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SubjectRe: process creation time increases linearly with shmem


On Fri, 26 Aug 2005, Hugh Dickins wrote:
>
> I see some flaws in the various patches posted, including Rik's.
> Here's another version - doing it inside copy_page_range, so this
> kind of vma special-casing is over in mm/ rather than kernel/.

I like this approach better, but I don't understand your particular
choice of bits.

> + * Assume the fork will probably exec: don't waste time copying
> + * ptes where a page fault will fill them correctly afterwards.
> + */
> + if ((vma->vm_flags & (VM_MAYSHARE|VM_HUGETLB|VM_NONLINEAR|VM_RESERVED))
> + == VM_MAYSHARE)
> + return 0;
> +
> if (is_vm_hugetlb_page(vma))
> return copy_hugetlb_page_range(dst_mm, src_mm, vma);

First off, if you just did it below the hugetlb check, you'd not need to
check hugetlb again. And while I understand VM_NONLINEAR and VM_RESERVED,
can you please comment on why VM_MAYSHARE is so important, and why no
other information matters.

Now, VM_MAYSHARE is a sign of the mapping being a shared mapping. Fair
enough. But afaik, a shared anonymous mapping absolutely needs its page
tables copied, because those page tables contains either the pointers to
the shared pages, or the swap entries.

So I really think you need to verify that it's a file mapping too.

Also, arguably, there are other cases that may or may not be worth
worrying about. What about non-shared non-writable file mappings? What
about private mappings that haven't been COW'ed?

So I think that in addition to your tests, you should test for
"vma->vm_file", and you could toy with testing for "vma->anon_vma" being
NULL (the latter will cause a _lot_ of hits, because any read-only private
mapping will trigger, but it's a good stress-test and conceptually
interesting, even if I suspect it will kill any performance gain through
extra minor faults in the child).

Linus
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