Re: [PATCH 3/4] mm: prevent KSM from completely breaking VMA merging

[David Hildenbrand <david@redhat.com>]

On 19.05.25 21:02, Lorenzo Stoakes wrote:
> On Mon, May 19, 2025 at 08:04:22PM +0200, David Hildenbrand wrote:
>>
>>>> +/*
>>>> + * Are we guaranteed no driver can change state such as to preclude KSM merging?
>>>> + * If so, let's set the KSM mergeable flag early so we don't break VMA merging.
>>>> + *
>>>> + * This is applicable when PR_SET_MEMORY_MERGE has been set on the mm_struct via
>>>> + * prctl() causing newly mapped VMAs to have the KSM mergeable VMA flag set.
>>>> + *
>>>> + * If this is not the case, then we set the flag after considering mergeability,
>>>> + * which will prevent mergeability as, when PR_SET_MEMORY_MERGE is set, a new
>>>> + * VMA will not have the KSM mergeability VMA flag set, but all other VMAs will,
>>>> + * preventing any merge.
>>>
>>> Hmmm, so an ordinary MAP_PRIVATE of any file (executable etc.) will get
>>> VM_MERGEABLE set but not be able to merge?
>>>
>>> Probably these are not often expected to be merged ...
>>>
>>> Preventing merging should really only happen because of VMA flags that
>>> are getting set: VM_PFNMAP, VM_MIXEDMAP, VM_DONTEXPAND, VM_IO.
>>>
>>>
>>> I am not 100% sure why we bail out on special mappings: all we have to
>>> do is reliably identify anon pages, and we should be able to do that.
>>>
>>> GUP does currently refuses any VM_PFNMAP | VM_IO, and KSM uses GUP,
>>> which might need a tweak then (maybe the solution could be to ... not
>>> use GUP but a folio_walk).
>>
>> Oh, someone called "David" already did that. Nice :)
>>
>> So we *should* be able to drop
>>
>> * VM_PFNMAP: we correctly identify CoWed pages
>> * VM_MIXEDMAP: we correctly identify CoWed pages
>> * VM_IO: should not affect CoWed pages
>> * VM_DONTEXPAND: no idea why that should even matter here
> 
> I objected in the other thread but now realise I forgot we're talking about
> MAP_PRIVATE... So we can do the CoW etc. Right.
> 
> Then we just need to be able to copy the thing on CoW... but what about
> write-through etc. cache settings? I suppose we don't care once CoW'd...

Yes. It's ordinary kernel-managed memory.

> 
> But is this common enough of a use case to be worth the hassle of checking this
> is all ok?

The reason I bring it up is because

1) Just because some drivers do weird mmap() things, we cannot merge any 
MAP_PRIVATE file mappings (except shmem ;) and mmap_prepare).

2) The whole "early_ksm" checks/handling would go away, making this 
patch significantly simpler :)

-- 
Cheers,

David / dhildenb