svn commit: r232071 - head/sys/vm

[Alexey Dokuchaev]

On Thu, Feb 23, 2012 at 09:07:16PM +0000, Konstantin Belousov wrote:
> New Revision: 232071
> URL: http://svn.freebsd.org/changeset/base/232071
> 
> Log:
>   Account the writeable shared mappings backed by file in the vnode
>   v_writecount.  Keep the amount of the virtual address space used by
>   the mappings in the new vm_object un_pager.vnp.writemappings
>   counter. The vnode v_writecount is incremented when writemappings gets
>   non-zero value, and decremented when writemappings is returned to
>   zero.
>   
>   Writeable shared vnode-backed mappings are accounted for in vm_mmap(),
>   and vm_map_insert() is instructed to set MAP_ENTRY_VN_WRITECNT flag on
>   the created map entry.  During deferred map entry deallocation,
>   vm_map_process_deferred() checks for MAP_ENTRY_VN_WRITECOUNT and
>   decrements writemappings for the vm object.
>   
>   Now, the writeable mount cannot be demoted to read-only while
>   writeable shared mappings of the vnodes from the mount point
>   exist. Also, execve(2) fails for such files with ETXTBUSY, as it
>   should be.
>   
> ...
> Modified: head/sys/vm/vnode_pager.c
> ==============================================================================
> --- head/sys/vm/vnode_pager.c	Thu Feb 23 20:58:52 2012	(r232070)
> +++ head/sys/vm/vnode_pager.c	Thu Feb 23 21:07:16 2012	(r232071)
> @@ -1215,3 +1222,81 @@ vnode_pager_undirty_pages(vm_page_t *ma,
>  	}
>  	VM_OBJECT_UNLOCK(obj);
>  }
> +
> +void
> +vnode_pager_update_writecount(vm_object_t object, vm_offset_t start,
> +    vm_offset_t end)

So, it is first `start, then `end', but below...

> +{
> +	struct vnode *vp;
> +	vm_ooffset_t old_wm;
> +
> +	...
> +}
> +
> +void
> +vnode_pager_release_writecount(vm_object_t object, vm_offset_t start,
> +    vm_offset_t end)
> +{
> +	struct vnode *vp;
> +	struct mount *mp;
> +	vm_offset_t inc;
> +	int vfslocked;
> +
> +	VM_OBJECT_LOCK(object);
> +
> +	/*
> +	 * First, recheck the object type to account for the race when
> +	 * the vnode is reclaimed.
> +	 */
> +	if (object->type != OBJT_VNODE) {
> +		VM_OBJECT_UNLOCK(object);
> +		return;
> +	}
> +
> +	/*
> +	 * Optimize for the case when writemappings is not going to
> +	 * zero.
> +	 */
> +	inc = end - start;
> +	if (object->un_pager.vnp.writemappings != inc) {
> +		object->un_pager.vnp.writemappings -= inc;
> +		VM_OBJECT_UNLOCK(object);
> +		return;
> +	}
> +
> +	vp = object->handle;
> +	vhold(vp);
> +	VM_OBJECT_UNLOCK(object);
> +	vfslocked = VFS_LOCK_GIANT(vp->v_mount);
> +	mp = NULL;
> +	vn_start_write(vp, &mp, V_WAIT);
> +	vn_lock(vp, LK_EXCLUSIVE | LK_RETRY);
> +
> +	/*
> +	 * Decrement the object's writemappings, by swapping the start
> +	 * and end arguments for vnode_pager_update_writecount().  If
> +	 * there was not a race with vnode reclaimation, then the
> +	 * vnode's v_writecount is decremented.
> +	 */
> +	vnode_pager_update_writecount(object, end, start);

... here, first `end' is passed, then `start'.  Is this intentional?

PVS Studio complains:

/usr/src/sys/vm/vnode_pager.c:1584:1: warning: V764 Possible
incorrect order of arguments passed to 'vnode_pager_update_writecount'
function: 'end' and 'start'.

> +	VOP_UNLOCK(vp, 0);
> +	vdrop(vp);
> +	if (mp != NULL)
> +		vn_finished_write(mp);
> +	VFS_UNLOCK_GIANT(vfslocked);
> +}