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

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Message-ID: <c48b719.112d.196b84367d1.Coremail.00107082@163.com>
Date: Sat, 10 May 2025 11:35:47 +0800 (CST)
From: "David Wang" <00107082@....com>
To: "Tim Chen" <tim.c.chen@...ux.intel.com>
Cc: "Suren Baghdasaryan" <surenb@...gle.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


At 2025-05-10 03:46:06, "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, 
>then print_header will be initialized to true.
>Will there be subsequent calls of allocinfo_start() with *pos !=0,
>but priv->print_header stays at 0? 

Header only needs to be printed at line #0:
Normal seq_file operates;
    start(0)
    show(0)   <---print header here
    show(1)
    stop()
    start(2)   <----no need to print header
    show(2)
    show(3)
   ...

If start(0)/show(0) happened twice, two header would be printed; or if there is no start(0) nothing would be printed.
If those two scenario happens,  I think it is a bug in seq_file.

Thanks
David
 

>
>Tim
>> 
>> > 
>> >         } else {
>> >                 priv->print_header = false;
>> >         }
>> > 
>> > Tim
>> > 
>> > > +     return priv->iter.ct ? priv : NULL;
>> > >  }
>> > > 
>> >