Re: vma_needs_copy always true for VM_HUGETLB ?

[Hugh Dickins <hughd@google.com>]

On Wed, 18 May 2022, Mike Kravetz wrote:

> For most non-anonymous vmas, we do not copy page tables at fork time, but
> rather lazily populate the tables after fork via faults.  The routine
> vma_needs_copy() is used to make this decision. For VM_HUGETLB vmas, it always
> returns true.

"vma_needs_copy()" is *very* recent coinage, not reached Linus yet.

> 
> Anyone know/remember why?  The code was added more than 15 years ago and
> my search for why hugetlb vmas were excluded came up empty.
> 
> I do not see a reason why VM_HUGETLB is in this list.  Initial testing did
> not reveal any problems when I removed the VM_HUGETLB check.
> 
> FYI - I am looking at the performance of fork and exec (unmap) of processes
> with very large hugetlb mappings.  Skipping the copy at fork time would
> certainly speed things up.  Of course, there could some users who would
> notice if hugetlb page tables are not copied at fork time.  However, this
> is the behavior for 'normal' mappings.  I am inclined to make hugetlb be
> 'more normal'.

Good question, not obvious to me either: but I've found the answer.

The commit was of course Nick's d992895ba2b2 ("[PATCH] Lazy page table
copies in fork()") in 2.6.14; but it doesn't explain why VM_HUGETLB is
there in the test, and goes on to be copied.

I haven't re-read through the whole mail thread which led to that
commit, but I think you'll find the crucial observation comes from
Andi in https://lore.kernel.org/lkml/200508251756.07849.ak@suse.de/#t

"Actually I disabled it for hugetlbfs (... !is_huge...vma). The reason 
is that lazy faulting for huge pages is still not in mainline."

and indeed, look at the 2.6.13 or 2.6.14 mm/hugetlb.c and you find
/*
 * We cannot handle pagefaults against hugetlb pages at all.  They cause
 * handle_mm_fault() to try to instantiate regular-sized pages in the
 * hugegpage VMA.  do_page_fault() is supposed to trap this, so BUG is we get
 * this far.
 */
static struct page *hugetlb_nopage(struct vm_area_struct *vma,
				unsigned long address, int *unused)
{
	BUG();
	return NULL;
}

Oh, and that pretty much still exists to this day, to cover that path
to a fault; but 2.6.16 implemented hugetlb_no_page(), which is what
then actually got used to satisfy a hugetlb fault.

So the reason for fork copying VM_HUGETLB appears to have gone away
in 2.6.16.

(I haven't a clue on private hugetlb mappings and reservations and
whether anon_vma means the same on hugetlb, but you know all that.)

Hugh