Re: [RFC/PATCH] mm/futex: Fix futex writes on archs with SW trackingof dirty & young
From: Shan Hai
Date: Tue Jul 19 2011 - 01:36:07 EST
On 07/19/2011 01:24 PM, Benjamin Herrenschmidt wrote:
On Tue, 2011-07-19 at 13:17 +0800, Shan Hai wrote:
The patch works, but I have certain confusions,
- Do we want to handle_mm_fault on each futex_lock_pi
even though in most cases there is no write permission
fixup's needed?
Don't we only ever call this when futex_atomic_op_inuser() failed ?
Which means a fixup -is- needed .... The fast path is still there.
What you said is another path, that is futex_wake_op(),
but what about futex_lock_pi in which my test case failed?
your patch will call handle_mm_fault on every futex contention
in the futex_lock_pi path.
futex_lock_pi()
ret = futex_lock_pi_atomic(uaddr, hb, &q.key, &q.pi_state, current, 0);
case -EFAULT:
goto uaddr_faulted;
...
uaddr_faulted:
ret = fault_in_user_writeable(uaddr);
- How about let the archs do their own write permission
fixup as what I did in my original
Why ? This is generic and will fix all archs at once with generic code
which is a significant improvement in my book and a lot more
maintainable :-)
If the overhead in the futex_lock_pi path is not considerable yes fix it up
generally is nice :-)
"[PATCH 1/1] Fixup write permission of TLB on powerpc e500 core"?
(I will fix the stupid errors in my original patch if the concept
is acceptable)
in this way we could decrease the overhead of handle_mm_fault
in the path which does not need write permission fixup.
Which overhead ? gup does handle_mm_fault() as well if needed.
it does it *if needed*, and this requirement is rare in my opinion.
Thanks
Shan Hai
What I do is I replace what is arguably an abuse of gup() in the case
where a fixup -is- needed with a dedicated function designed to perform
the said fixup ... and do it properly which gup() didn't :-)
Cheers,
Ben.
Thanks
Shan Hai
Cheers,
Ben.
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 9670f71..1036614 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -985,6 +985,8 @@ int get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
int get_user_pages_fast(unsigned long start, int nr_pages, int write,
struct page **pages);
struct page *get_dump_page(unsigned long addr);
+extern int fixup_user_fault(struct task_struct *tsk, struct mm_struct *mm,
+ unsigned long address, unsigned int fault_flags);
extern int try_to_release_page(struct page * page, gfp_t gfp_mask);
extern void do_invalidatepage(struct page *page, unsigned long offset);
diff --git a/kernel/futex.c b/kernel/futex.c
index fe28dc2..7a0a4ed 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -355,8 +355,8 @@ static int fault_in_user_writeable(u32 __user *uaddr)
int ret;
down_read(&mm->mmap_sem);
- ret = get_user_pages(current, mm, (unsigned long)uaddr,
- 1, 1, 0, NULL, NULL);
+ ret = fixup_user_fault(current, mm, (unsigned long)uaddr,
+ FAULT_FLAG_WRITE);
up_read(&mm->mmap_sem);
return ret< 0 ? ret : 0;
diff --git a/mm/memory.c b/mm/memory.c
index 40b7531..b967fb0 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1815,7 +1815,64 @@ next_page:
}
EXPORT_SYMBOL(__get_user_pages);
-/**
+/*
+ * fixup_user_fault() - manually resolve a user page fault
+ * @tsk: the task_struct to use for page fault accounting, or
+ * NULL if faults are not to be recorded.
+ * @mm: mm_struct of target mm
+ * @address: user address
+ * @fault_flags:flags to pass down to handle_mm_fault()
+ *
+ * This is meant to be called in the specific scenario where for
+ * locking reasons we try to access user memory in atomic context
+ * (within a pagefault_disable() section), this returns -EFAULT,
+ * and we want to resolve the user fault before trying again.
+ *
+ * Typically this is meant to be used by the futex code.
+ *
+ * The main difference with get_user_pages() is that this function
+ * will unconditionally call handle_mm_fault() which will in turn
+ * perform all the necessary SW fixup of the dirty and young bits
+ * in the PTE, while handle_mm_fault() only guarantees to update
+ * these in the struct page.
+ *
+ * This is important for some architectures where those bits also
+ * gate the access permission to the page because their are
+ * maintained in software. On such architecture, gup() will not
+ * be enough to make a subsequent access succeed.
+ *
+ * This should be called with the mm_sem held for read.
+ */
+int fixup_user_fault(struct task_struct *tsk, struct mm_struct *mm,
+ unsigned long address, unsigned int fault_flags)
+{
+ struct vm_area_struct *vma;
+ int ret;
+
+ vma = find_extend_vma(mm, address);
+ if (!vma || address< vma->vm_start)
+ return -EFAULT;
+
+ ret = handle_mm_fault(mm, vma, address, fault_flags);
+ if (ret& VM_FAULT_ERROR) {
+ if (ret& VM_FAULT_OOM)
+ return -ENOMEM;
+ if (ret& (VM_FAULT_HWPOISON | VM_FAULT_HWPOISON_LARGE))
+ return -EHWPOISON;
+ if (ret& VM_FAULT_SIGBUS)
+ return -EFAULT;
+ BUG();
+ }
+ if (tsk) {
+ if (ret& VM_FAULT_MAJOR)
+ tsk->maj_flt++;
+ else
+ tsk->min_flt++;
+ }
+ return 0;
+}
+
+/*
* get_user_pages() - pin user pages in memory
* @tsk: the task_struct to use for page fault accounting, or
* NULL if faults are not to be recorded.
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