Re: [PATCH 4/5] mm: Do early cow for pinned pages during fork() for ptes

Oleg Nesterov
2020-09-22T11:49:18Z

On 09/21, Peter Xu wrote:
>
> @@ -859,6 +989,25 @@ static int copy_pte_range(struct mm_struct *dst_mm, struct mm_struct *src_mm,
> spin_needbreak(src_ptl) || spin_needbreak(dst_ptl))
> break;
> }
> +
> + if (unlikely(data.cow_new_page)) {
> + /*
> + * If cow_new_page set, we must be at the 2nd round of
> + * a previous COPY_MM_BREAK_COW. Try to arm the new
> + * page now. Note that in all cases page_break_cow()
> + * will properly release the objects in copy_mm_data.
> + */
> + WARN_ON_ONCE(copy_ret != COPY_MM_BREAK_COW);
> + if (pte_install_copied_page(dst_mm, new, src_pte,
> + dst_pte, addr, rss,
> + &data)) {
> + /* We installed the pte successfully; move on */
> + progress++;
> + continue;

I'm afraid I misread this patch too ;)

But it seems to me in this case the main loop can really "leak"
COPY_MM_BREAK_COW. Suppose the the next 31 pte's are pte_none() and
need_resched() is true.

No?

Oleg.