Re: [PATCH v4 3/3] vsprintf: dump full information of page flags in pGp

[Petr Mladek]

On Wed 2021-02-10 00:21:37, Yafang Shao wrote:
> On Tue, Feb 9, 2021 at 9:53 PM Petr Mladek <pmladek@xxxxxxxx> wrote:
> >
> > On Tue 2021-02-09 18:56:13, Yafang Shao wrote:
> > > Currently the pGp only shows the names of page flags, rather than
> > > the full information including section, node, zone, last cpupid and
> > > kasan tag. While it is not easy to parse these information manually
> > > because there're so many flavors. Let's interpret them in pGp as well.
> > >
> > > To be compitable with the existed format of pGp, the new introduced ones
> > > also use '|' as the separator, then the user tools parsing pGp won't
> > > need to make change, suggested by Matthew. The new information is
> > > tracked onto the end of the existed one.
> > >
> > > On example of the output in mm/slub.c as follows,
> > > - Before the patch,
> > > [ 6343.396602] Slab 0x000000004382e02b objects=33 used=3 fp=0x000000009ae06ffc flags=0x17ffffc0010200(slab|head)
> > >
> > > - After the patch,
> > > [ 8838.835456] Slab 0x000000002828b78a objects=33 used=3 fp=0x00000000d04efc88 flags=0x17ffffc0010200(slab|head|node=0|zone=2|lastcpupid=0x1fffff)
> > >
> > > The documentation and test cases are also updated. The output of the
> > > test cases as follows,
> > > [  501.485081] test_printf: loaded.
> > > [  501.485768] test_printf: all 388 tests passed
> > > [  501.488762] test_printf: unloaded.
> > >
> >
> > > diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> > > index 14c9a6af1b23..3f26611adb34 100644
> > > --- a/lib/vsprintf.c
> > > +++ b/lib/vsprintf.c
> > > @@ -1916,6 +1916,66 @@ char *format_flags(char *buf, char *end, unsigned long flags,
> > >       return buf;
> > >  }
> > >
> > > +struct page_flags_layout {
> > > +     int width;
> > > +     int shift;
> > > +     int mask;
> > > +     const struct printf_spec *spec;
> > > +     const char *name;
> > > +};
> > > +
> > > +static const struct page_flags_layout pfl[] = {
> > > +     {SECTIONS_WIDTH, SECTIONS_PGSHIFT, SECTIONS_MASK,
> > > +      &default_dec_spec, "section"},
> > > +     {NODES_WIDTH, NODES_PGSHIFT, NODES_MASK,
> > > +      &default_dec_spec, "node"},
> > > +     {ZONES_WIDTH, ZONES_PGSHIFT, ZONES_MASK,
> > > +      &default_dec_spec, "zone"},
> > > +     {LAST_CPUPID_WIDTH, LAST_CPUPID_PGSHIFT, LAST_CPUPID_MASK,
> > > +      &default_flag_spec, "lastcpupid"},
> > > +     {KASAN_TAG_WIDTH, KASAN_TAG_PGSHIFT, KASAN_TAG_MASK,
> > > +      &default_flag_spec, "kasantag"},
> > > +};
> > > +
> > > +static
> > > +char *format_page_flags(char *buf, char *end, unsigned long flags)
> > > +{
> > > +     DECLARE_BITMAP(mask, ARRAY_SIZE(pfl));
> > > +     unsigned long last;
> > > +     int i;
> > > +
> > > +     if (flags & (BIT(NR_PAGEFLAGS) - 1)) {
> > > +             if (buf < end)
> > > +                     *buf = '|';
> > > +             buf++;
> > > +     }
> >
> > This is far from obvious. You print '|' here because you printed
> > something somewhere else. See below.
> >
> > > +
> > > +     for (i = 0; i < ARRAY_SIZE(pfl); i++)
> > > +             __assign_bit(i, mask, pfl[i].width);
> >
> > The bitmap looks like an overkill. If I get it correctly, it is a
> > tricky way to handle only flags defined by the used build
> > configuration. See below.
> >
> > > +     last = find_last_bit(mask, ARRAY_SIZE(pfl));
> > > +
> > > +     for_each_set_bit(i, mask, ARRAY_SIZE(pfl)) {
> > > +             /* Format: Flag Name + '=' (equals sign) + Number + '|' (separator) */
> > > +             buf = string(buf, end, pfl[i].name, *pfl[i].spec);
> > > +
> > > +             if (buf < end)
> > > +                     *buf = '=';
> > > +             buf++;
> > > +             buf = number(buf, end, (flags >> pfl[i].shift) & pfl[i].mask,
> > > +                          *pfl[i].spec);
> > > +
> > > +             /* No separator for the last entry */
> > > +             if (i != last) {
> > > +                     if (buf < end)
> > > +                             *buf = '|';
> > > +                     buf++;
> > > +             }
> > > +     }
> > > +
> > > +     return buf;
> > > +}
> > > +
> > >  static noinline_for_stack
> > >  char *flags_string(char *buf, char *end, void *flags_ptr,
> > >                  struct printf_spec spec, const char *fmt)
> > > @@ -1929,10 +1989,10 @@ char *flags_string(char *buf, char *end, void *flags_ptr,
> > >       switch (fmt[1]) {
> > >       case 'p':
> > >               flags = *(unsigned long *)flags_ptr;
> > > -             /* Remove zone id */
> > > -             flags &= (1UL << NR_PAGEFLAGS) - 1;
> > >               names = pageflag_names;
> >
> > The "names" variable is needed only with "break;" when using the final
> > format_flags(buf, end, flags, names);
> >
> > > -             break;
> > > +             buf = format_flags(buf, end, flags & (BIT(NR_PAGEFLAGS) - 1), names);
> > > +             buf = format_page_flags(buf, end, flags);
> >
> > I am sorry for my ignorance. I am not familiar with MM.
> > But it is pretty hard to understand what call does what.
> >
> > I have found the following comment in include/linux/page_flags.h:
> >
> >  * The page flags field is split into two parts, the main flags area
> >  * which extends from the low bits upwards, and the fields area which
> >  * extends from the high bits downwards.
> >
> > Sigh, I know that you already reworked this several times because
> > people "nitpicked" about the code style. But it seems that it
> > rather diverged instead of converged.
> >
> > What about the following?
> >
> > Note: It is inpired by the names "main area" and "fields area"
> >       mentioned in the above comment from page_flags.h.
> >       I have later realized that "page_flags_layout" actually made
> >       sense as well. Feel free to rename page_flags_fileds
> >       back to page_flags_layout.
> >
> > Anyway, this is my proposal:
> >
> 
> This proposal is similar to v2.
> I don't mind changing it back with your additional better naming.

Great.

> By the way, it will be better to make a little change per Joe's
> suggestion on v2 that using a pointer instead of the index, for
> example,
> 
>  for (p = pff; p < pff + ARRAY_SIZE(pff); p++) {

This looks a bit non-standard. IMHO, Joe was not against using index.
He proposed:

	for (i = 0; i < ARRAY_SIZE(pfl) && buf < end; i++) {

, see
https://lore.kernel.org/lkml/e5ea9e8b1190c2a397a1b84dd55bb9c706dc7058.camel@xxxxxxxxxxx/

I am not sure about the (buf < end) check. It might be some
optimization or it did fit the the old code.

Anyway, I like the currently used:

	for (i = 0; i < ARRAY_SIZE(pff); i++) {

It is standard, easy to understand, and thus more safe. I am sure that
compiler will optimize it very well.

Best Regards,
Petr