Re: [RFC v2][PATCH 2/9] General infrastructure for checkpoint restart

Dave Hansen
2008-08-26T16:42:41Z

On Sun, 2008-08-24 at 22:47 -0400, Oren Laadan wrote:
> > I replaced all of the uses of these with kmalloc()/kfree() and stack
> > allocations. I'm really not sure what these buy us since our allocators
> > are already so fast. tbuf, especially, worries me if it gets used in
> > any kind of nested manner we're going to get some really fun bugs.
>
> I disagree with putting some of the cr_hdr_... on the stack: first, if they
> grow in size, it's easy to forget to change the allocation to stack.

I can buy that.

> Second,
> using a standard code/handling for all cr_hdr_... makes the code more readable
> and easier for others to extend by simply following existing code.

It actually makes it harder for others to jump into because they see
something which is basically a kmalloc() to the rest of the kernel. We
don't have ext2_alloc() or usb_alloc(), so I'm not sure we should have
cr_alloc(), effectively.

> The main argument for ->hbuf is that eventually we would like to buffer
> data in the kernel to avoid potentially slow writing/reading when processes
> are frozen during checkpoint/restart.

Um, we're writing to a file descriptor and kmap()'ing. Those can both
potentially be very, very slow. I don't think that a few kmalloc()s
thrown in there are going to be noticeable.

> By using the simple cr_get_hbuf() and
> cr_put_hbuf() we prepare for that: now they just allocate room in ->hbuf,
> but later they will provide a pointer directly in the data buffer. Moreover,
> it simplifies the error path since cleanup (of ->hbuf) is implicit.

It simplifies *one* error path. But, it complicates the ctx creation
and makes *that* error path more complex. Pick your poison, I guess.

> Also,
> it is designed to allow checkpoint and restart function to be called in a
> nested manner, again simplifying the code. Finally, my experience was that
> it can impact performance if you are after very short downtimes, especially
> for the checkpoint.

Right, but I'm comparing it to kmalloc() Certainly kmalloc()s can be
used in a nested manner.

> >> +/* read the checkpoint header */
> >> +static int cr_read_hdr(struct cr_ctx *ctx)
> >> +{
> >> + struct cr_hdr_head *hh = cr_hbuf_get(ctx, sizeof(*hh));
> >> + int ret;
> >> +
> >> + ret = cr_read_obj_type(ctx, hh, sizeof(*hh), CR_HDR_HEAD);
> >> + if (ret < 0)
> >> + return ret;
> >> + else if (ret != 0)
> >> + return -EINVAL;
> >
> > How about just making cr_read_obj_type() stop returning positive values?
> > Is it ever valid for it to return >0?
>
> The idea with cr_read_obj_type() is that it returns the 'ptag' - parent tag -
> field of the object that it reads in. The 'ptag' is the tag of the parent
> object, and is useful in several places. For instance, the 'ptag' of an MM
> header identifies (is equal to) the 'tag' of TASK to which it belongs. In
> this case, the 'ptag' should be zero because it has no parent object.
>
> I'll change the variable name from 'ret' to 'ptag' to make it more obvious.

Since this ptag isn't actually used, I can't really offer a suggestion.
I don't see the whole picture.

-- Dave