linux-kernel - Re: [PATCH v2] mm/mmap: return early if it can't merge in vma

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Date: Thu, 22 Feb 2024 15:47:04 +0800
From: Yajun Deng <yajun.deng@...ux.dev>
To: Lorenzo Stoakes <lstoakes@...il.com>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>, akpm@...ux-foundation.org,
 vbabka@...e.cz, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm/mmap: return early if it can't merge in vma_merge()


On 2024/2/22 04:41, Lorenzo Stoakes wrote:
> On Wed, Feb 21, 2024 at 10:38:27AM -0500, Liam R. Howlett wrote:
>> * Yajun Deng <yajun.deng@...ux.dev> [240221 04:15]:
>>> In most cases, the range of the area is valid. But in do_mprotect_pkey(),
>>> the minimum value of end and vma->vm_end is passed to mprotect_fixup().
>>> This will lead to the end is less than the end of prev.
>>>
>>> In this case, the curr will be NULL, but the next will be equal to the
>>> prev. So it will attempt to merge before, the vm_pgoff check will cause
>>> this case to fail.
>>>
>>> To avoid the process described above and reduce unnecessary operations.
>>> Add a check to immediately return NULL if the end is less than the end of
>>> prev.
>> If it's only one caller, could we stop that caller instead of checking
>> an almost never case for all callers?  Would this better fit in
>> vma_modify()?  Although that's not just for this caller at this point.
>> Maybe there isn't a good place?
> I definitely agree with Liam that this should not be in vma_merge(), as
> it's not going to be relevant to _most_ callers.
>
> I am not sure vma_modify() is much better, this would be the only early
> exit check in that function and makes what is very simple and
> straightforward now more confusing.


There are two paths that will cause this case. One is in 
mprotect_fixup(), the other is in

madvise_update_vma().


One way is to separate out the split_vma() from vma_modify(). And create 
a new helper function.

We can call it directly at source, but we need to do this in both 
paths.  It's more complicated.


The other way is to add a check in vma_modify(). Like the following:

diff --git a/mm/mmap.c b/mm/mmap.c
index 0fccd23f056e..f93f1d3939f2 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2431,11 +2431,15 @@ struct vm_area_struct *vma_modify(struct 
vma_iterator *vmi,
         pgoff_t pgoff = vma->vm_pgoff + ((start - vma->vm_start) >> 
PAGE_SHIFT);
         struct vm_area_struct *merged;

+       if (prev && end < prev->vm_end)
+               goto cannot_merge;
+
         merged = vma_merge(vmi, prev, vma, start, end, vm_flags,
                            pgoff, policy, uffd_ctx, anon_name);
         if (merged)
                 return merged;

+cannot_merge:
         if (vma->vm_start < start) {
                 int err = split_vma(vmi, vma, start, 1);


> And I think this is the crux of it - it's confusing that we special case
> this one particular non-merge scenario, but no others (all of which we then
> deem ok to be caught by the usual rules).
>
> I think it's simpler (and more efficient) to just keep things the way they
> are.
>
>> Or are there other reasons this may happen and is better done in this
>> function?
>>
>> Often, this is called the "punch a hole" scenario; where an operation
>> creates two entries from the old data and either leaves an empty space
>> or fills the space with a new VMA.
>>
>>> Signed-off-by: Yajun Deng <yajun.deng@...ux.dev>
>>> ---
>>> v2: remove the case label.
>>> v1: https://lore.kernel.org/all/20240218085028.3294332-1-yajun.deng@linux.dev/
>>> ---
>>>   mm/mmap.c | 3 +++
>>>   1 file changed, 3 insertions(+)
>>>
>>> diff --git a/mm/mmap.c b/mm/mmap.c
>>> index 0fccd23f056e..7668854d2246 100644
>>> --- a/mm/mmap.c
>>> +++ b/mm/mmap.c
>>> @@ -890,6 +890,9 @@ static struct vm_area_struct
>>>   	if (vm_flags & VM_SPECIAL)
>>>   		return NULL;
>>>
>>> +	if (prev && end < prev->vm_end)
>>> +		return NULL;
>>> +
>>>   	/* Does the input range span an existing VMA? (cases 5 - 8) */
>>>   	curr = find_vma_intersection(mm, prev ? prev->vm_end : 0, end);
>>>
>>> --
>>> 2.25.1
>>>
> So overall I don't think this check makes much sense anywhere.
>
> I think a better solution would be to prevent it happening _at source_ if
> you can find a logical way of doing so.
>
> I do plan to do some cleanup passes over this stuff once again so maybe I
> can figure something out that better handles non-merge scenarios.
>
> This is a great find though overall even if a patch doesn't make sense
> Yajun, thanks for this, it's really made me think about this case (+ others
> like it) :)