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

[David Hildenbrand <david@redhat.com>]

>> +/*
>> + * 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

-- 
Cheers,

David / dhildenb