Re: [PATCH] pagemap: fix wrong KPF_THP on slab pages

[Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>]

On Wed, Sep 26, 2012 at 03:38:41PM +0800, Fengguang Wu wrote:
> On Wed, Sep 26, 2012 at 02:06:08AM -0400, Naoya Horiguchi wrote:
> > On Wed, Sep 26, 2012 at 12:02:34AM -0400, Naoya Horiguchi wrote:
> > ...
> > > > > +	 * page is a thp, not a non-huge compound page.
> > > > > +	 */
> > > > > +	else if (PageTransCompound(page) && !PageSlab(page))
> > > > >  		u |= 1 << KPF_THP;
> > > > 
> > > > Good catch!
> > > > 
> > > > Will this report THP for the various drivers that do __GFP_COMP
> > > > page allocations?
> > > 
> > > I'm afraid it will. I think of checking PageLRU as an alternative,
> > > but it needs compound_head() to report tail pages correctly.
> > > In this context, pages are not pinned or locked, so it's unsafe to
> > > use compound_head() because it can return a dangling pointer.
> > > Maybe it's a thp's/hugetlbfs's (not kpageflags specific) problem,
> > > so going forward with compound_head() expecting that it will be
> > > fixed in the future work can be an option.
> > 
> > It seems that compound_trans_head() solves this problem, so I'll
> > simply use it.
> 
> Naoya, in fact I didn't quite catch your concerns. Why not just test
> 
>         PageTransCompound(page) && PageLRU(page)

If we simply check PageLRU, tail pages in thp only show KPF_COMPOUND_TAIL
and we can't distinguish them from tail pages in non-huge compound pages.

Moreover this behavior is not consistent with that of hugetlbfs tail
pages where tail pages also have KPF_HUGE and are distinct from non-huge
compound pages. I show the output of page-types:

  offset  len     flags
  ...
  2d400   1       ___U_lA____Ma_bH______t____________ (thp head)
  2d401   1ff     ________________T__________________ (thp tail) # no KPF_THP
  ...
  77000   1       ___U_______Ma__H_G_________________ (hugetlbfs head)
  77001   1ff     ________________TG_________________ (hugetlbfs tail)
  ...
  11fb50  1       _______________H___________________ (compound head)
  11fb51  3       ________________T__________________ (compound tail)
  ...
  11fb58  1       _______S_______H___________________ (slab head)
  11fb59  7       ________________T__________________ (slab tail)

    H: KPF_COMPOUND_HEAD   T: KPF_COMPOUND_TAIL
    G: KPF_HUGE            t: KPF_THP

So I think it's better to set KPF_THP on thp tail pages.
  
> The whole page flag report thing is inherently racy and it's fine to
> report wrong values due to races. The "__GFP_COMP reported as THP",
> however, should be avoided because it will make consistent wrong
> reporting of page flags.

Yes, I agree with this point.

Thanks,
Naoya

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