Re: locking question: do_mmap(), do_munmap()

Kanoj Sarcar
1999-10-13T01:25:42Z

>
> Hi,
>
> On Mon, 11 Oct 1999 12:07:08 -0700 (PDT), kanoj@google.engr.sgi.com
> (Kanoj Sarcar) said:
>
> >> What about something like a rw-semaphore which protects the vma list:
> >> vma-list modifiers [ie merge_segments(), insert_vm_struct() and
> >> do_munmap()] grab it exclusive, swapper grabs it "shared, starve
> >> exclusive".
> >> All other vma-list readers are protected by mm->mmap_sem.
>
> It will deadlock.
>

Not sure why you say that. Yes, I did see the nesting semdaphore grabbing
scenario that you posted, but I think you can prevent that from
happening if you follow some rules.

Here's a primitive patch showing the direction I am thinking of. I do not
have any problem with a spinning lock, but I coded this against 2.2.10,
where insert_vm_struct could go to sleep, hence I had to use sleeping
locks to protect the vma chain. Slight change is needed to vmscan.c
to use spinning locks.

Kanoj

This is a skeleton of the solution that prevents kswapd from walking
down a vma chain without protections. I am trying to get comments on
this approach before I try a full blown implementation.

The rules:
1. To modify the vmlist (add/delete), you must hold mmap_sem to
guard against clones doing mmap/munmap/faults, (ie all vm system
calls and faults), and from ptrace, swapin due to swap deletion
etc.
2. To modify the vmlist (add/delete), you must also hold
vmlist_modify_lock, to guard against page stealers scanning the
list.
3. To scan the vmlist, you must either
a. grab mmap_sem, which should be all cases except page stealer.
or
b. grab vmlist_access_lock, only done by page stealer.
4. While holding the vmlist_modify_lock, you must be able to guarantee
that no code path will lead to page stealing.
5. You must be able to guarantee that while holding vmlist_modify_lock
or vmlist_access_lock of mm A, you will not try to get either lock
for mm B.

The assumptions:
1. No code path reachable thru insert_vm_struct and merge_segments
sleep for memory.

--- /usr/tmp/p_rdiff_a00368/vmscan.c Tue Oct 12 17:58:50 1999
+++ mm/vmscan.c Tue Oct 12 16:55:13 1999
@@ -295,6 +295,7 @@
/*
* Find the proper vm-area
*/
+ vmlist_access_lock(mm);
vma = find_vma(mm, address);
if (vma) {
if (address < vma->vm_start)
@@ -302,8 +303,10 @@

for (;;) {
int result = swap_out_vma(vma, address, gfp_mask);
- if (result)
+ if (result) {
+ vmlist_access_unlock(mm);
return result;
+ }
vma = vma->vm_next;
if (!vma)
break;
@@ -310,6 +313,7 @@
address = vma->vm_start;
}
}
+ vmlist_access_unlock(mm);

/* We didn't find anything for the process */
mm->swap_cnt = 0;
--- /usr/tmp/p_rdiff_a0036H/mlock.c Tue Oct 12 17:59:06 1999
+++ mm/mlock.c Tue Oct 12 16:35:25 1999
@@ -13,7 +13,9 @@

static inline int mlock_fixup_all(struct vm_area_struct * vma, int newflags)
{
+ vmlist_modify_lock(vma->vm_mm);
vma->vm_flags = newflags;
+ vmlist_modify_unlock(vma->vm_mm);
return 0;
}

@@ -26,15 +28,17 @@
if (!n)
return -EAGAIN;
*n = *vma;
- vma->vm_start = end;
n->vm_end = end;
- vma->vm_offset += vma->vm_start - n->vm_start;
n->vm_flags = newflags;
if (n->vm_file)
get_file(n->vm_file);
if (n->vm_ops && n->vm_ops->open)
n->vm_ops->open(n);
+ vmlist_modify_lock(vma->vm_mm);
+ vma->vm_offset += end - vma->vm_start;
+ vma->vm_start = end;
insert_vm_struct(current->mm, n);
+ vmlist_modify_unlock(vma->vm_mm);
return 0;
}

@@ -47,7 +51,6 @@
if (!n)
return -EAGAIN;
*n = *vma;
- vma->vm_end = start;
n->vm_start = start;
n->vm_offset += n->vm_start - vma->vm_start;
n->vm_flags = newflags;
@@ -55,7 +58,10 @@
get_file(n->vm_file);
if (n->vm_ops && n->vm_ops->open)
n->vm_ops->open(n);
+ vmlist_modify_lock(vma->vm_mm);
+ vma->vm_end = start;
insert_vm_struct(current->mm, n);
+ vmlist_modify_unlock(vma->vm_mm);
return 0;
}

@@ -75,10 +81,7 @@
*left = *vma;
*right = *vma;
left->vm_end = start;
- vma->vm_start = start;
- vma->vm_end = end;
right->vm_start = end;
- vma->vm_offset += vma->vm_start - left->vm_start;
right->vm_offset += right->vm_start - left->vm_start;
vma->vm_flags = newflags;
if (vma->vm_file)
@@ -88,8 +91,14 @@
vma->vm_ops->open(left);
vma->vm_ops->open(right);
}
+ vmlist_modify_lock(vma->vm_mm);
+ vma->vm_offset += start - vma->vm_start;
+ vma->vm_start = start;
+ vma->vm_end = end;
+ vma->vm_flags = newflags;
insert_vm_struct(current->mm, left);
insert_vm_struct(current->mm, right);
+ vmlist_modify_unlock(vma->vm_mm);
return 0;
}

@@ -168,7 +177,9 @@
break;
}
}
+ vmlist_modify_lock(current->mm);
merge_segments(current->mm, start, end);
+ vmlist_modify_unlock(current->mm);
return error;
}

@@ -240,7 +251,9 @@
if (error)
break;
}
+ vmlist_modify_lock(current->mm);
merge_segments(current->mm, 0, TASK_SIZE);
+ vmlist_modify_unlock(current->mm);
return error;
}