Re: [PATCH v4 1/3] mm/mprotect: Fix soft-dirty check in can_change_pte_writable()

[David Hildenbrand <david@redhat.com>]

On 28.12.22 15:14, Muhammad Usama Anjum wrote:
> On 12/19/22 5:19 PM, Muhammad Usama Anjum wrote:
>> Addition of vma_soft_dirty_enabled() has tinkered with the soft-dirty PTE
>> bit status setting. The internal behavior has changed. The test case was
>> shared by David
>> (https://lore.kernel.org/all/bfcae708-db21-04b4-0bbe-712badd03071@redhat.com/).
>> The explanation is as following:
>>
>> _Before_ addition of this patch(76aefad628aae),
>> m = mmap(2 pages)
>> clear_softdirty()
>> mremap(m + pag_size)
>> mprotect(READ)
>> mprotect(READ | WRITE);
>> memset(m)
>> After memset(),
>> 			PAGE-1		PAGE-2
>> VM_SOFTDIRTY		set		set
>> PTE softdirty flag	set		set
>> /proc//pagemap view	set		set
>>
>>
>> _After_ addition of this patch(76aefad628aae)
>> m = mmap(2 pages)
>> clear_softdirty()
>> mremap(m + page_size)
>> mprotect(READ)
>> mprotect(READ | WRITE);
>> memset(m)
>> After memset(),
>> 			PAGE-1		PAGE-2
>> VM_SOFTDIRTY		set		set
>> PTE softdirty flag	*not set*	set
>> /proc//pagemap view	set		set
>>
>> The user's point of view hasn't changed. But internally after this patch,
>> the soft-dirty tracking in PTEs gets turn off if VM_SOFTDIRTY is set. The
>> soft-dirty tracking in the PTEs shouldn't be just turned off when mprotect
>> is used. Why? Because soft-dirty tracking in the PTEs is always enabled
>> regardless of VM_SOFTDIRTY is set or not. Example:
>>
>> m = mem(2 pages)
>> At this point:
>> 			PAGE-1		PAGE-2
>> VM_SOFTDIRTY		set		set
>> PTE softdirty flag	not set		not set
>> /proc//pagemap view	set		set
>> memset(m)
>> At this point:
>> 			PAGE-1		PAGE-2
>> VM_SOFTDIRTY		set		set
>> PTE softdirty flag	set		set
>> /proc//pagemap view	set		set
>>
>> This example proves that soft-dirty flag on the PTE is set regardless of
>> the VM_SOFTDIRTY.
> 
> Hi Andrew and Cyrill,
> 
> Peter doesn't agree with me here that this change in behavior should be
> reverted etc. Please comment.

For the records, I agree with Peter: As 76aefad628aa ("mm/mprotect: fix 
soft-dirty check in can_change_pte_writable()") documents, this patch 
fixed real problems.

/proc/pagemap works as expected right now such that we don't have an 
under-indication. Internal representation is an implementation detail.

Whatever we do, there must not be an under-indication of softdirty. That 
is the ABI guaranteed (especially for anonymous memory). "No 
over-indication" was never the ABI guarantee.

For your use case, you want to reduce over-indication. I suggested 
looked into alternatives.

-- 
Thanks,

David / dhildenb