Re: [PATCH] fs: ramfs: make chunk counting in truncate() better readable

[Sascha Hauer <s.hauer@pengutronix.de>]

On Wed, Sep 26, 2018 at 01:22:28PM +0200, Marcin Niestrój wrote:
> 
> I think I was a little bit too early with review :) Below I have some
> comments.
> 
> Marcin Niestrój <m.niestroj@grinn-global.com> writes:
> 
> > Sascha Hauer <s.hauer@pengutronix.de> writes:
> >
> >> In ramfs_truncate() "newchunks" denotes the number of chunks we
> >> want to have after the call. We decrease that number while iterating
> >> over the existing chunks and decrease it further with every newly
> >> allocated chunk until "newchunks" is zero.
> >> This is a bit hard to read. Instead we drop the decreasing while
> >> iterating over existing chunks and increase "oldchunks" while allocating
> >> until it reaches "newchunks".
> >>
> >> This is mainly done to make the next patch easier.
> >>
> >> Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
> >> ---
> >>  fs/ramfs.c | 9 ++++-----
> >>  1 file changed, 4 insertions(+), 5 deletions(-)
> >>
> >> diff --git a/fs/ramfs.c b/fs/ramfs.c
> >> index 09dafe02ae..8ba8d77de9 100644
> >> --- a/fs/ramfs.c
> >> +++ b/fs/ramfs.c
> >> @@ -384,19 +384,18 @@ static int ramfs_truncate(struct device_d *dev, FILE *f, ulong size)
> >>  			if (!node->data)
> >>  				return -ENOMEM;
> >>  			data = node->data;
> >> +			newchunks = 1;
> 
> What is the reason of this instruction? What if 'size' == 16384 and we
> do it on freshly opened file (with truncate(fd, 16384)? 'newchunk'
> should be 2 in that case, or not?

Yes, you're right. It should be "oldchunks = 1" instead. Then we have:

>		if (!data) {
>			node->data = ramfs_get_chunk();
>			if (!node->data)
>				return -ENOMEM;
>			data = node->data;
>			oldchunks = 1;
>		}

!data we have no chunks allocated. We allocate one and set oldchunks to one. When we
do:

>		while (newchunks > oldchunks) {
>			data->next = ramfs_get_chunk();
>			if (!data->next)
>				return -ENOMEM;
>			data = data->next;
>			oldchunks++;
>		}

In your example above we execute this loop once, allocate the second chunk, oldchunks
will become two which is the same as newchunks and then we go out.

I hope this is correct now ;)

Sascha

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