On Tue, Oct 15, 2019 at 2:38 PM Chengguang Xu <cgxu519@xxxxxxxxxxxx> wrote:
>
> ---- 在 星期五, 2019-10-04 21:20:30 Chengguang Xu <cgxu519@xxxxxxxxxxxx> 撰写 ----
> > Current copy-up is not efficient for sparse file,
> > It's not only slow but also wasting more disk space
> > when the target lower file has huge hole inside.
> > This patch tries to recognize file hole and skip it
> > during copy-up.
> >
> > In detail, this optimization checks the hole according
> > to copy-up chunk size so it may not recognize all kind
> > of holes in the file. However, it is easy to implement
> > and will be enough for most of the time.
I must say I do not see how aligning to copy-up chunk size
simplifies the change. Why is that more complicated?
if (old_next_data_pos >= old_pos) {
hole_len = old_next_data_pos - old_pos;
...
It can still copy hole up to this_len, because there is no
SEEK_HOLE, so you can document that.
> >
> > Additionally, this optimization relies on lseek(2)
> > SEEK_DATA implementation, so for some specific
> > filesystems which do not support this feature
> > will behave as before on copy-up.
> >
I am not sure if we support any lower fs with no f_op->llseek
in that case, copy up will not behave as before - it will return
-ESPIPE and will be a regression.
> > Signed-off-by: Chengguang Xu <cgxu519@xxxxxxxxxxxx>
> > ---
>
> Any better idea or suggestion for this?
This change should be accompanied with proper xfstests examining
all sorts of sparse files.
See overlay/001 and _run_seek_sanity_test for inspiration.
Perhaps you can run all _run_seek_sanity_test tests on
lower fs, then mount overlay. copy up all the sanity test
files and then check something???
Thanks,
Amir.
>
>
> > fs/overlayfs/copy_up.c | 15 ++++++++++++++-
> > 1 file changed, 14 insertions(+), 1 deletion(-)
> >
> > diff --git a/fs/overlayfs/copy_up.c b/fs/overlayfs/copy_up.c
> > index b801c6353100..028033c9f021 100644
> > --- a/fs/overlayfs/copy_up.c
> > +++ b/fs/overlayfs/copy_up.c
> > @@ -144,10 +144,11 @@ static int ovl_copy_up_data(struct path *old, struct path *new, loff_t len)
> > goto out;
> > /* Couldn't clone, so now we try to copy the data */
> >
> > - /* FIXME: copy up sparse files efficiently */
> > while (len) {
> > size_t this_len = OVL_COPY_UP_CHUNK_SIZE;
> > long bytes;
> > + loff_t old_next_data_pos;
> > + loff_t hole_len;
> >
> > if (len < this_len)
> > this_len = len;
> > @@ -157,6 +158,18 @@ static int ovl_copy_up_data(struct path *old, struct path *new, loff_t len)
> > break;
> > }
> >
> > + old_next_data_pos = vfs_llseek(old_file, old_pos, SEEK_DATA);
> > + if (old_next_data_pos >= old_pos + OVL_COPY_UP_CHUNK_SIZE) {
> > + hole_len = (old_next_data_pos - old_pos) /
> > + OVL_COPY_UP_CHUNK_SIZE * OVL_COPY_UP_CHUNK_SIZE;
> > + old_pos += hole_len;
> > + new_pos += hole_len;
> > + len -= hole_len;
> > + continue;
> > + } else if (old_next_data_pos == -ENXIO) {
> > + break;
> > + }
> > +
> > bytes = do_splice_direct(old_file, &old_pos,
> > new_file, &new_pos,
> > this_len, SPLICE_F_MOVE);
> > --
