On Sat, Sep 26, 2020 at 4:23 PM Jason Gunthorpe <jgg@xxxxxxxx> wrote:
>
> Linus's version doesn't do pte_sw_mkyoung(), but looks OK to have it
I don't think it matters. But I don't think it should make it young,
since there's no access, but it's not like it's a big deal.
> > + pte = maybe_mkwrite(pte_mkdirty(pte), new);
>
> maybe_mkwrite() was not in Linus's version but it is wp_page_copy().
Actually, it is in my version too, just in a different form.
I did it using
if (vma->vm_flags & VM_WRITE)
*src_pte = pte_mkwrite(*src_pte);
instead, ie avoiding the write to src_pte if it wasn't a writable vma
(and I had checked that it was dirty and not done this at all if not,
so no need for the mkdirty).
> It seemed like mk_pte() should set the proper write
> bit already from the vm_page_prot?
No, vm_page_prot won't have the writable bit for a COW mapping.
The write bit gets set when the write happens (which may be on the
first access, of course), by the code that makes sure it's a private
copy.
> Perhaps this is harmless but redundant?
No, the pte_mkwrite() is required in some form, whether it's that
"maybe_mkwrite()" that looks at the vm flags, or in that explicit
form.
> > + page_add_new_anon_rmap(new_page, new, addr, false);
> > + rss[mm_counter(new_page)]++;
> > + set_pte_at(dst_mm, addr, dst_pte, pte);
>
> Linus's patch had a lru_cache_add_inactive_or_unevictable() here, like
> wp_page_copy()
Yeah, I do think that is needed so that we have the new page on the
LRU and it gets properly evicted under memory pressure.
Linus
