Re: [RFC PATCH 02/26] hugetlb: sort hstates in hugetlb_init_hstates

[Muchun Song <songmuchun@bytedance.com>]

On Tue, Jun 28, 2022 at 08:40:27AM -0700, James Houghton wrote:
> On Mon, Jun 27, 2022 at 11:42 AM Mike Kravetz <mike.kravetz@oracle.com> wrote:
> >
> > On 06/24/22 17:36, James Houghton wrote:
> > > When using HugeTLB high-granularity mapping, we need to go through the
> > > supported hugepage sizes in decreasing order so that we pick the largest
> > > size that works. Consider the case where we're faulting in a 1G hugepage
> > > for the first time: we want hugetlb_fault/hugetlb_no_page to map it with
> > > a PUD. By going through the sizes in decreasing order, we will find that
> > > PUD_SIZE works before finding out that PMD_SIZE or PAGE_SIZE work too.
> > >
> > > Signed-off-by: James Houghton <jthoughton@google.com>
> > > ---
> > >  mm/hugetlb.c | 40 +++++++++++++++++++++++++++++++++++++---
> > >  1 file changed, 37 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > > index a57e1be41401..5df838d86f32 100644
> > > --- a/mm/hugetlb.c
> > > +++ b/mm/hugetlb.c
> > > @@ -33,6 +33,7 @@
> > >  #include <linux/migrate.h>
> > >  #include <linux/nospec.h>
> > >  #include <linux/delayacct.h>
> > > +#include <linux/sort.h>
> > >
> > >  #include <asm/page.h>
> > >  #include <asm/pgalloc.h>
> > > @@ -48,6 +49,10 @@
> > >
> > >  int hugetlb_max_hstate __read_mostly;
> > >  unsigned int default_hstate_idx;
> > > +/*
> > > + * After hugetlb_init_hstates is called, hstates will be sorted from largest
> > > + * to smallest.
> > > + */
> > >  struct hstate hstates[HUGE_MAX_HSTATE];
> > >
> > >  #ifdef CONFIG_CMA
> > > @@ -3144,14 +3149,43 @@ static void __init hugetlb_hstate_alloc_pages(struct hstate *h)
> > >       kfree(node_alloc_noretry);
> > >  }
> > >
> > > +static int compare_hstates_decreasing(const void *a, const void *b)
> > > +{
> > > +     const int shift_a = huge_page_shift((const struct hstate *)a);
> > > +     const int shift_b = huge_page_shift((const struct hstate *)b);
> > > +
> > > +     if (shift_a < shift_b)
> > > +             return 1;
> > > +     if (shift_a > shift_b)
> > > +             return -1;
> > > +     return 0;
> > > +}
> > > +
> > > +static void sort_hstates(void)
> > > +{
> > > +     unsigned long default_hstate_sz = huge_page_size(&default_hstate);
> > > +
> > > +     /* Sort from largest to smallest. */
> > > +     sort(hstates, hugetlb_max_hstate, sizeof(*hstates),
> > > +          compare_hstates_decreasing, NULL);
> > > +
> > > +     /*
> > > +      * We may have changed the location of the default hstate, so we need to
> > > +      * update it.
> > > +      */
> > > +     default_hstate_idx = hstate_index(size_to_hstate(default_hstate_sz));
> > > +}
> > > +
> > >  static void __init hugetlb_init_hstates(void)
> > >  {
> > >       struct hstate *h, *h2;
> > >
> > > -     for_each_hstate(h) {
> > > -             if (minimum_order > huge_page_order(h))
> > > -                     minimum_order = huge_page_order(h);
> > > +     sort_hstates();
> > >
> > > +     /* The last hstate is now the smallest. */
> > > +     minimum_order = huge_page_order(&hstates[hugetlb_max_hstate - 1]);
> > > +
> > > +     for_each_hstate(h) {
> > >               /* oversize hugepages were init'ed in early boot */
> > >               if (!hstate_is_gigantic(h))
> > >                       hugetlb_hstate_alloc_pages(h);
> >
> > This may/will cause problems for gigantic hugetlb pages allocated at boot
> > time.  See alloc_bootmem_huge_page() where a pointer to the associated hstate
> > is encoded within the allocated hugetlb page.  These pages are added to
> > hugetlb pools by the routine gather_bootmem_prealloc() which uses the saved
> > hstate to add prep the gigantic page and add to the correct pool.  Currently,
> > gather_bootmem_prealloc is called after hugetlb_init_hstates.  So, changing
> > hstate order will cause errors.
> >
> > I do not see any reason why we could not call gather_bootmem_prealloc before
> > hugetlb_init_hstates to avoid this issue.
> 
> Thanks for catching this, Mike. Your suggestion certainly seems to
> work, but it also seems kind of error prone. I'll have to look at the
> code more closely, but maybe it would be better if I just maintained a
> separate `struct hstate *sorted_hstate_ptrs[]`, where the original

I don't think this is a good idea.  If you really rely on the order of
the initialization in this patch.  The easier solution is changing
huge_bootmem_page->hstate to huge_bootmem_page->hugepagesz. Then we
can use size_to_hstate(huge_bootmem_page->hugepagesz) in
gather_bootmem_prealloc().

Thanks.

> locations of the hstates remain unchanged, as to not break
> gather_bootmem_prealloc/other things.
> 
> > --
> > Mike Kravetz
>