linux-kernel - Re: [RFC v2 3/3] mm, printk: introduce new format %pGt for page

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Date:   Mon, 7 Nov 2022 15:20:34 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     Hyeonggon Yoo <42.hyeyoo@...il.com>, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Naoya Horiguchi <naoya.horiguchi@....com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Matthew Wilcox <willy@...radead.org>,
        Minchan Kim <minchan@...nel.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Hugh Dickins <hughd@...gle.com>,
        Muchun Song <songmuchun@...edance.com>,
        David Hildenbrand <david@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Jonathan Corbet <corbet@....net>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC v2 3/3] mm, printk: introduce new format %pGt for page_type

On Mon 2022-11-07 13:18:52, Andy Shevchenko wrote:
> On Sun, Nov 06, 2022 at 11:03:55PM +0900, Hyeonggon Yoo wrote:
> > dump_page() uses %pGp format to print 'flags' field of struct page.
> > As some page flags (e.g. PG_buddy, see page-flags.h for more details)
> > are set in page_type field, introduce %pGt format which provides
> > human readable output of page_type. And use it in dump_page().
> > 
> > Note that the sense of bits are different in page_type. if page_type is
> > 0xffffffff, no flags are set. if PG_slab (0x00100000) flag is set,
> > page_type is 0xffefffff. Clearing a bit means we set the bit. Bits in
> > page_type are inverted when printing type names.
> > 
> > Below is examples of dump_page(). One is just after alloc_pages() and
> > the other is after __SetPageSlab().
> > 
> > [    1.814728] page:ffffea000415e200 refcount:1 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x105788
> > [    1.815961] flags: 0x17ffffc0000000(node=0|zone=2|lastcpupid=0x1fffff)
> 
> > [    1.816443] page_type: 0xffffffff()
> 
> Why do we have empty parentheses? I would expect either something there, or no
> parentheses at all.

This looks fine. format_page_flags() does the same for %pGp.

> ...
> 
> > +	%pGt	0xffefffff(slab)
> 
> No space before ( ?

Also looks fine. %pGp does the same.

> ...
> 
> > +static
> > +char *format_page_type(char *buf, char *end, unsigned int page_type)
> > +{
> > +	if (!(page_type & PAGE_TYPE_BASE))
> > +		return string(buf, end, "no type for user-mapped page", default_str_spec);
> 
> It's too long, can we make it shorten?

I wonder if it would help to write the value. Something like:

      page_type: 0x0ace5768(no type)

That said. I am not familiar with the page types and am not sure
about the semantic of this value. MM people should decide what they
want to see in this case.

> ...
> 
> >  	pr_warn("%sflags: %pGp%s\n", type, &head->flags,
> >  		page_cma ? " CMA" : "");
> > +	pr_warn("page_type: %pGt\n", &head->page_type);
> > +
> >  	print_hex_dump(KERN_WARNING, "raw: ", DUMP_PREFIX_NONE, 32,
> >  			sizeof(unsigned long), page,
> >  			sizeof(struct page), false);
> > diff --git a/mm/internal.h b/mm/internal.h
> > index cb4c663a714e..956eaa9f12c0 100644
> > --- a/mm/internal.h
> > +++ b/mm/internal.h
> > @@ -773,6 +773,7 @@ static inline void flush_tlb_batched_pending(struct mm_struct *mm)
> >  #endif /* CONFIG_ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH */
> >  
> >  extern const struct trace_print_flags pageflag_names[];
> > +extern const struct trace_print_flags pagetype_names[];
> >  extern const struct trace_print_flags vmaflag_names[];
> >  extern const struct trace_print_flags gfpflag_names[];
> 
> I would split this to a separate change, but it's up to PRINTK maintainers.

I guess that you are talking about the line:

+	pr_warn("page_type: %pGt\n", &head->page_type);


Yes, it would be better to have implementation of %pGt modifier
in one patch and add the user in another one.

Best Regards,
Petr