Re: [PATCH] mm: swap: check for xa_zero_entry() on vma in swapoff path

[David Hildenbrand]

On 11.08.25 14:14, Lorenzo Stoakes wrote:
> On Mon, Aug 11, 2025 at 03:13:14PM +0530, Charan Teja Kalla wrote:
> > Thanks David, for the reply!!
> > On 8/8/2025 5:34 PM, David Hildenbrand wrote:
> > > >          if (mpnt) {
> > > >              mas_set_range(&vmi.mas, mpnt->vm_start, mpnt->vm_end - 1);
> > > >              mas_store(&vmi.mas, XA_ZERO_ENTRY);
> > > >              /* Avoid OOM iterating a broken tree */
> > > >              set_bit(MMF_OOM_SKIP, &mm->flags);
> > > >          }
> > > >          /*
> > > >           * The mm_struct is going to exit, but the locks will be dropped
> > > >           * first.  Set the mm_struct as unstable is advisable as it is
> > > >           * not fully initialised.
> > > >           */
> > > >          set_bit(MMF_UNSTABLE, &mm->flags);
> > > >      }
> > > >
> > > > Shouldn't we just remove anything from the tree here that was not copied
> > > > immediately?
> > >
> > > Another fix would be to just check MMF_UNSTABLE in unuse_mm(). But
> > > having these MMF_UNSTABLE checks all over the place feels a bit like
> > > whack-a-mole.
> > Seems MMF_UNSTABLE is the expectation per the commit,
> > 64c37e134b12("kernel: be more careful about dup_mmap() failures and
> > uprobe registering"). Excerpt(s) from the commit message:
>
> This really is whack-a-mole yeah.
>
> > This patch sets the MMF_OOM_SKIP to avoid the iteration of the vmas on
> > the oom side (even though this is extremely unlikely to be selected as
> > an oom victim in the race window), and __sets MMF_UNSTABLE to avoid
> > other potential users from using a partially initialised mm_struct.
>
> But... maybe this is better for the _hotfix_ version as a nicer way of
> doing this.

I would prefer using MMF_UNSTABLE as a hotfix.

> > When registering vmas for uprobe, skip the vmas in an mm that is marked
> > unstable.  Modifying a vma in an unstable mm may cause issues if the mm
> > isn't fully initialised.__
> >
> > > Is there anything preventing us from just leaving a proper tree that
> > > reflects reality in place before we drop the write lock?
> >
> > When you mean proper tree, is this about the your previous question? --
> > Shouldn't we just remove anything from the tree here that was not copied
> > immediately?
>
> Commit d24062914837 (" fork: use __mt_dup() to duplicate maple tree in
> dup_mmap()") did this for efficiency, so it'd be a regression to do this.

We're talking about the case where fork *fails*. That cannot possibly be 
relevant for performance, can it? :)

--
Cheers,

David / dhildenb