Tmpfs elimination of double-copy

[Kostik Belousov]

On Mon, Jun 21, 2010 at 04:15:22PM -0400, John Baldwin wrote:
> On Monday 21 June 2010 2:49:28 pm Kostik Belousov wrote:
> > On Mon, Jun 21, 2010 at 10:30:55AM -0400, John Baldwin wrote:
> > > On Monday 21 June 2010 8:58:25 am Kostik Belousov wrote:
> > > > Hi,
> > > > Below is the patch that eliminates second copy of the data kept by tmpfs
> > > > in case a file is mapped. Also, it removes potential deadlocks due to
> > > > tmpfs doing copyin/out while page is busy. It is possible that patch
> > > > also fixes known issue with sendfile(2) of tmpfs file, but I did not
> > > > verified this.
> > > > 
> > > > Patch essentially consists of three parts:
> > > > - move of vm_object' vnp_size from the type-discriminated union to the
> > > >   vm_object proper;
> > > > - making vm not choke when vm object held in the struct vnode' v_object
> > > >   is default or swap object instead of vnode object;
> > > > - use of the swap object that keeps data for tmpfs VREG file, also as
> > > >   v_object.
> > > > 
> > > > Peter Holm helped me with the patch, apparently we survive fsx and 
> stress2.
> > > 
> > > Why did you have to move vnp_size out of the union?  Is tmpfs using a non-
> > > OBJT_VNODE object to hold file data?
> > Tmpfs uses OBJT_SWAP object to keep the data pages for the files.
> > Current code allocates another object of type OBJT_VNODE, assigned
> > to vp->v_object, to satisfy VM interface for mapping the file, using
> > vnode_create_vobject. The objects do not share the pages (I do not think
> > this can be easily achieved without serious changes to VM). Thus most,
> > if not all, the data is present in two sets of pages.
> > 
> > When such file is written to, tmpfs copies user buffer both to the swap
> > object, and to the v_object.
> > 
> > Patch I posted assigns the swap object to the vp->v_object. I had to
> > make small change to vm_mmap_vnode() to not allocate the vnode pager
> > and to not increment vnode use counter when v_object is the swap
> > object.
> > 
> > vnp_size has to be provided on the object layer because our swap
> > object is used to e.g. mmap the executables from tmpfs, and image
> > activation code relies on vnp_size instead of slower VOP_GETATTR().
> > I think this route is easier then converting all vnp_size users to
> > VOP_GETATTR for only tmpfs benefit.
> 
> Ok, thanks for the expanded explanation. :) It seems a shame to have
> to move vnp_size out of the pager-specific data. Maybe add a comment
> in vm_object.h to say that vnp_size is used by multiple object types
> which is why it can't be vnode-specific anymore?

Thanks for you note. I put the following comment into vm_object declaration.
	/*
	 * Current size of file for vnode pager.
	 *
	 * The tmpfs uses OBJT_SWAP object for vnode v_object.  To
	 * satisfy vm_object consumers that use vnp_size for v_object,
	 * tmpfs maintain vnp_size, and it have to be put outside
	 * un_pager union.
	 */
	off_t vnp_size;

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