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Subject[RFC,PATCH 3/3] kill PF_BORROWED_MM in favour of PF_KTHREAD
Kill PF_BORROWED_MM. Change use_mm/unuse_mm to not play with ->flags, and
do s/PF_BORROWED_MM/PF_KTHREAD/ for a couple of other users.

No functional changes yet. But this allows us to do further fixes.

oom_kill/ptrace/etc often check "p->mm != NULL" to filter out the kthreads,
this is wrong because of use_mm(). The problem with PF_BORROWED_MM is that
we need task_lock() to avoid races. With this patch we can check PF_KTHREAD
directly, or use a simple lockless helper:

/* The result must not be dereferenced !!! */
struct mm_struct *__get_task_mm(struct task_struct *tsk)
{
if (tsk->flags & PF_KTHREAD)
return NULL;
return tsk->mm;
}

It could be used, for example by zap_threads(). Currently it is buggy, we
can hit a kthread with PF_BORROWED_MM.

Note also that ecard_task(). It runs with ->mm != NULL, but it is the kernel
thread without PF_BORROWED_MM.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>

include/linux/sched.h | 3 +--
fs/aio.c | 2 --
kernel/fork.c | 4 ++--
kernel/power/process.c | 2 +-
4 files changed, 4 insertions(+), 7 deletions(-)

--- 26-rc2/include/linux/sched.h~3_USE_PF_KTHREAD 2008-05-18 15:44:16.000000000 +0400
+++ 26-rc2/include/linux/sched.h 2008-05-18 19:11:44.000000000 +0400
@@ -1500,7 +1500,7 @@ static inline void put_task_struct(struc
#define PF_KSWAPD 0x00040000 /* I am kswapd */
#define PF_SWAPOFF 0x00080000 /* I am in swapoff */
#define PF_LESS_THROTTLE 0x00100000 /* Throttle me less: I clean memory */
-#define PF_BORROWED_MM 0x00200000 /* I am a kthread doing use_mm */
+#define PF_KTHREAD 0x00200000 /* I am a kernel thread */
#define PF_RANDOMIZE 0x00400000 /* randomize virtual address space */
#define PF_SWAPWRITE 0x00800000 /* Allowed to write to swap */
#define PF_SPREAD_PAGE 0x01000000 /* Spread page cache over cpuset */
@@ -1508,7 +1508,6 @@ static inline void put_task_struct(struc
#define PF_MEMPOLICY 0x10000000 /* Non-default NUMA mempolicy */
#define PF_MUTEX_TESTER 0x20000000 /* Thread belongs to the rt mutex tester */
#define PF_FREEZER_SKIP 0x40000000 /* Freezer should not count it as freezeable */
-#define PF_KTHREAD 0x80000000 /* I am a kernel thread */

/*
* Only the _current_ task can read/write to tsk->flags, but other
--- 26-rc2/fs/aio.c~3_USE_PF_KTHREAD 2008-05-18 17:20:42.000000000 +0400
+++ 26-rc2/fs/aio.c 2008-05-18 19:19:07.000000000 +0400
@@ -586,7 +586,6 @@ static void use_mm(struct mm_struct *mm)
struct task_struct *tsk = current;

task_lock(tsk);
- tsk->flags |= PF_BORROWED_MM;
active_mm = tsk->active_mm;
atomic_inc(&mm->mm_count);
tsk->mm = mm;
@@ -610,7 +609,6 @@ static void unuse_mm(struct mm_struct *m
struct task_struct *tsk = current;

task_lock(tsk);
- tsk->flags &= ~PF_BORROWED_MM;
tsk->mm = NULL;
/* active_mm is still 'mm' */
enter_lazy_tlb(mm, tsk);
--- 26-rc2/kernel/fork.c~3_USE_PF_KTHREAD 2008-05-18 15:44:18.000000000 +0400
+++ 26-rc2/kernel/fork.c 2008-05-18 19:21:02.000000000 +0400
@@ -447,7 +447,7 @@ EXPORT_SYMBOL_GPL(mmput);
/**
* get_task_mm - acquire a reference to the task's mm
*
- * Returns %NULL if the task has no mm. Checks PF_BORROWED_MM (meaning
+ * Returns %NULL if the task has no mm. Checks PF_KTHREAD (meaning
* this kernel workthread has transiently adopted a user mm with use_mm,
* to do its AIO) is not set and if so returns a reference to it, after
* bumping up the use count. User must release the mm via mmput()
@@ -460,7 +460,7 @@ struct mm_struct *get_task_mm(struct tas
task_lock(task);
mm = task->mm;
if (mm) {
- if (task->flags & PF_BORROWED_MM)
+ if (task->flags & PF_KTHREAD)
mm = NULL;
else
atomic_inc(&mm->mm_users);
--- 26-rc2/kernel/power/process.c~3_USE_PF_KTHREAD 2008-05-18 15:42:44.000000000 +0400
+++ 26-rc2/kernel/power/process.c 2008-05-18 19:22:40.000000000 +0400
@@ -86,7 +86,7 @@ static void fake_signal_wake_up(struct t

static int has_mm(struct task_struct *p)
{
- return (p->mm && !(p->flags & PF_BORROWED_MM));
+ return (p->mm && !(p->flags & PF_KTHREAD));
}

/**


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