Re: [PATCH] ovl: copy-up optimization for truncate

[Miklos Szeredi <miklos@szeredi.hu>]

On Thu, Apr 1, 2021 at 1:15 PM Chengguang Xu <cgxu519@mykernel.net> wrote:
>
>  ---- 在 星期一, 2021-03-29 23:13:52 Miklos Szeredi <miklos@szeredi.hu> 撰写 ----
>  > On Mon, Mar 8, 2021 at 12:17 PM Chengguang Xu <cgxu519@mykernel.net> wrote:
>  > >
>  > > Currently copy-up will copy whole lower file to upper
>  > > regardless of the data range which is needed for further
>  > > operation. This patch avoids unnecessary copy when truncate
>  > > size is smaller than the file size.
>  >
>  > This doesn't look right.   If copy up succeeds, resulting in a
>  > truncated file, then we should return success there and then.   Doing
>  > the truncate again and failing (unlikely, but I wouldn't think it
>  > impossible) wouldn't be nice.
>
> Hi Miklos
>
> I noticed a problem here, if we just return success after copy-up then mtime
> keeps the same as lower file. I think doing the truncate again would be better
> than manually updating the upper file's mtime. What do you think for this?

Let's simplify instead:  skip the mtime restore on copy-up.   Not sure
how that's handled on O_TRUNC opens, maybe it's relevant to that case
too.

Thanks,
Miklos