Re: [PATCH V4 3/4] mm/sparse-vmemmap: Generalise vmemmap_populate_hugepages()

[Will Deacon <will@kernel.org>]

On Thu, Jul 21, 2022 at 10:08:10AM +0800, Huacai Chen wrote:
> On Wed, Jul 20, 2022 at 5:34 PM David Hildenbrand <david@redhat.com> wrote:
> > On 14.07.22 14:34, Huacai Chen wrote:
> > > On Fri, Jul 8, 2022 at 5:47 PM Huacai Chen <chenhuacai@kernel.org> wrote:
> > >> On Thu, Jul 7, 2022 at 12:17 AM Will Deacon <will@kernel.org> wrote:
> > >>> On Tue, Jul 05, 2022 at 09:07:59PM +0800, Huacai Chen wrote:
> > >>>> On Tue, Jul 5, 2022 at 5:29 PM Will Deacon <will@kernel.org> wrote:
> > >>>>> On Mon, Jul 04, 2022 at 07:25:25PM +0800, Huacai Chen wrote:
> > >>>>>> +int __meminit vmemmap_populate_hugepages(unsigned long start, unsigned long end,
> > >>>>>> +                                      int node, struct vmem_altmap *altmap)
> > >>>>>> +{
> > >>>>>> +     unsigned long addr;
> > >>>>>> +     unsigned long next;
> > >>>>>> +     pgd_t *pgd;
> > >>>>>> +     p4d_t *p4d;
> > >>>>>> +     pud_t *pud;
> > >>>>>> +     pmd_t *pmd;
> > >>>>>> +
> > >>>>>> +     for (addr = start; addr < end; addr = next) {
> > >>>>>> +             next = pmd_addr_end(addr, end);
> > >>>>>> +
> > >>>>>> +             pgd = vmemmap_pgd_populate(addr, node);
> > >>>>>> +             if (!pgd)
> > >>>>>> +                     return -ENOMEM;
> > >>>>>> +
> > >>>>>> +             p4d = vmemmap_p4d_populate(pgd, addr, node);
> > >>>>>> +             if (!p4d)
> > >>>>>> +                     return -ENOMEM;
> > >>>>>> +
> > >>>>>> +             pud = vmemmap_pud_populate(p4d, addr, node);
> > >>>>>> +             if (!pud)
> > >>>>>> +                     return -ENOMEM;
> > >>>>>> +
> > >>>>>> +             pmd = pmd_offset(pud, addr);
> > >>>>>> +             if (pmd_none(READ_ONCE(*pmd))) {
> > >>>>>> +                     void *p;
> > >>>>>> +
> > >>>>>> +                     p = vmemmap_alloc_block_buf(PMD_SIZE, node, altmap);
> > >>>>>> +                     if (p) {
> > >>>>>> +                             vmemmap_set_pmd(pmd, p, node, addr, next);
> > >>>>>> +                             continue;
> > >>>>>> +                     } else if (altmap)
> > >>>>>> +                             return -ENOMEM; /* no fallback */
> > >>>>>
> > >>>>> Why do you return -ENOMEM if 'altmap' here? That seems to be different to
> > >>>>> what we currently have on arm64 and it's not clear to me why we're happy
> > >>>>> with an altmap for the pmd case, but not for the pte case.
> > >>>> The generic version is the same as X86. It seems that ARM64 always
> > >>>> fallback whether there is an altmap, but X86 only fallback in the no
> > >>>> altmap case. I don't know the reason of X86, can Dan Williams give
> > >>>> some explaination?
> > >>>
> > >>> Right, I think we need to understand the new behaviour here before we adopt
> > >>> it on arm64.
> > >> Hi, Dan,
> > >> Could you please tell us the reason? Thanks.
> > >>
> > >> And Sudarshan,
> > >> You are the author of adding a fallback mechanism to ARM64,  do you
> > >> know why ARM64 is different from X86 (only fallback in no altmap
> > >> case)?
> >
> > I think that's a purely theoretical issue: I assume that in any case we
> > care about, the altmap should be reasonably sized and aligned such that
> > this will always succeed.
> >
> > To me it even sounds like the best idea to *consistently* fail if there
> > is no more space in the altmap, even if we'd have to fallback to PTE
> > (again, highly unlikely that this is relevant in practice). Could
> > indicate an altmap-size configuration issue.
> 
> Does David's explanation make things clear? Moreover, I think Dan's
> dedicated comments "no fallback" implies that his design is carefully
> considered. So I think the generic version using the X86 logic is just
> OK.

I think the comment isn't worth the metaphorical paper that it's written
on! If you can bulk it up a bit based on David's reasoning, then that would
help. But yes, I'm happy with the code now, thanks both.

Will