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

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Message-ID: <9178bf98-2ea7-4ad8-ad43-cdcc02ab863d@redhat.com>
Date: Mon, 11 Aug 2025 15:03:36 +0200
From: David Hildenbrand <david@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Charan Teja Kalla <quic_charante@...cinc.com>
Cc: 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 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