Re: [PATCH 1/4] mm: allow guard regions in file-backed and read-only mappings

[David Hildenbrand <david@redhat.com>]

On 18.02.25 17:49, Lorenzo Stoakes wrote:
> On Tue, Feb 18, 2025 at 05:27:24PM +0100, David Hildenbrand wrote:
>> On 18.02.25 17:21, Lorenzo Stoakes wrote:
>>> On Tue, Feb 18, 2025 at 05:17:20PM +0100, David Hildenbrand wrote:
>>>> On 18.02.25 17:12, Lorenzo Stoakes wrote:
>>>>> On Tue, Feb 18, 2025 at 05:01:16PM +0100, David Hildenbrand wrote:
>>>>>> On 13.02.25 19:17, Lorenzo Stoakes wrote:
>>>>>>> There is no reason to disallow guard regions in file-backed mappings -
>>>>>>> readahead and fault-around both function correctly in the presence of PTE
>>>>>>> markers, equally other operations relating to memory-mapped files function
>>>>>>> correctly.
>>>>>>>
>>>>>>> Additionally, read-only mappings if introducing guard-regions, only
>>>>>>> restrict the mapping further, which means there is no violation of any
>>>>>>> access rights by permitting this to be so.
>>>>>>>
>>>>>>> Removing this restriction allows for read-only mapped files (such as
>>>>>>> executable files) correctly which would otherwise not be permitted.
>>>>>>>
>>>>>>> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
>>>>>>> ---
>>>>>>>      mm/madvise.c | 8 +-------
>>>>>>>      1 file changed, 1 insertion(+), 7 deletions(-)
>>>>>>>
>>>>>>> diff --git a/mm/madvise.c b/mm/madvise.c
>>>>>>> index 6ecead476a80..e01e93e179a8 100644
>>>>>>> --- a/mm/madvise.c
>>>>>>> +++ b/mm/madvise.c
>>>>>>> @@ -1051,13 +1051,7 @@ static bool is_valid_guard_vma(struct vm_area_struct *vma, bool allow_locked)
>>>>>>>      	if (!allow_locked)
>>>>>>>      		disallowed |= VM_LOCKED;
>>>>>>> -	if (!vma_is_anonymous(vma))
>>>>>>> -		return false;
>>>>>>> -
>>>>>>> -	if ((vma->vm_flags & (VM_MAYWRITE | disallowed)) != VM_MAYWRITE)
>>>>>>> -		return false;
>>>>>>> -
>>>>>>> -	return true;
>>>>>>> +	return !(vma->vm_flags & disallowed);
>>>>>>>      }
>>>>>>>      static bool is_guard_pte_marker(pte_t ptent)
>>>>>>
>>>>>> Acked-by: David Hildenbrand <david@redhat.com>
>>>>>
>>>>> Thanks!
>>>>>
>>>>>>
>>>>>> I assume these markers cannot completely prevent us from allocating
>>>>>> pages/folios for these underlying file/pageache ranges of these markers in
>>>>>> case of shmem during page faults, right?
>>>>>
>>>>> If the markers are in place, then page faulting will result in a
>>>>> segfault. If we faulted in a shmem page then installed markers (which would
>>>>> zap the range), then the page cache will be populated, but obviously
>>>>> subject to standard reclaim.
>>>>
>>>> Well, yes, (a) if there is swap and (b), if the noswap option was not
>>>> specified for tmpfs.
>>>>
>>>
>>> Right, yeah if you don't have it set up such that dropping a reference to the
>>> folio doesn't drop the page altogether.
>>>
>>> I think this matches expectation though in that you'd get the same results from
>>> an MADV_DONTNEED followed by faulting the page again.
>>
>> It might make sense to document that: installing a guard behaves just like
>> MADV_DONTNEED; in case of a file, that means that the pagecache is left
>> untouched.
> 
> More docs noooo! :P I will update the man pages when this is more obviously
> heading for landing in 6.15 accordingly.
> 
> Current man page documentation on this is:
> 
> 'If the region maps memory pages those mappings will be replaced as part of
> the operation'
> 
> I think something like:
> 
> 'If the region maps pages those mappings will be replaced as part of the
> operation. When guard regions are removed via MADV_GUARD_REMOVE, faulting
> in the page will behave as if that region had MADV_DONTNEED applied to it,
> that is anonymous ranges will be backed by newly allocated zeroed pages and
> file-backed ranges will be backed by the underlying file pages.'
> 
> Probably something less wordy than this...

Yeah, but sounds good to me.

> 
>>
>>>
>>>> Okay, so installing a guard entry might require punshing a hole to get rid
>>>> of any already-existing memory. But readahead (below) might mess it up.
>>>
>>> Only if you are so concerned about avoiding the page cache being populated there
>>> that you want to do this :)
>>>
>>> Readahead I think will not readahead into a holepunched region as the hole
>>> punching extends to the fs layer _I believe_ I have not checked the code for
>>> this, but I believe it actually changes the underlying file too right to say
>>> 'this part of the file is empty'?
>>
>> Well, we are talking about shmem here ... not your ordinary fs backed by an
>> actual file :)
> 
> I am talking about both, I multitask ;)

For !shmem, we should indeed not be messing with a sparse file structure.


-- 
Cheers,

David / dhildenb