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

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Date:   Tue, 22 Sep 2020 18:52:17 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        Kirill Shutemov <kirill@...temov.name>,
        Jann Horn <jannh@...gle.com>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        Hugh Dickins <hughd@...gle.com>,
        Leon Romanovsky <leonro@...dia.com>, Jan Kara <jack@...e.cz>,
        John Hubbard <jhubbard@...dia.com>,
        Christoph Hellwig <hch@....de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [PATCH 4/5] mm: Do early cow for pinned pages during fork() for
 ptes

On 09/22, Peter Xu wrote:
>
> On Tue, Sep 22, 2020 at 02:40:14PM +0200, Oleg Nesterov wrote:
> > On 09/22, Oleg Nesterov wrote:
> > >
> > > 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?
>
> I still think it's a no...
>
> Note that now we'll reset "progress" every time before the do loop, so we'll
> never reach need_resched() (since progress<32) before pte_install_copied_page()
> when needed.

Yes. But copy_ret is still COPY_MM_BREAK_COW after pte_install_copied_page().
Now suppose that the next 31 pte's are pte_none(), progress will be incremented
every time.

> I explicitly put the pte_install_copied_page() into the loop just...
...
> >  	progress = 0;
> > +	if (unlikely(copy_ret == COPY_MM_BREAK_COW)) {
> > +		/*
> > +		 * Note that in all cases pte_install_copied_page()
> > +		 * will properly release the objects in copy_mm_data.
> > +		 */
> > +		copy_ret = COPY_MM_DONE;
> > +		if (pte_install_copied_page(dst_mm, new, src_pte,
> > +					    dst_pte, addr, rss,
> > +					    &data)) {
> > +			/* We installed the pte successfully; move on */
> > +			progress++;
> > +			goto next;
>
> ... to avoid jumps like this because I think it's really tricky. :)

To me it looks better before the main loop because we know that
data.cow_new_page != NULL is only possible at the 1st iterattion after
restart ;)

But I agree, this is subjective, please ignore. However, I still think
it is better to rely on the copy_ret == COPY_MM_BREAK_COW check rather
than data.cow_new_page != NULL.

> >  	case COPY_MM_SWAP_CONT:
> >  		if (add_swap_count_continuation(data.entry, GFP_KERNEL) < 0)
> >  			return -ENOMEM;
> > -		break;
> > +		copy_ret = COPY_MM_DONE;
>
> Kind of a continuation of the discussion from previous patch - I think we'd
> better reset copy_ret not only for this case, but move it after the switch
> (just in case there'll be new ones).  The new BREAK_COW uses goto so it's quite
> special.
>
> > +		goto again;
>
> I feel like this could go wrong without the "addr != end" check later, when
> this is the last pte to check.

How? We know that copy_one_pte() failed and returned COPY_MM_SWAP_CONT
before addr = end.

And this matters "case COPY_MM_BREAK_COW" below which does "goto again"
without the "addr != end" check.

Oleg.