Re: [PATCH v4 05/13] mm/arch: Provide pud_pfn() fallback

Peter Xu
2024-04-02T23:36:09Z

On Tue, Apr 02, 2024 at 07:53:20PM -0300, Jason Gunthorpe wrote:
> On Tue, Apr 02, 2024 at 06:43:56PM -0400, Peter Xu wrote:
>
> > I actually tested this without hitting the issue (even though I didn't
> > mention it in the cover letter..). I re-kicked the build test, it turns
> > out my "make alldefconfig" on loongarch will generate a config with both
> > HUGETLB=n && THP=n, while arch/loongarch/configs/loongson3_defconfig has
> > THP=y (which I assume was the one above build used). I didn't further
> > check how "make alldefconfig" generated the config; a bit surprising that
> > it didn't fetch from there.
>
> I suspect it is weird compiler variations.. Maybe something is not
> being inlined.
>
> > (and it also surprises me that this BUILD_BUG can trigger.. I used to try
> > triggering it elsewhere but failed..)
>
> As the pud_leaf() == FALSE should result in the BUILD_BUG never being
> called and the optimizer removing it.

Good point, for some reason loongarch defined pud_leaf() without defining
pud_pfn(), which does look strange.

#define pud_leaf(pud) ((pud_val(pud) & _PAGE_HUGE) != 0)

But I noticed at least MIPS also does it.. Logically I think one arch
should define either none of both.

>
> Perhaps the issue is that the pud_leaf() is too far from the pud_pfn?

My understanding is follow_pud_mask() should completely get optimized and
follow_huge_pud() will be dropped in the compiler output if pud_leaf()==false.

Thanks,

--
Peter Xu