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

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Message-ID: <CAJuCfpF=u-LpR6S+XmwPe8a6h4knzP2Nu5WFp=Rdvqa14vOzDA@mail.gmail.com>
Date: Fri, 9 May 2025 13:46:34 -0700
From: Suren Baghdasaryan <surenb@...gle.com>
To: Tim Chen <tim.c.chen@...ux.intel.com>
Cc: David Wang <00107082@....com>, kent.overstreet@...ux.dev, akpm@...ux-foundation.org, 
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v2 2/2] alloc_tag: keep codetag iterator active between
 read() calls

On Fri, May 9, 2025 at 12:46 PM Tim Chen <tim.c.chen@...ux.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@...ux.intel.com> wrote:
> > >
> > > On Sat, 2025-05-10 at 01:39 +0800, David Wang wrote:
> > > >
> > > >
> > > > Signed-off-by: David Wang <00107082@....com>
> >
> > Acked-by: Suren Baghdasaryan <surenb@...gle.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.

> 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;
> > > >  }
> > > >
> > >
>