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

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:

quoted

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.

quoted

(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