Re: [PATCH v2 2/2] alloc_tag: keep codetag iterator active between read() calls

Suren Baghdasaryan
2025-05-10T04:03:21Z

On Fri, May 9, 2025 at 8:58 PM David Wang <00107082@163.com> wrote:
>
>
>
> At 2025-05-10 11:30:50, "Suren Baghdasaryan" <surenb@google.com> wrote:
> >On Fri, May 9, 2025 at 8:10 PM David Wang <00107082@163.com> wrote:
> >>
> >>
> >> At 2025-05-10 05:15:43, "Suren Baghdasaryan" <surenb@google.com> wrote:
> >> >On Fri, May 9, 2025 at 1:46 PM Suren Baghdasaryan <surenb@google.com> wrote:
> >> >>
> >> >> On Fri, May 9, 2025 at 12:46 PM Tim Chen <tim.c.chen@linux.intel.com> wrote:
> >> >> >
> >> >> > On Fri, 2025-05-09 at 12:36 -0700, Suren Baghdasaryan wrote:
> >> >> > > On Fri, May 9, 2025 at 11:33 AM Tim Chen <tim.c.chen@linux.intel.com> wrote:
> >> >> > > >
> >> >> > > > On Sat, 2025-05-10 at 01:39 +0800, David Wang wrote:
> >> >> > > > >
> >> >> > > > >
> >> >> > > > > Signed-off-by: David Wang <00107082@163.com>
> >> >> > >
> >> >> > > Acked-by: Suren Baghdasaryan <surenb@google.com>
> >> >> > >
> >> >> > > > > ---
> >> >> > > > > lib/alloc_tag.c | 29 ++++++++++-------------------
> >> >> > > > > 1 file changed, 10 insertions(+), 19 deletions(-)
> >> >> > > > >
> >> >> > > > > diff --git a/lib/alloc_tag.c b/lib/alloc_tag.c
> >> >> > > > > index 25ecc1334b67..fdd5887769a6 100644
> >> >> > > > > --- a/lib/alloc_tag.c
> >> >> > > > > +++ b/lib/alloc_tag.c
> >> >> > > > > @@ -45,21 +45,16 @@ struct allocinfo_private {
> >> >> > > > > static void *allocinfo_start(struct seq_file *m, loff_t *pos)
> >> >> > > > > {
> >> >> > > > > struct allocinfo_private *priv;
> >> >> > > > > - struct codetag *ct;
> >> >> > > > > loff_t node = *pos;
> >> >> > > > >
> >> >> > > > > - priv = kzalloc(sizeof(*priv), GFP_KERNEL);
> >> >> > > > > - m->private = priv;
> >> >> > > > > - if (!priv)
> >> >> > > > > - return NULL;
> >> >> > > > > -
> >> >> > > > > - priv->print_header = (node == 0);
> >> >> > > > > + priv = (struct allocinfo_private *)m->private;
> >> >> > > > > codetag_lock_module_list(alloc_tag_cttype, true);
> >> >> > > > > - priv->iter = codetag_get_ct_iter(alloc_tag_cttype);
> >> >> > > > > - while ((ct = codetag_next_ct(&priv->iter)) != NULL && node)
> >> >> > > > > - node--;
> >> >> > > > > -
> >> >> > > > > - return ct ? priv : NULL;
> >> >> > > > > + if (node == 0) {
> >> >> > > > > + priv->print_header = true;
> >> >> > > > > + priv->iter = codetag_get_ct_iter(alloc_tag_cttype);
> >> >> > > > > + codetag_next_ct(&priv->iter);
> >> >> > > > > + }
> >> >> > > >
> >> >> > > > Do you need to skip print header when *pos != 0? i.e add
> >> >> > >
> >> >> > > Technically not needed since proc_create_seq_private() allocates
> >> >> > > seq->private using kzalloc(), so the initial value of
> >> >> > > priv->print_header is always false.
> >> >> >
> >> >> > But we'll start with first call to allocinfo_start() with *pos == 0,
> >> >>
> >> >> Usually but not always if we do lseek() to a non-zero position beforehand.
> >> >
> >> >Actually, this change will break the lseek() case. We can't always
> >> >assume that we start reading from *pos == 0. Current patch will fail
> >> >to initialize priv if we start reading with *pos != 0.
> >> >priv->iter should be tracking current position and allocinfo_start()
> >> >should detect a mismatch between *pos and iter->pos and re-walk the
> >> >tags if there was a position change.
> >>
> >> seq_file works line by line, I think even if it support lseek, seq_file would still start with line #0,
> >> since seq_file have on clue the byte size for each line.
> >>
> >> I will check the code, make some tests and update later.
> >
> >Ah, yes. You are correct.
> >seq_lseek() will traverse restarting from 0:
> >https://elixir.bootlin.com/linux/v6.14.6/source/fs/seq_file.c#L323.
> >Position jumps are similarly handled with traversal from 0:
> >https://elixir.bootlin.com/linux/v6.14.6/source/fs/seq_file.c#L194.
> >
>
> Actually I was expecting EOPNOTSUPP when lseek on seq files, surprised to see it works...... :)
>
> If seq_file somehow skips start(0), then nothing would be displayed since
> priv->iter.ct would be 0 and `return priv->iter.ct ? priv : NULL;` would return NULL;
> But I think that case would be seq_file's bug, starting with 0 is kind of protocol promised by seq_file.

Yes, I think that is the correct assumption. Even pread() with a jump
in position should work the same way, restarting at 0.

>
>
> >>
>
> >>
> >> >
> >> >>
> >> >> > then print_header will be initialized to true.
> >> >>
> >> >> After the first call to allocinfo_show() print_header will be reset
> >> >> back to false.
> >> >>
> >> >> > Will there be subsequent calls of allocinfo_start() with *pos !=0,
> >> >> > but priv->print_header stays at 0?
> >> >>
> >> >> Yes, there will be subsequent calls to allocinfo_start() with *pos !=0
> >> >> and priv->print_header=false, which is what we want, right? We want to
> >> >> print the header only at the beginning of the file (node == 0).
> >> >>
> >> >> >
> >> >> > Tim
> >> >> > >
> >> >> > > >
> >> >> > > > } else {
> >> >> > > > priv->print_header = false;
> >> >> > > > }
> >> >> > > >
> >> >> > > > Tim
> >> >> > > >
> >> >> > > > > + return priv->iter.ct ? priv : NULL;
> >> >> > > > > }
> >> >> > > > >
> >> >> > > >
> >> >> >