On 3/5/2014 5:07 AM, Konstantin Belousov wrote:
> On Wed, Mar 05, 2014 at 11:41:24AM +0200, Andriy Gapon wrote:
>> on 04/03/2014 18:45 John Baldwin said the following:
>>> So I'm not sure how to fix this. The crash is in this code in
>>> vm_object_deallocate():
>>>
>>> if (object->type == OBJT_SWAP &&
>>> (object->flags & OBJ_TMPFS) != 0) {
>>> vp = object->un_pager.swp.swp_tmpfs;
>>> vhold(vp);
>>> VM_OBJECT_WUNLOCK(object);
>>> vn_lock(vp, LK_EXCLUSIVE | LK_RETRY);
>>> vdrop(vp);
>>> VM_OBJECT_WLOCK(object);
>>> if (object->type == OBJT_DEAD ||
>>> object->ref_count != 1) {
>>> VM_OBJECT_WUNLOCK(object);
>>> VOP_UNLOCK(vp, 0);
>>> return;
>>> }
>>> if ((object->flags & OBJ_TMPFS) != 0)
>>> VOP_UNSET_TEXT(vp);
>>> VOP_UNLOCK(vp, 0);
>>> }
>>>
>>> The vdrop() is dropping the count to zero and trying to free the vnode.
>>> The
>>> real problem I think is that swp_tmpfs doesn't have an implicit vhold() on
>>> the
>>> vnode, so in this case, the code is doing a vhold/vn_lock/vdrop of an
>>> already-
>>> free vnode. For OBJT_VNODE objects, the reference from the object back to
>>> the
>>> vnode holds a vref() that gets released by a vput() in
>>> vm_object_vndeallocate().
>>>
>>> One fix might be to chagne smp_tmpfs to hold a vhold reference. This is
>>> untested but might work (but I'm also not sure that this is the right thing
>>> in
>>> that I don't know what other effects it might have).
>>
>> I agree with your analysis, but I don't think that a filesystem holding its
>> own
>> vnode is a good idea. If I am not mistaken, that would prevent tmpfs vnodes
>> from going to free list.
>> I'd rather try to modify vm_object_deallocate() code. E.g. vdrop() could be
>> called after VOP_UNLOCK(). Alternatively, the code could handle a doomed
>> vnode
>> in a different way.
>
> I agree with Andrey, it is just a bug to vdrop() before unlock.
> Please try this.
>
> diff --git a/sys/vm/vm_object.c b/sys/vm/vm_object.c
> index 8683e2f..787b18b 100644
> --- a/sys/vm/vm_object.c
> +++ b/sys/vm/vm_object.c
> @@ -536,17 +536,18 @@ vm_object_deallocate(vm_object_t object)
> vhold(vp);
> VM_OBJECT_WUNLOCK(object);
> vn_lock(vp, LK_EXCLUSIVE | LK_RETRY);
> - vdrop(vp);
> VM_OBJECT_WLOCK(object);
> if (object->type == OBJT_DEAD ||
> object->ref_count != 1) {
> VM_OBJECT_WUNLOCK(object);
> VOP_UNLOCK(vp, 0);
> + vdrop(vp);
> return;
> }
> if ((object->flags & OBJ_TMPFS) != 0)
> VOP_UNSET_TEXT(vp);
> VOP_UNLOCK(vp, 0);
> + vdrop(vp);
> }
> if (object->shadow_count == 0 &&
> object->handle == NULL &&
> Ok I will try this. Note that I cannot easily reproduce the issue. So if this seems to be right logically I suggest just committing after stress testing. I'll report back after a few builds to let you know if it makes it any worse. Thanks all. -- Regards, Bryan Drewery
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