linux-kernel - Re: [PATCH v9 06/10] mm/memory.c: Allow different return codes for copy_nonpresent

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Date:   Wed, 26 May 2021 21:44:22 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Alistair Popple <apopple@...dia.com>
Cc:     linux-mm@...ck.org, akpm@...ux-foundation.org,
        nouveau@...ts.freedesktop.org, bskeggs@...hat.com,
        rcampbell@...dia.com, linux-doc@...r.kernel.org,
        jhubbard@...dia.com, bsingharora@...il.com,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        hch@...radead.org, jglisse@...hat.com, willy@...radead.org,
        jgg@...dia.com, hughd@...gle.com
Subject: Re: [PATCH v9 06/10] mm/memory.c: Allow different return codes for
 copy_nonpresent_pte()

On Thu, May 27, 2021 at 11:20:36AM +1000, Alistair Popple wrote:
> On Thursday, 27 May 2021 5:50:05 AM AEST Peter Xu wrote:
> > On Mon, May 24, 2021 at 11:27:21PM +1000, Alistair Popple wrote:
> > > Currently if copy_nonpresent_pte() returns a non-zero value it is
> > > assumed to be a swap entry which requires further processing outside the
> > > loop in copy_pte_range() after dropping locks. This prevents other
> > > values being returned to signal conditions such as failure which a
> > > subsequent change requires.
> > > 
> > > Instead make copy_nonpresent_pte() return an error code if further
> > > processing is required and read the value for the swap entry in the main
> > > loop under the ptl.
> > > 
> > > Signed-off-by: Alistair Popple <apopple@...dia.com>
> > > 
> > > ---
> > > 
> > > v9:
> > > 
> > > New for v9 to allow device exclusive handling to occur in
> > > copy_nonpresent_pte().
> > > ---
> > > 
> > >  mm/memory.c | 12 +++++++-----
> > >  1 file changed, 7 insertions(+), 5 deletions(-)
> > > 
> > > diff --git a/mm/memory.c b/mm/memory.c
> > > index 2fb455c365c2..e061cfa18c11 100644
> > > --- a/mm/memory.c
> > > +++ b/mm/memory.c
> > > @@ -718,7 +718,7 @@ copy_nonpresent_pte(struct mm_struct *dst_mm, struct
> > > mm_struct *src_mm,> 
> > >       if (likely(!non_swap_entry(entry))) {
> > >       
> > >               if (swap_duplicate(entry) < 0)
> > > 
> > > -                     return entry.val;
> > > +                     return -EAGAIN;
> > > 
> > >               /* make sure dst_mm is on swapoff's mmlist. */
> > >               if (unlikely(list_empty(&dst_mm->mmlist))) {
> > > 
> > > @@ -974,11 +974,13 @@ copy_pte_range(struct vm_area_struct *dst_vma,
> > > struct vm_area_struct *src_vma,> 
> > >                       continue;
> > >               
> > >               }
> > >               if (unlikely(!pte_present(*src_pte))) {
> > > 
> > > -                     entry.val = copy_nonpresent_pte(dst_mm, src_mm,
> > > -                                                     dst_pte, src_pte,
> > > -                                                     src_vma, addr, rss);
> > > -                     if (entry.val)
> > > +                     ret = copy_nonpresent_pte(dst_mm, src_mm,
> > > +                                             dst_pte, src_pte,
> > > +                                             src_vma, addr, rss);
> > > +                     if (ret == -EAGAIN) {
> > > +                             entry = pte_to_swp_entry(*src_pte);
> > > 
> > >                               break;
> > > 
> > > +                     }
> > > 
> > >                       progress += 8;
> > >                       continue;
> > >               
> > >               }
> > 
> > Note that -EAGAIN was previously used by copy_present_page() for early cow
> > use.  Here later although we check entry.val first:
> > 
> >         if (entry.val) {
> >                 if (add_swap_count_continuation(entry, GFP_KERNEL) < 0) {
> >                         ret = -ENOMEM;
> >                         goto out;
> >                 }
> >                 entry.val = 0;
> >         } else if (ret) {
> >                 WARN_ON_ONCE(ret != -EAGAIN);
> >                 prealloc = page_copy_prealloc(src_mm, src_vma, addr);
> >                 if (!prealloc)
> >                         return -ENOMEM;
> >                 /* We've captured and resolved the error. Reset, try again.
> > */ ret = 0;
> >         }
> > 
> > We didn't reset "ret" in entry.val case (maybe we should?). Then in the next
> > round of "goto again" if "ret" is unluckily untouched, it could reach the
> > 2nd if check, and I think it could cause an unexpected
> > page_copy_prealloc().
> 
> Thanks, I had considered that but saw "ret" was always set either by 
> copy_nonpresent_pte() or copy_present_pte(). However missed the "unlucky" case 
> at the start of the loop:
> 
> 	if (progress >= 32) {
> 		progress = 0;
> 		if (need_resched() ||
> 				spin_needbreak(src_ptl) || pin_needbreak(dst_ptl))
> 			break;
> 
> Looking at this again though checking different variables to figure out what 
> to do outside the locks and reusing error codes seems error prone. I reused -
> EAGAIN for copy_nonpresent_pte() simply because that seemed the most sensible 
> error code, but I don't think that aids readability and it might be better to 
> use a unique error code for each case needing extra handling.
> 
> So it might be better if I update this patch to:
> 1) Use unique error codes for each case requiring special handling outside the 
> lock.
> 2) Only check "ret" to determine what to do outside locks (ie. not entry.val)
> 3) Document these.
> 4) Always reset ret after handling.
> 
> Thoughts?

Looks good to me.  Thanks,

-- 
Peter Xu