Re: [PATCH] erofs: fix compacted_{4b_initial, 2b} when compacted_4b_initial > totalidx

From: Gao Xiang
Date: Mon Sep 13 2021 - 05:11:31 EST


On Mon, Sep 13, 2021 at 05:00:16PM +0800, Yue Hu wrote:
> On Mon, 13 Sep 2021 16:48:45 +0800
> Gao Xiang <hsiangkao@xxxxxxxxxxxxxxxxx> wrote:
>
> > Hi Yue,
> >
> > On Mon, Sep 13, 2021 at 03:24:05PM +0800, Yue Hu wrote:
> > > From: Yue Hu <huyue2@xxxxxxxxxx>
> > >
> > > mkfs.erofs will treat compacted_4b_initial & compacted_2b as 0 if
> > > compacted_4b_initial > totalidx, kernel should be aligned with it
> > > accordingly.
> >
> > There is no difference between compacted_4b_initial or compacted_4b_end
> > for compacted 4B. Since in this way totalidx for compact 2B won't larger
> > than 16 (number of lclusters in a compacted 2B pack.)
>
> However, we can see compacted_2b is a big number for this case. It should
> be pointless.

Does it has some real impact?

compacted_4b_initial is only used for the alignment use for the
first compacted_2b so that each compacted_2b pack won't cross
the block (page) boundary. And compacted_4b_end is for the last
lclusters aren't fitted in any compacted_2b pack.

If compacted_4b_initial > totalidx, I think the whole indexes
would be compacted 4B and handled in

if (lcn < compacted_4b_initial) {
amortizedshift = 2;
goto out;
}

Thanks,
Gao Xiang

>
> Thanks.
>
> >
> > So it can be handled in either compacted_4b_initial or compacted_4b_end
> > cases, because there are all compacted 4B.
> >
> > Thanks,
> > Gao Xiang
> >
> > >
> > > Signed-off-by: Yue Hu <huyue2@xxxxxxxxxx>
> > > ---
> > > fs/erofs/zmap.c | 5 ++++-
> > > 1 file changed, 4 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/fs/erofs/zmap.c b/fs/erofs/zmap.c
> > > index 9fb98d8..4f941b6 100644
> > > --- a/fs/erofs/zmap.c
> > > +++ b/fs/erofs/zmap.c
> > > @@ -369,7 +369,10 @@ static int compacted_load_cluster_from_disk(struct z_erofs_maprecorder *m,
> > > if (compacted_4b_initial == 32 / 4)
> > > compacted_4b_initial = 0;
> > >
> > > - if (vi->z_advise & Z_EROFS_ADVISE_COMPACTED_2B)
> > > + if (compacted_4b_initial > totalidx) {
> > > + compacted_4b_initial = 0;
> > > + compacted_2b = 0;
> > > + } else if (vi->z_advise & Z_EROFS_ADVISE_COMPACTED_2B)
> > > compacted_2b = rounddown(totalidx - compacted_4b_initial, 16);
> > > else
> > > compacted_2b = 0;
> > > --
> > > 1.9.1