Re: [PATCH v2 01/12] mm/debug_vm_pgtable: Introduce struct pgtable_debug_args

[Gavin Shan]

Hi Anshuman,

On 7/21/21 2:50 PM, Anshuman Khandual wrote:
> On 7/21/21 4:59 AM, Gavin Shan wrote:
> > On 7/20/21 4:42 PM, Anshuman Khandual wrote:
> > > On 7/19/21 6:31 PM, Gavin Shan wrote:
> > > > > +    if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) &&
> > > > > +        has_transparent_hugepage()) {
> > > > > +        page = alloc_pages(GFP_KERNEL, HPAGE_PMD_ORDER);
> > > > > +        if (page) {
> > > > > +            args->pmd_pfn = page_to_pfn(page);
> > > > > +            args->pte_pfn = args->pmd_pfn;
> > > > > +            return 0;
> > > > > +        }
> > > > > +    }
> > > > > +
> > > >
> > > > As syzbot reported against v1 series, we could allocate pages larger than (1 << (MAX_ORDER - 1)) here.
> > > > So __GFP_NOWARN is needed here. I will fix it in v3 series.
> > >
> > > I could find the following build error reported from lkp on V2.
> > >
> > > mm/debug_vm_pgtable.c:445:8: warning: variable 'pud' set but not used [-Wunused-but-set-variable]
> >
> > Yes, The following line is missed in PATCH[v2 09/12] and fixed in
> > PATCH[v3 09/12]: WARN_ON(!pud_none(pud)). With this line added,
> > the variable @pud is used in v3.
> >
> > > Could you please point to the syzbot reported problem on V1 as you
> > > have mentioned above. Are there configs where HPAGE_[PMD|PUD]_ORDER
> > > is greater than (MAX_ORDER - 1) ? If yes, how adding __GFP_NOWARN
> > > solves the problem ?
> >
> > https://syzkaller.appspot.com/bug?extid=8730ec44a441a434a2c8
> > https://syzkaller.appspot.com/x/.config?x=29a82c885e192046
> >
> > The kernel config has the following options:
> >
> > CONFIG_X86_64=y
> > CONFIG_TRANSPARENT_HUGEPAGE=y
> > CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD=y
> > #define PUD_SHIFT        30
> > #define PMD_SHIFT        21
> >
> > CONFIG_FORCE_MAX_ZONEORDER=n
> > #define MAX_ORDER    11
> >
> > (HPAGE_PUD_SHIFT - PAGE_SHIFT) >= (1 << MAX_ORDER)
> > (HPAGE_PMD_ORDER)              <  (1 << MAX_ORDER)
> >
> > The warning is triggered in the following path, __GFP_NOWARN helps to
> > avoid the WARNING_ON_ONCE(), but NULL is returned as expected.
> >
> >     alloc_pages
> >       __alloc_pages
> >
> >         if (unlikely(order >= MAX_ORDER)) {
> >                  WARN_ON_ONCE(!(gfp & __GFP_NOWARN));
> >                  return NULL;
> >          }
>
> But then that does not allocate the PUD element for the test which
> subsequently will be skipped. Isn't it ? So if the order is greater
> than MAX_ORDER, allocation needs to happen via alloc_contig_pages()
> or something similar.

Yes, the corresponding (modifying) tests will be skipped if we fail
to allocate the PUD huge page. And we need to use alloc_contig_pages()
when CONFIG_CONTIG_ALLOC is enabled. Otherwise, alloc_pages() is still
used as best-effort before we fail completely.

It's explained to you when we're discussion on PATCH[v3 01/12].

Thanks,
Gavin