Date: Thu, 22 Feb 2024 16:37:44 +0800
From: Yajun Deng <yajun.deng@...ux.dev>
To: Lorenzo Stoakes <lstoakes@...il.com>
Cc: "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 16:31, Lorenzo Stoakes wrote:
> On Thu, Feb 22, 2024 at 03:47:04PM +0800, Yajun Deng wrote:
>> 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.
> Absolutely not. I wrote the vma_modify() patch series explicitly to expose
> _less_ not more.
>
>> 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:
> As I said above, I really don't think this is a good idea, you're just
> special casing one non-merge case but not any others + adding an
> unnecessary branch.
>
>> 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) :)
> I guess maybe again I've not been clear enough on this, so unless
> compelling reasoning can otherwise be provided, I feel this check should
> not be added _anywhere_.


Okay, I got it. Thank you!

> Therefore, NACK.

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