[RFC PATCH 07/13] kernel/fork: Split and export 'mm_alloc' and 'mm_init'

Till Smejkal
2017-03-13T22:14:49Z

The only way until now to create a new memory map was via the exported
function 'mm_alloc'. Unfortunately, this function not only allocates a new
memory map, but also completely initializes it. However, with the
introduction of first class virtual address spaces, some initialization
steps done in 'mm_alloc' are not applicable to the memory maps needed for
this feature and hence would lead to errors in the kernel code.

Instead of introducing a new function that can allocate and initialize
memory maps for first class virtual address spaces and potentially
duplicate some code, I decided to split the mm_alloc function as well as
the 'mm_init' function that it uses.

Now there are four functions exported instead of only one. The new
'mm_alloc' function only allocates a new mm_struct and zeros it out. If one
want to have the old behavior of mm_alloc one can use the newly introduced
function 'mm_alloc_and_setup' which not only allocates a new mm_struct but
also fully initializes it.

The old 'mm_init' function which fully initialized a mm_struct was split up
into two separate functions. The first one - 'mm_setup' - does all the
initialization of the mm_struct that is not related to the mm_struct
belonging to a particular task. This part of the initialization is done in
the 'mm_set_task' function. This way it is possible to create memory maps
that don't have any task-specific information as needed by the first class
virtual address space feature. Both functions, 'mm_setup' and 'mm_set_task'
are also exported, so that they can be used in all files in the source
tree.

Signed-off-by: Till Smejkal <till.smejkal@gmail.com>
---
arch/arm/mach-rpc/ecard.c | 2 +-
fs/exec.c | 2 +-
include/linux/sched.h | 7 +++++-
kernel/fork.c | 64 +++++++++++++++++++++++++++++++++++++----------
4 files changed, 59 insertions(+), 16 deletions(-)

diff --git a/arch/arm/mach-rpc/ecard.c b/arch/arm/mach-rpc/ecard.c
index dc67a7fb3831..15845e8abd7e 100644
--- a/arch/arm/mach-rpc/ecard.c
+++ b/arch/arm/mach-rpc/ecard.c
@@ -245,7 +245,7 @@ static void ecard_init_pgtables(struct mm_struct *mm)

static int ecard_init_mm(void)
{
- struct mm_struct * mm = mm_alloc();
+ struct mm_struct *mm = mm_alloc_and_setup();
struct mm_struct *active_mm = current->active_mm;

if (!mm)
diff --git a/fs/exec.c b/fs/exec.c
index e57946610733..68d7908a1e5a 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -380,7 +380,7 @@ static int bprm_mm_init(struct linux_binprm *bprm)
int err;
struct mm_struct *mm = NULL;

- bprm->mm = mm = mm_alloc();
+ bprm->mm = mm = mm_alloc_and_setup();
err = -ENOMEM;
if (!mm)
goto err;
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 42b9b93a50ac..7955adc00397 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -2922,7 +2922,12 @@ static inline unsigned long sigsp(unsigned long sp, struct ksignal *ksig)
/*
* Routines for handling mm_structs
*/
-extern struct mm_struct * mm_alloc(void);
+extern struct mm_struct *mm_setup(struct mm_struct *mm);
+extern struct mm_struct *mm_set_task(struct mm_struct *mm,
+ struct task_struct *p,
+ struct user_namespace *user_ns);
+extern struct mm_struct *mm_alloc(void);
+extern struct mm_struct *mm_alloc_and_setup(void);

/* mmdrop drops the mm and the page tables */
extern void __mmdrop(struct mm_struct *);
diff --git a/kernel/fork.c b/kernel/fork.c
index 11c5c8ab827c..9209f6d5d7c0 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -747,8 +747,10 @@ static void mm_init_owner(struct mm_struct *mm, struct task_struct *p)
#endif
}

-static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
- struct user_namespace *user_ns)
+/**
+ * Initialize all the task-unrelated fields of a mm_struct.
+ **/
+struct mm_struct *mm_setup(struct mm_struct *mm)
{
mm->mmap = NULL;
mm->mm_rb = RB_ROOT;
@@ -767,24 +769,37 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
spin_lock_init(&mm->page_table_lock);
mm_init_cpumask(mm);
mm_init_aio(mm);
- mm_init_owner(mm, p);
mmu_notifier_mm_init(mm);
clear_tlb_flush_pending(mm);
#if defined(CONFIG_TRANSPARENT_HUGEPAGE) && !USE_SPLIT_PMD_PTLOCKS
mm->pmd_huge_pte = NULL;
#endif

+ mm->flags = default_dump_filter;
+ mm->def_flags = 0;
+
+ if (mm_alloc_pgd(mm))
+ goto fail_nopgd;
+
+ return mm;
+
+fail_nopgd:
+ free_mm(mm);
+ return NULL;
+}
+
+/**
+ * Initialize all the task-related fields of a mm_struct.
+ **/
+struct mm_struct *mm_set_task(struct mm_struct *mm, struct task_struct *p,
+ struct user_namespace *user_ns)
+{
if (current->mm) {
mm->flags = current->mm->flags & MMF_INIT_MASK;
mm->def_flags = current->mm->def_flags & VM_INIT_DEF_MASK;
- } else {
- mm->flags = default_dump_filter;
- mm->def_flags = 0;
}

- if (mm_alloc_pgd(mm))
- goto fail_nopgd;
-
+ mm_init_owner(mm, p);
if (init_new_context(p, mm))
goto fail_nocontext;

@@ -793,11 +808,21 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,

fail_nocontext:
mm_free_pgd(mm);
-fail_nopgd:
free_mm(mm);
return NULL;
}

+static struct mm_struct *mm_setup_all(struct mm_struct *mm,
+ struct task_struct *p,
+ struct user_namespace *user_ns)
+{
+ mm = mm_setup(mm);
+ if (!mm)
+ return NULL;
+
+ return mm_set_task(mm, p, user_ns);
+}
+
static void check_mm(struct mm_struct *mm)
{
int i;
@@ -822,10 +847,22 @@ static void check_mm(struct mm_struct *mm)
#endif
}

+struct mm_struct *mm_alloc(void)
+{
+ struct mm_struct *mm;
+
+ mm = allocate_mm();
+ if (!mm)
+ return NULL;
+
+ memset(mm, 0, sizeof(*mm));
+ return mm;
+}
+
/*
* Allocate and initialize an mm_struct.
*/
-struct mm_struct *mm_alloc(void)
+struct mm_struct *mm_alloc_and_setup(void)
{
struct mm_struct *mm;

@@ -834,9 +871,10 @@ struct mm_struct *mm_alloc(void)
return NULL;

memset(mm, 0, sizeof(*mm));
- return mm_init(mm, current, current_user_ns());
+ return mm_setup_all(mm, current, current_user_ns());
}

+
/*
* Called when the last reference to the mm
* is dropped: either by a lazy thread or by
@@ -1131,7 +1169,7 @@ static struct mm_struct *dup_mm(struct task_struct *tsk)

memcpy(mm, oldmm, sizeof(*mm));

- if (!mm_init(mm, tsk, mm->user_ns))
+ if (!mm_setup_all(mm, tsk, mm->user_ns))
goto fail_nomem;

err = dup_mmap(mm, oldmm);
--
2.12.0