Messages in this thread | | | From | Linus Torvalds <> | Date | Sat, 26 Sep 2020 17:04:38 -0700 | Subject | Re: [PATCH v2 3/4] mm: Do early cow for pinned pages during fork() for ptes |
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On Sat, Sep 26, 2020 at 4:23 PM Jason Gunthorpe <jgg@ziepe.ca> 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
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