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

[Peter Xu]

On Tue, Sep 22, 2020 at 06:52:17PM +0200, Oleg Nesterov wrote:
> 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.

Yes, I think you're right - I'll need to reset that.

> 
> > 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.

Thanks.  For simplicity, I'll keep the code majorly as is.  But I'm still open
to change if e.g. someone else still perfers the other way.

> 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.

Yes.  Logically we should check both, but now as I'm written it as:

        if (unlikely(data.cow_new_page)) {
                WARN_ON_ONCE(copy_ret != COPY_MM_BREAK_COW);
                ...
        }

I think it's even safer because it's actually checking both, but also warn if
only cow_new_page is set, which should never happen anyways.

Or I can also do it in inverted order if you think better:

        if (unlikely(copy_ret == COPY_MM_BREAK_COW)) {
                WARN_ON_ONCE(!data.cow_new_page);
                ...
        }

> 
> > >  	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.

I think you're right, again. :)

Thanks,

-- 
Peter Xu