[PATCH 2/6] fs/direct-io.c: Use DIV_ROUND_UP

[PATCH 2/6] fs/direct-io.c: Use DIV_ROUND_UP

[Julia Lawall]

From: Julia Lawall <julia@diku.dk>

The kernel.h macro DIV_ROUND_UP performs the computation (((n) + (d) - 1) /
(d)) but is perhaps more readable.

An extract of the semantic patch that makes this change is as follows:
(http://www.emn.fr/x-info/coccinelle/)

// <smpl>
@haskernel@
@@

#include <linux/kernel.h>

@depends on haskernel@
expression n,d;
@@

(
- (n + d - 1) / d
+ DIV_ROUND_UP(n,d)
|
- (n + (d - 1)) / d
+ DIV_ROUND_UP(n,d)
)

@depends on haskernel@
expression n,d;
@@

- DIV_ROUND_UP((n),d)
+ DIV_ROUND_UP(n,d)

@depends on haskernel@
expression n,d;
@@

- DIV_ROUND_UP(n,(d))
+ DIV_ROUND_UP(n,d)
// </smpl>

Signed-off-by: Julia Lawall <julia@diku.dk>

---

diff -u -p a/fs/direct-io.c b/fs/direct-io.c
--- a/fs/direct-io.c 2008-02-08 08:58:17.000000000 +0100
+++ b/fs/direct-io.c 2008-02-13 20:58:53.000000000 +0100
@@ -976,7 +976,7 @@ direct_io_worker(int rw, struct kiocb *i
 	for (seg = 0; seg < nr_segs; seg++) {
 		user_addr = (unsigned long)iov[seg].iov_base;
 		dio->pages_in_io +=
-			((user_addr+iov[seg].iov_len +PAGE_SIZE-1)/PAGE_SIZE
+			(DIV_ROUND_UP(user_addr + iov[seg].iov_len, PAGE_SIZE)
 				- user_addr/PAGE_SIZE);
 	}

@@ -998,7 +998,7 @@ direct_io_worker(int rw, struct kiocb *i
 			dio->total_pages++;
 			bytes -= PAGE_SIZE - (user_addr & (PAGE_SIZE - 1));
 		}
-		dio->total_pages += (bytes + PAGE_SIZE - 1) / PAGE_SIZE;
+		dio->total_pages += DIV_ROUND_UP(bytes, PAGE_SIZE);
 		dio->curr_user_address = user_addr;

 		ret = do_direct_IO(dio);

Re: [PATCH 2/6] fs/direct-io.c: Use DIV_ROUND_UP

[Zach Brown]

Julia Lawall wrote:
> From: Julia Lawall <julia@diku.dk>
> 
> The kernel.h macro DIV_ROUND_UP performs the computation (((n) + (d) - 1) /
> (d)) but is perhaps more readable.

It certainly looks better to me, thanks for doing this.

> Signed-off-by: Julia Lawall <julia@diku.dk>

Acked-By: Zach Brown <zach.brown@oracle.com>

- z

Re: [PATCH 2/6] fs/direct-io.c: Use DIV_ROUND_UP

[Nishanth Aravamudan]

On 14.02.2008 [16:14:33 +0100], Julia Lawall wrote:
> From: Julia Lawall <julia@diku.dk>
> 
> The kernel.h macro DIV_ROUND_UP performs the computation (((n) + (d) - 1) /
> (d)) but is perhaps more readable.
> 
> An extract of the semantic patch that makes this change is as follows:
> (http://www.emn.fr/x-info/coccinelle/)
> 
> // <smpl>
> @haskernel@
> @@
> 
> #include <linux/kernel.h>
> 
> @depends on haskernel@
> expression n,d;
> @@
> 
> (
> - (n + d - 1) / d
> + DIV_ROUND_UP(n,d)
> |
> - (n + (d - 1)) / d
> + DIV_ROUND_UP(n,d)
> )
> 
> @depends on haskernel@
> expression n,d;
> @@
> 
> - DIV_ROUND_UP((n),d)
> + DIV_ROUND_UP(n,d)
> 
> @depends on haskernel@
> expression n,d;
> @@
> 
> - DIV_ROUND_UP(n,(d))
> + DIV_ROUND_UP(n,d)
> // </smpl>
> 
> Signed-off-by: Julia Lawall <julia@diku.dk>
> 
> ---
> 
> diff -u -p a/fs/direct-io.c b/fs/direct-io.c
> --- a/fs/direct-io.c 2008-02-08 08:58:17.000000000 +0100
> +++ b/fs/direct-io.c 2008-02-13 20:58:53.000000000 +0100
> @@ -976,7 +976,7 @@ direct_io_worker(int rw, struct kiocb *i
>  	for (seg = 0; seg < nr_segs; seg++) {
>  		user_addr = (unsigned long)iov[seg].iov_base;
>  		dio->pages_in_io +=
> -			((user_addr+iov[seg].iov_len +PAGE_SIZE-1)/PAGE_SIZE
> +			(DIV_ROUND_UP(user_addr + iov[seg].iov_len, PAGE_SIZE)
>  				- user_addr/PAGE_SIZE);

Is it just me, or does

	((user_addr + iov[seg].iov_len + PAGE_SIZE - 1)/PAGE_SIZE - user_addr/PAGE_SIZE)

not simplify to

	= ((iov[seg].iov_len + PAGE_SIZE - 1)/PAGE_SIZE + user_addr/PAGE_SIZE - user_addr/PAGE_SIZE)

	= ((iov[seg].iov_len + PAGE_SIZE - 1)/PAGE_SIZE)

	= DIV_ROUND_UP(iov[seg].iov_len, PAGE_SIZE)

CMIIW.

Thanks,
Nish

-- 
Nishanth Aravamudan <nacc@us.ibm.com>
IBM Linux Technology Center

Re: [PATCH 2/6] fs/direct-io.c: Use DIV_ROUND_UP

[Pekka Enberg]

Hi Nishanth,

On Thu, Feb 14, 2008 at 7:38 PM, Nishanth Aravamudan <nacc@us.ibm.com> wrote:
>  Is it just me, or does
>
>         ((user_addr + iov[seg].iov_len + PAGE_SIZE - 1)/PAGE_SIZE - user_addr/PAGE_SIZE)
>
>  not simplify to
>
>         = ((iov[seg].iov_len + PAGE_SIZE - 1)/PAGE_SIZE + user_addr/PAGE_SIZE - user_addr/PAGE_SIZE)
>
>         = ((iov[seg].iov_len + PAGE_SIZE - 1)/PAGE_SIZE)
>
>         = DIV_ROUND_UP(iov[seg].iov_len, PAGE_SIZE)
>
>  CMIIW.

I double-checked this and I believe you're correct. It's simpler to
see when you do:

  x = user_addr
  y = iov[seg].iov_len
  z = PAGE_SIZE

So

  (x + y + z - 1)/z - x/z

  = [x + (y + z - 1)]/z - x/z

  = [xz + z(y + z - 1)]/z^2 - x/z

  = x/z + (y + z - 1)/z - x/z

And the rest follows from your simplifications.

                                      Pekka

Re: [PATCH 2/6] fs/direct-io.c: Use DIV_ROUND_UP

[Julia Lawall]

On Thu, 14 Feb 2008, Pekka Enberg wrote:

> Hi Nishanth,
> 
> On Thu, Feb 14, 2008 at 7:38 PM, Nishanth Aravamudan <nacc@us.ibm.com> wrote:
> >  Is it just me, or does
> >
> >         ((user_addr + iov[seg].iov_len + PAGE_SIZE - 1)/PAGE_SIZE - user_addr/PAGE_SIZE)
> >
> >  not simplify to
> >
> >         = ((iov[seg].iov_len + PAGE_SIZE - 1)/PAGE_SIZE + user_addr/PAGE_SIZE - user_addr/PAGE_SIZE)
> >
> >         = ((iov[seg].iov_len + PAGE_SIZE - 1)/PAGE_SIZE)
> >
> >         = DIV_ROUND_UP(iov[seg].iov_len, PAGE_SIZE)
> >
> >  CMIIW.
> 
> I double-checked this and I believe you're correct. It's simpler to
> see when you do:
> 
>   x = user_addr
>   y = iov[seg].iov_len
>   z = PAGE_SIZE
> 
> So
> 
>   (x + y + z - 1)/z - x/z
> 
>   = [x + (y + z - 1)]/z - x/z
> 
>   = [xz + z(y + z - 1)]/z^2 - x/z
> 
>   = x/z + (y + z - 1)/z - x/z
> 
> And the rest follows from your simplifications.

It doesn't work:

 ((3+4+5-1)/5) - (3/5) = 2
 ((4+5-1)/5) = 1

julia

Re: [PATCH 2/6] fs/direct-io.c: Use DIV_ROUND_UP

[Mariusz Kozlowski]

Hello,

> > >  Is it just me, or does
> > >
> > >         ((user_addr + iov[seg].iov_len + PAGE_SIZE - 1)/PAGE_SIZE - user_addr/PAGE_SIZE)
> > >
> > >  not simplify to
> > >
> > >         = ((iov[seg].iov_len + PAGE_SIZE - 1)/PAGE_SIZE + user_addr/PAGE_SIZE - user_addr/PAGE_SIZE)
> > >
> > >         = ((iov[seg].iov_len + PAGE_SIZE - 1)/PAGE_SIZE)
> > >
> > >         = DIV_ROUND_UP(iov[seg].iov_len, PAGE_SIZE)
> > >
> > >  CMIIW.
> > 
> > I double-checked this and I believe you're correct. It's simpler to
> > see when you do:
> > 
> >   x = user_addr
> >   y = iov[seg].iov_len
> >   z = PAGE_SIZE
> > 
> > So
> > 
> >   (x + y + z - 1)/z - x/z
> > 
> >   = [x + (y + z - 1)]/z - x/z
> > 
> >   = [xz + z(y + z - 1)]/z^2 - x/z
> > 
> >   = x/z + (y + z - 1)/z - x/z
> > 
> > And the rest follows from your simplifications.
> 
> It doesn't work:
> 
>  ((3+4+5-1)/5) - (3/5) = 2
>  ((4+5-1)/5) = 1

Logic was wrong but conclusion was right :) and so was Nishanth:

(a + b) + c = a + (b + c)

and

k   l   k + l
- + - = -----
m   m     m

thus:

(user_addr + iov[seg].iov_len + PAGE_SIZE - 1) = user_addr + (iov[seg].iov_len + PAGE_SIZE - 1)

and


user_addr + iov[seg].iov_len + PAGE_SIZE - 1   user_addr
-------------------------------------------- - --------- =
                 PAGE_SIZE                     PAGE_SIZE


user_addr   iov[seg].iov_len + PAGE_SIZE - 1   user_addr
--------- + -------------------------------- - --------- = 
PAGE_SIZE               PAGE_SIZE              PAGE_SIZE


iov[seg].iov_len + PAGE_SIZE - 1
--------------------------------
           PAGE_SIZE


or even more:


iov[seg].iov_len - 1
-------------------- + 1
     PAGE_SIZE


Regards,

	Mariusz

Re: [PATCH 2/6] fs/direct-io.c: Use DIV_ROUND_UP

[Nishanth Aravamudan]

On 14.02.2008 [23:16:44 +0100], Mariusz Kozlowski wrote:
> Hello,
> 
> > > >  Is it just me, or does
> > > >
> > > >         ((user_addr + iov[seg].iov_len + PAGE_SIZE - 1)/PAGE_SIZE - user_addr/PAGE_SIZE)
> > > >
> > > >  not simplify to
> > > >
> > > >         = ((iov[seg].iov_len + PAGE_SIZE - 1)/PAGE_SIZE + user_addr/PAGE_SIZE - user_addr/PAGE_SIZE)
> > > >
> > > >         = ((iov[seg].iov_len + PAGE_SIZE - 1)/PAGE_SIZE)
> > > >
> > > >         = DIV_ROUND_UP(iov[seg].iov_len, PAGE_SIZE)
> > > >
> > > >  CMIIW.
> > > 
> > > I double-checked this and I believe you're correct. It's simpler to
> > > see when you do:
> > > 
> > >   x = user_addr
> > >   y = iov[seg].iov_len
> > >   z = PAGE_SIZE
> > > 
> > > So
> > > 
> > >   (x + y + z - 1)/z - x/z
> > > 
> > >   = [x + (y + z - 1)]/z - x/z
> > > 
> > >   = [xz + z(y + z - 1)]/z^2 - x/z
> > > 
> > >   = x/z + (y + z - 1)/z - x/z
> > > 
> > > And the rest follows from your simplifications.
> > 
> > It doesn't work:
> > 
> >  ((3+4+5-1)/5) - (3/5) = 2
> >  ((4+5-1)/5) = 1
> 
> Logic was wrong but conclusion was right :) and so was Nishanth:

<snip>

As I replied to Julia off-list -- the math I and others have given is
correct, but assumes we're dealing with real-typed values. However, we
only have integer types in the kernel, meaning the divisions truncate.

Thus, while mathematically equivalent, the simplification is not
computationally so.

Thanks,
Nish

-- 
Nishanth Aravamudan <nacc@us.ibm.com>
IBM Linux Technology Center