linux-kernel - Re: [PATCH] mm: swap: check for xa_zero

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Message-ID: <904f85d0-acd6-4f47-ab45-fbf18b80f1c6@lucifer.local>
Date: Mon, 11 Aug 2025 14:08:35 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: David Hildenbrand <david@...hat.com>
Cc: Charan Teja Kalla <quic_charante@...cinc.com>, akpm@...ux-foundation.org,
        shikemeng@...weicloud.com, kasong@...cent.com, nphamcs@...il.com,
        bhe@...hat.com, baohua@...nel.org, chrisl@...nel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH] mm: swap: check for xa_zero_entry() on vma in swapoff
 path

On Mon, Aug 11, 2025 at 03:03:36PM +0200, David Hildenbrand wrote:
> 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.

Yeah, I think MMF_UNSTABLE is probably the way go, this is what I was trying to
say :P

>
> >
> > > 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? :)

I think it optimises the overall operation, but as a product of that, has to
handle this edge case, and that necessitated this rather horrble stuff.

Obviously we don't need to optimise a 'we are about to die' case :)

See https://lore.kernel.org/linux-mm/20231016032226.59199-1-zhangpeng.00@bytedance.com/
for details.

Cheers, Lorenzo