Re: [PATCH v2 5/9] mm/mprotect: keep writable entries writable on permission upgrades

["David Hildenbrand (Arm)" <david@kernel.org>]

On 11/5/26 18:06, David Hildenbrand (Arm) wrote:
> On 11/5/26 17:50, Carlos López wrote:
>> change_protection() applies vma->vm_page_prot, which lacks the write bit
>> for CoW and write-notified mappings, so it write-protects every entry
>> in them. When an entry would simply be made writable on fault without
>> further checks (e.g. dirty PTEs in shared mappings, exclusive anonymous
>> pages), MM_CP_TRY_CHANGE_WRITABLE makes the entry writable again to
>> avoid the future fault.
>>
>> However, writable entries in shared mappings that are not dirty still
>> become read-only. For example, a uffd-wp unprotect makes clean shmem
>> PTEs read-only even if they were never write-protected, e.g. when they
>> were mapped by a read fault before uffd-wp was registered. The next
>> write to each of them causes a fault that only restores the write bit.
>>
>> Now that MM_CP_PERM_UPGRADE indicates whether mprotect() only adds
>> permissions, change_protection() can tell, together with the
>> userfaultfd unprotect flags, when a change revokes no access. Keep
>> writable entries writable in these cases, at the PTE, PMD and PUD
>> levels. This is consistent with the write fault handler, as an entry
>> that is already writable needs no write fault.
>>
>> Signed-off-by: Carlos López <clopez@suse.de>
>> ---
>>  mm/huge_memory.c | 7 +++++++
>>  mm/internal.h    | 7 +++++++
>>  mm/mprotect.c    | 4 ++++
>>  3 files changed, 18 insertions(+)
>>
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index 506e1e61c0fd..6e23d0e4ae66 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -2809,6 +2809,10 @@ int change_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
>>  	if (userfaultfd_rwp(vma) && pmd_uffd(entry))
>>  		entry = pmd_modify(entry, PAGE_NONE);
>>  
>> +	/* See change_present_ptes(). */
>> +	if (mm_cp_is_perm_upgrade(cp_flags) && pmd_write(oldpmd))
>> +		entry = pmd_mkwrite(entry, vma);
>> +
>>  	/* See change_pte_range(). */
>>  	if ((cp_flags & MM_CP_TRY_CHANGE_WRITABLE) && !pmd_write(entry) &&
>>  	    can_change_pmd_writable(vma, addr, entry))
>> @@ -2885,6 +2889,9 @@ int change_huge_pud(struct mmu_gather *tlb, struct vm_area_struct *vma,
>>  	 */
>>  	oldpud = pudp_invalidate(vma, addr, pudp);
>>  	entry = pud_modify(oldpud, newprot);
>> +	/* See change_present_ptes(). */
>> +	if (mm_cp_is_perm_upgrade(cp_flags) && pud_write(oldpud))
>> +		entry = pud_mkwrite(entry);
> 
> That looks completely hacked on. We do have can_change_pud_writable() below to
> make such decisions.
> 
> Whaatever you propose here is either wrong or way too shaky, sorry.
For some reason, your reply neither reached my inbox through CC nor the
mailing list folder through the mailing list.

I see that you send a v3 where you keep it simple for now, good.

>> 
>> That looks completely hacked on. We do have can_change_pud_writable() below to
>> make such decisions.
>
> Okay, for v3 I can pass the old entry to can_change_{pmd,pud}_writable()
> and return true if it was writable, after the existing write-tracking
> checks (uffd-wp/soft-dirty). For PTEs, can_change_pte_writable() is not
> used on the change_protection() path, so the same check would go in
> set_write_prot_commit_flush_ptes().
>
> Does that sound more reasonable?

Let's keep it simple for now. Whenever we mess with protection changes
there is the chance that we mess up and cause more trouble, either now
or in the future when we change calling code.

>
> Whaatever you propose here is either wrong or way too shaky, sorry.
>
> The idea is that MM_CP_TRY_CHANGE_WRITABLE relies on the dirty bit to
> make entries in shared mappings writable, which misses clean entries
> that were already writable (e.g. shmem read-faulted before uffd-wp
> registration). Such entries can stay writable, as anything that relies
> on write faults write-protects the entry first.

Right, but that should be optimized separately with a clear motivation why
we even care about optimizing this.

Not hidden in a patch set that tries to achieve something completely different.

-- 
Cheers,

David