On 5/18/22 18:30, Hugh Dickins wrote:
> 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.
Thank you Hugh! You went above and beyond as usual.
> 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@xxxxxxx/#t
Sorry, that I did not find the entire thread. There were only a couple
pieces on linux-mm and that is the only place I looked.
>
> "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.
Yes, that is the likely reason. Functionality was not originally
supported, and when it was added this 'optimization' was not enabled.
> (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.)
Yes, I believe anon_vma means the same on hugetlb for this purpose.
Although, I do need to look closer just to make sure there are not
hidden surprises.
Thanks again,
--
Mike Kravetz
