numa so we must check the pmd isn't constantly
@@ -138,6 +141,7 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
ptent = pte_mkwrite(ptent);
}
ptep_modify_prot_commit(vma, addr, pte, oldpte, ptent);
+ tlb_flush_pte_range(tlb, addr, PAGE_SIZE);
pages++;
} else if (is_swap_pte(oldpte)) {
swp_entry_t entry = pte_to_swp_entry(oldpte);
@@ -219,9 +223,9 @@ static inline int pmd_none_or_clear_bad_unless_trans_huge(pmd_t *pmd)
return 0;
}

-static inline unsigned long change_pmd_range(struct vm_area_struct *vma,
- pud_t *pud, unsigned long addr, unsigned long end,
- pgprot_t newprot, unsigned long cp_flags)
+static inline unsigned long change_pmd_range(struct mmu_gather *tlb,
+ struct vm_area_struct *vma, pud_t *pud, unsigned long addr,
+ unsigned long end, pgprot_t newprot, unsigned long cp_flags)
{
pmd_t *pmd;
unsigned long next;
@@ -261,8 +265,12 @@ static inline unsigned long change_pmd_range(struct vm_area_struct *vma,
if (next - addr != HPAGE_PMD_SIZE) {
__split_huge_pmd(vma, pmd, addr, false, NULL);
} else {
- int nr_ptes = change_huge_pmd(vma, pmd, addr,
- newprot, cp_flags);
+ /*
+ * change_huge_pmd() does not defer TLB flushes,
+ * so no need to propagate the tlb argument.
+ */
+ int nr_ptes = change_huge_pmd(tlb, vma, pmd,
+ addr, newprot, cp_flags);

if (nr_ptes) {
if (nr_ptes == HPAGE_PMD_NR) {
@@ -276,8 +284,8 @@ static inline unsigned long change_pmd_range(struct vm_area_struct *vma,
}
/* fall through, the trans huge pmd just split */
}
- this_pages = change_pte_range(vma, pmd, addr, next, newprot,
- cp_flags);
+ this_pages = change_pte_range(tlb, vma, pmd, addr, next,
+ newprot, cp_flags);
pages += this_pages;
next:
cond_resched();
@@ -291,9 +299,9 @@ static inline unsigned long change_pmd_range(struct vm_area_struct *vma,
return pages;
}

-static inline unsigned long change_pud_range(struct vm_area_struct *vma,
- p4d_t *p4d, unsigned long addr, unsigned long end,
- pgprot_t newprot, unsigned long cp_flags)
+static inline unsigned long change_pud_range(struct mmu_gather *tlb,
+ struct vm_area_struct *vma, p4d_t *p4d, unsigned long addr,
+ unsigned long end, pgprot_t newprot, unsigned long cp_flags)
{
pud_t *pud;
unsigned long next;
@@ -304,16 +312,16 @@ static inline unsigned long change_pud_range(struct vm_area_struct *vma,
next = pud_addr_end(addr, end);
if (pud_none_or_clear_bad(pud))
continue;
- pages += change_pmd_range(vma, pud, addr, next, newprot,
+ pages += change_pmd_range(tlb, vma, pud, addr, next, newprot,
cp_flags);
} while (pud++, addr = next, addr != end);

return pages;
}

-static inline unsigned long change_p4d_range(struct vm_area_struct *vma,
- pgd_t *pgd, unsigned long addr, unsigned long end,
- pgprot_t newprot, unsigned long cp_flags)
+static inline unsigned long change_p4d_range(struct mmu_gather *tlb,
+ struct vm_area_struct *vma, pgd_t *pgd, unsigned long addr,
+ unsigned long end, pgprot_t newprot, unsigned long cp_flags)
{
p4d_t *p4d;
unsigned long next;
@@ -324,44 +332,40 @@ static inline unsigned long change_p4d_range(struct vm_area_struct *vma,
next = p4d_addr_end(addr, end);
if (p4d_none_or_clear_bad(p4d))
continue;
- pages += change_pud_range(vma, p4d, addr, next, newprot,
+ pages += change_pud_range(tlb, vma, p4d, addr, next, newprot,
cp_flags);
} while (p4d++, addr = next, addr != end);

return pages;
}

-static unsigned long change_protection_range(struct vm_area_struct *vma,
- unsigned long addr, unsigned long end, pgprot_t newprot,
- unsigned long cp_flags)
+static unsigned long change_protection_range(struct mmu_gather *tlb,
+ struct vm_area_struct *vma, unsigned long addr,
+ unsigned long end, pgprot_t newprot, unsigned long cp_flags)
{
struct mm_struct *mm = vma->vm_mm;
pgd_t *pgd;
unsigned long next;
- unsigned long start = addr;
unsigned long pages = 0;

BUG_ON(addr >= end);
pgd = pgd_offset(mm, addr);
- flush_cache_range(vma, addr, end);
- inc_tlb_flush_pending(mm);
+ tlb_start_vma(tlb, vma);
do {
next = pgd_addr_end(addr, end);
if (pgd_none_or_clear_bad(pgd))
continue;
- pages += change_p4d_range(vma, pgd, addr, next, newprot,
+ pages += change_p4d_range(tlb, vma, pgd, addr, next, newprot,
cp_flags);
} while (pgd++, addr = next, addr != end);

- /* Only flush the TLB if we actually modified any entries: */
- if (pages)
- flush_tlb_range(vma, start, end);
- dec_tlb_flush_pending(mm);
+ tlb_end_vma(tlb, vma);

return pages;
}

-unsigned long change_protection(struct vm_area_struct *vma, unsigned long start,
+unsigned long change_protection(struct mmu_gather *tlb,
+ struct vm_area_struct *vma, unsigned long start,
unsigned long end, pgprot_t newprot,
unsigned long cp_flags)
{
@@ -372,7 +376,7 @@ unsigned long change_protection(struct vm_area_struct *vma, unsigned long start,
if (is_vm_hugetlb_page(vma))
pages = hugetlb_change_protection(vma, start, end, newprot);
else
- pages = change_protection_range(vma, start, end, newprot,
+ pages = change_protection_range(tlb, vma, start, end, newprot,
cp_flags);

return pages;
@@ -406,8 +410,9 @@ static const struct mm_walk_ops prot_none_walk_ops = {
};

int
-mprotect_fixup(struct vm_area_struct *vma, struct vm_area_struct **pprev,
- unsigned long start, unsigned long end, unsigned long newflags)
+mprotect_fixup(struct mmu_gather *tlb, struct vm_area_struct *vma,
+ struct vm_area_struct **pprev, unsigned long start,
+ unsigned long end, unsigned long newflags)
{
struct mm_struct *mm = vma->vm_mm;
unsigned long oldflags = vma->vm_flags;
@@ -494,7 +499,7 @@ mprotect_fixup(struct vm_area_struct *vma, struct vm_area_struct **pprev,
dirty_accountable = vma_wants_writenotify(vma, vma->vm_page_prot);
vma_set_page_prot(vma);

- change_protection(vma, start, end, vma->vm_page_prot,
+ change_protection(tlb, vma, start, end, vma->vm_page_prot,
dirty_accountable ? MM_CP_DIRTY_ACCT : 0);

/*
@@ -528,6 +533,7 @@ static int do_mprotect_pkey(unsigned long start, size_t len,
const int grows = prot & (PROT_GROWSDOWN|PROT_GROWSUP);
const bool rier = (current->personality & READ_IMPLIES_EXEC) &&
(prot & PROT_READ);
+ struct mmu_gather tlb;

start = untagged_addr(start);

@@ -584,6 +590,7 @@ static int do_mprotect_pkey(unsigned long start, size_t len,
if (start > vma->vm_start)
prev = vma;

+ tlb_gather_mmu(&tlb, current->mm);
for (nstart = start ; ; ) {
unsigned long mask_off_old_flags;
unsigned long newflags;
@@ -610,18 +617,18 @@ static int do_mprotect_pkey(unsigned long start, size_t len,
/* newflags >> 4 shift VM_MAY% in place of VM_% */
if ((newflags & ~(newflags >> 4)) & VM_ACCESS_FLAGS) {
error = -EACCES;
- goto out;
+ goto out_tlb;
}

/* Allow architectures to sanity-check the new flags */
if (!arch_validate_flags(newflags)) {
error = -EINVAL;
- goto out;
+ goto out_tlb;
}

error = security_file_mprotect(vma, reqprot, prot);
if (error)
- goto out;
+ goto out_tlb;

tmp = vma->vm_end;
if (tmp > end)
@@ -630,27 +637,29 @@ static int do_mprotect_pkey(unsigned long start, size_t len,
if (vma->vm_ops && vma->vm_ops->mprotect) {
error = vma->vm_ops->mprotect(vma, nstart, tmp, newflags);
if (error)
- goto out;
+ goto out_tlb;
}

- error = mprotect_fixup(vma, &prev, nstart, tmp, newflags);
+ error = mprotect_fixup(&tlb, vma, &prev, nstart, tmp, newflags);
if (error)
- goto out;
+ goto out_tlb;

nstart = tmp;

if (nstart < prev->vm_end)
nstart = prev->vm_end;
if (nstart >= end)
- goto out;
+ goto out_tlb;

vma = prev->vm_next;
if (!vma || vma->vm_start != nstart) {
error = -ENOMEM;
- goto out;
+ goto out_tlb;
}
prot = reqprot;
}
+out_tlb:
+ tlb_finish_mmu(&tlb);
out:
mmap_write_unlock(current->mm);
return error;
diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index ac6f036298cd..15a20bb35868 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -16,6 +16,7 @@
#include <linux/hugetlb.h>
#include <linux/shmem_fs.h>
#include <asm/tlbflush.h>
+#include <asm/tlb.h>
#include "internal.h"

static __always_inline
@@ -674,6 +675,7 @@ int mwriteprotect_range(struct mm_struct *dst_mm, unsigned long start,
atomic_t *mmap_changing)
{
struct vm_area_struct *dst_vma;
+ struct mmu_gather tlb;
pgprot_t newprot;
int err;

@@ -715,8 +717,10 @@ int mwriteprotect_range(struct mm_struct *dst_mm, unsigned long start,
else
newprot = vm_get_page_prot(dst_vma->vm_flags);

- change_protection(dst_vma, start, start + len, newprot,
+ tlb_gather_mmu(&tlb, dst_mm);
+ change_protection(&tlb, dst_vma, start, start + len, newprot,
enable_wp ? MM_CP_UFFD_WP : MM_CP_UFFD_WP_RESOLVE);
+ tlb_finish_mmu(&tlb);

err = 0;
out_unlock:
--
2.25.1