Re: [PATCH v4 3/4] mm: Optimize mprotect() by PTE-batching

[Ryan Roberts]

On 30/06/2025 13:52, Lorenzo Stoakes wrote:
> On Sat, Jun 28, 2025 at 05:04:34PM +0530, Dev Jain wrote:
>> Use folio_pte_batch to batch process a large folio. Reuse the folio from
>> prot_numa case if possible.
>>
>> For all cases other than the PageAnonExclusive case, if the case holds true
>> for one pte in the batch, one can confirm that that case will hold true for
>> other ptes in the batch too; for pte_needs_soft_dirty_wp(), we do not pass
>> FPB_IGNORE_SOFT_DIRTY. modify_prot_start_ptes() collects the dirty
>> and access bits across the batch, therefore batching across
>> pte_dirty(): this is correct since the dirty bit on the PTE really is
>> just an indication that the folio got written to, so even if the PTE is
>> not actually dirty (but one of the PTEs in the batch is), the wp-fault
>> optimization can be made.
>>
>> The crux now is how to batch around the PageAnonExclusive case; we must
>> check the corresponding condition for every single page. Therefore, from
>> the large folio batch, we process sub batches of ptes mapping pages with
>> the same PageAnonExclusive condition, and process that sub batch, then
>> determine and process the next sub batch, and so on. Note that this does
>> not cause any extra overhead; if suppose the size of the folio batch
>> is 512, then the sub batch processing in total will take 512 iterations,
>> which is the same as what we would have done before.
>>
>> Signed-off-by: Dev Jain <dev.jain@xxxxxxx>
>> ---
>>  mm/mprotect.c | 143 +++++++++++++++++++++++++++++++++++++++++---------
>>  1 file changed, 117 insertions(+), 26 deletions(-)
>>
>> diff --git a/mm/mprotect.c b/mm/mprotect.c
>> index 627b0d67cc4a..28c7ce7728ff 100644
>> --- a/mm/mprotect.c
>> +++ b/mm/mprotect.c
>> @@ -40,35 +40,47 @@
>>
>>  #include "internal.h"
>>
>> -bool can_change_pte_writable(struct vm_area_struct *vma, unsigned long addr,
>> -			     pte_t pte)
>> -{
>> -	struct page *page;
>> +enum tristate {
>> +	TRI_FALSE = 0,
>> +	TRI_TRUE = 1,
>> +	TRI_MAYBE = -1,
>> +};
> 
> Yeah no, absolutely not, this is horrible, I don't want to see an arbitrary type
> like this added, to a random file, and I absolutely think this adds confusion
> and does not in any way help clarify things.
> 
>>
>> +/*
>> + * Returns enum tristate indicating whether the pte can be changed to writable.
>> + * If TRI_MAYBE is returned, then the folio is anonymous and the user must
>> + * additionally check PageAnonExclusive() for every page in the desired range.
>> + */
>> +static int maybe_change_pte_writable(struct vm_area_struct *vma,
>> +				     unsigned long addr, pte_t pte,
>> +				     struct folio *folio)
>> +{
>>  	if (WARN_ON_ONCE(!(vma->vm_flags & VM_WRITE)))
>> -		return false;
>> +		return TRI_FALSE;
>>
>>  	/* Don't touch entries that are not even readable. */
>>  	if (pte_protnone(pte))
>> -		return false;
>> +		return TRI_FALSE;
>>
>>  	/* Do we need write faults for softdirty tracking? */
>>  	if (pte_needs_soft_dirty_wp(vma, pte))
>> -		return false;
>> +		return TRI_FALSE;
>>
>>  	/* Do we need write faults for uffd-wp tracking? */
>>  	if (userfaultfd_pte_wp(vma, pte))
>> -		return false;
>> +		return TRI_FALSE;
>>
>>  	if (!(vma->vm_flags & VM_SHARED)) {
>>  		/*
>>  		 * Writable MAP_PRIVATE mapping: We can only special-case on
>>  		 * exclusive anonymous pages, because we know that our
>>  		 * write-fault handler similarly would map them writable without
>> -		 * any additional checks while holding the PT lock.
>> +		 * any additional checks while holding the PT lock. So if the
>> +		 * folio is not anonymous, we know we cannot change pte to
>> +		 * writable. If it is anonymous then the caller must further
>> +		 * check that the page is AnonExclusive().
>>  		 */
>> -		page = vm_normal_page(vma, addr, pte);
>> -		return page && PageAnon(page) && PageAnonExclusive(page);
>> +		return (!folio || folio_test_anon(folio)) ? TRI_MAYBE : TRI_FALSE;
>>  	}
>>
>>  	VM_WARN_ON_ONCE(is_zero_pfn(pte_pfn(pte)) && pte_dirty(pte));
>> @@ -80,15 +92,61 @@ bool can_change_pte_writable(struct vm_area_struct *vma, unsigned long addr,
>>  	 * FS was already notified and we can simply mark the PTE writable
>>  	 * just like the write-fault handler would do.
>>  	 */
>> -	return pte_dirty(pte);
>> +	return pte_dirty(pte) ? TRI_TRUE : TRI_FALSE;
>> +}
> 
> Yeah not a fan of this at all.
> 
> This is squashing all the logic into one place when we don't really need to.
> 
> We can separate out the shared logic and just do something like:
> 
> ////// Lorenzo's suggestion //////
> 
> -bool can_change_pte_writable(struct vm_area_struct *vma, unsigned long addr,
> -			     pte_t pte)
> +static bool maybe_change_pte_writable(struct vm_area_struct *vma,
> +		pte_t pte)
>  {
> -	struct page *page;
> -
>  	if (WARN_ON_ONCE(!(vma->vm_flags & VM_WRITE)))
>  		return false;
> 
> @@ -60,16 +58,14 @@ bool can_change_pte_writable(struct vm_area_struct *vma, unsigned long addr,
>  	if (userfaultfd_pte_wp(vma, pte))
>  		return false;
> 
> -	if (!(vma->vm_flags & VM_SHARED)) {
> -		/*
> -		 * Writable MAP_PRIVATE mapping: We can only special-case on
> -		 * exclusive anonymous pages, because we know that our
> -		 * write-fault handler similarly would map them writable without
> -		 * any additional checks while holding the PT lock.
> -		 */
> -		page = vm_normal_page(vma, addr, pte);
> -		return page && PageAnon(page) && PageAnonExclusive(page);
> -	}
> +	return true;
> +}
> +
> +static bool can_change_shared_pte_writable(struct vm_area_struct *vma,
> +		pte_t pte)
> +{
> +	if (!maybe_change_pte_writable(vma, pte))
> +		return false;
> 
>  	VM_WARN_ON_ONCE(is_zero_pfn(pte_pfn(pte)) && pte_dirty(pte));
> 
> @@ -83,6 +79,33 @@ bool can_change_pte_writable(struct vm_area_struct *vma, unsigned long addr,
>  	return pte_dirty(pte);
>  }
> 
> +static bool can_change_private_pte_writable(struct vm_area_struct *vma,
> +		unsigned long addr, pte_t pte)
> +{
> +	struct page *page;
> +
> +	if (!maybe_change_pte_writable(vma, pte))
> +		return false;
> +
> +	/*
> +	 * Writable MAP_PRIVATE mapping: We can only special-case on
> +	 * exclusive anonymous pages, because we know that our
> +	 * write-fault handler similarly would map them writable without
> +	 * any additional checks while holding the PT lock.
> +	 */
> +	page = vm_normal_page(vma, addr, pte);
> +	return page && PageAnon(page) && PageAnonExclusive(page);
> +}
> +
> +bool can_change_pte_writable(struct vm_area_struct *vma,
> +		unsigned long addr, pte_t pte)
> +{
> +	if (vma->vm_flags & VM_SHARED)
> +		return can_change_shared_pte_writable(vma, pte);
> +
> +	return can_change_private_pte_writable(vma, addr, pte);
> +}
> +
> 
> ////// end of Lorenzo's suggestion //////
> 
> You can obviously modify this to change other stuff like whether you feed back
> the PAE or not in private case for use in your code.

This sugestion for this part of the problem looks much cleaner!

Sorry; this whole struct tristate thing was my idea. I never really liked it but
I was more focussed on trying to illustrate the big picture flow that I thought
would work well with a batch and sub-batches (which it seems below that you
hate... but let's talk about that down there).

> 
>> +
>> +/*
>> + * Returns the number of pages within the folio, starting from the page
>> + * indicated by pgidx and up to pgidx + max_nr, that have the same value of
>> + * PageAnonExclusive(). Must only be called for anonymous folios. Value of
>> + * PageAnonExclusive() is returned in *exclusive.
>> + */
>> +static int anon_exclusive_batch(struct folio *folio, int pgidx, int max_nr,
>> +				bool *exclusive)
> 
> Let's generalise it to something like count_folio_fungible_pages()
> 
> or maybe count_folio_batchable_pages()?
> 
> Yes naming is hard... :P but right now it reads like this is returning a batch
> or doing something with a batch.
> 
>> +{
>> +	struct page *page;
>> +	int nr = 1;
>> +
>> +	if (!folio) {
>> +		*exclusive = false;
>> +		return nr;
>> +	}
>> +
>> +	page = folio_page(folio, pgidx++);
>> +	*exclusive = PageAnonExclusive(page);
>> +	while (nr < max_nr) {
> 
> The C programming language asks why you don't like using for :)
> 
>> +		page = folio_page(folio, pgidx++);
>> +		if ((*exclusive) != PageAnonExclusive(page))
>> +			break;
>> +		nr++;
> 
> This *exclusive stuff makes me want to cry :)
> 
> Just set a local variable and hand it back at the end.
> 
>> +	}
>> +
>> +	return nr;
>> +}
>> +
>> +bool can_change_pte_writable(struct vm_area_struct *vma, unsigned long addr,
>> +			     pte_t pte)
>> +{
>> +	struct page *page;
>> +	int ret;
>> +
>> +	ret = maybe_change_pte_writable(vma, addr, pte, NULL);
>> +	if (ret == TRI_MAYBE) {
>> +		page = vm_normal_page(vma, addr, pte);
>> +		ret = page && PageAnon(page) && PageAnonExclusive(page);
>> +	}
>> +
>> +	return ret;
>>  }
> 
> See above comments on this stuff.
> 
>>
>>  static int mprotect_folio_pte_batch(struct folio *folio, unsigned long addr,
>> -		pte_t *ptep, pte_t pte, int max_nr_ptes)
>> +		pte_t *ptep, pte_t pte, int max_nr_ptes, fpb_t switch_off_flags)
> 
> This last parameter is pretty horrible. It's a negative mask so now you're
> passing 'ignore soft dirty' to the function meaning 'don't ignore it'. This is
> just planting land mines.
> 
> Obviously David's flag changes will also alter all this.
> 
> Just add a boolean re: soft dirty.

Dev had a boolean for this in the last round. I've seen various functions expand
over time with increasing numbers of bool flags. So I asked to convert to a
flags parameter and just pass in the flags we need. Then it's a bit more future
proof and self documenting. (For the record I dislike the "switch_off_flags"
approach taken here).

> 
>>  {
>> -	const fpb_t flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY;
>> +	fpb_t flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY;
>> +
>> +	flags &= ~switch_off_flags;
>>
>> -	if (!folio || !folio_test_large(folio) || (max_nr_ptes == 1))
>> +	if (!folio || !folio_test_large(folio))
>>  		return 1;
> 
> Why remove this last check?
> 
>>
>>  	return folio_pte_batch(folio, addr, ptep, pte, max_nr_ptes, flags,
>> @@ -154,7 +212,8 @@ static int prot_numa_skip_ptes(struct folio **foliop, struct vm_area_struct *vma
>>  	}
>>
>>  skip_batch:
>> -	nr_ptes = mprotect_folio_pte_batch(folio, addr, pte, oldpte, max_nr_ptes);
>> +	nr_ptes = mprotect_folio_pte_batch(folio, addr, pte, oldpte,
>> +					   max_nr_ptes, 0);
> 
> See above about flag param. If you change to boolean, please prefix this with
> e.g. /* set_soft_dirty= */ true or whatever the flag ends up being :)
> 
>>  out:
>>  	*foliop = folio;
>>  	return nr_ptes;
>> @@ -191,7 +250,10 @@ static long change_pte_range(struct mmu_gather *tlb,
>>  		if (pte_present(oldpte)) {
>>  			int max_nr_ptes = (end - addr) >> PAGE_SHIFT;
>>  			struct folio *folio = NULL;
>> -			pte_t ptent;
>> +			int sub_nr_ptes, pgidx = 0;
>> +			pte_t ptent, newpte;
>> +			bool sub_set_write;
>> +			int set_write;
>>
>>  			/*
>>  			 * Avoid trapping faults against the zero or KSM
>> @@ -206,6 +268,11 @@ static long change_pte_range(struct mmu_gather *tlb,
>>  					continue;
>>  			}
>>
>> +			if (!folio)
>> +				folio = vm_normal_folio(vma, addr, oldpte);
>> +
>> +			nr_ptes = mprotect_folio_pte_batch(folio, addr, pte, oldpte,
>> +							   max_nr_ptes, FPB_IGNORE_SOFT_DIRTY);
> 
> Don't we only care about S/D if pte_needs_soft_dirty_wp()?
> 
>>  			oldpte = modify_prot_start_ptes(vma, addr, pte, nr_ptes);
>>  			ptent = pte_modify(oldpte, newprot);
>>
>> @@ -227,15 +294,39 @@ static long change_pte_range(struct mmu_gather *tlb,
>>  			 * example, if a PTE is already dirty and no other
>>  			 * COW or special handling is required.
>>  			 */
>> -			if ((cp_flags & MM_CP_TRY_CHANGE_WRITABLE) &&
>> -			    !pte_write(ptent) &&
>> -			    can_change_pte_writable(vma, addr, ptent))
>> -				ptent = pte_mkwrite(ptent, vma);
>> -
>> -			modify_prot_commit_ptes(vma, addr, pte, oldpte, ptent, nr_ptes);
>> -			if (pte_needs_flush(oldpte, ptent))
>> -				tlb_flush_pte_range(tlb, addr, PAGE_SIZE);
>> -			pages++;
>> +			set_write = (cp_flags & MM_CP_TRY_CHANGE_WRITABLE) &&
>> +				    !pte_write(ptent);
>> +			if (set_write)
>> +				set_write = maybe_change_pte_writable(vma, addr, ptent, folio);
>> +
>> +			while (nr_ptes) {
>> +				if (set_write == TRI_MAYBE) {
>> +					sub_nr_ptes = anon_exclusive_batch(folio,
>> +						pgidx, nr_ptes, &sub_set_write);
>> +				} else {
>> +					sub_nr_ptes = nr_ptes;
>> +					sub_set_write = (set_write == TRI_TRUE);
>> +				}
>> +
>> +				if (sub_set_write)
>> +					newpte = pte_mkwrite(ptent, vma);
>> +				else
>> +					newpte = ptent;
>> +
>> +				modify_prot_commit_ptes(vma, addr, pte, oldpte,
>> +							newpte, sub_nr_ptes);
>> +				if (pte_needs_flush(oldpte, newpte))
>> +					tlb_flush_pte_range(tlb, addr,
>> +						sub_nr_ptes * PAGE_SIZE);
>> +
>> +				addr += sub_nr_ptes * PAGE_SIZE;
>> +				pte += sub_nr_ptes;
>> +				oldpte = pte_advance_pfn(oldpte, sub_nr_ptes);
>> +				ptent = pte_advance_pfn(ptent, sub_nr_ptes);
>> +				nr_ptes -= sub_nr_ptes;
>> +				pages += sub_nr_ptes;
>> +				pgidx += sub_nr_ptes;
>> +			}
> 
> I hate hate hate having this loop here, let's abstract this please.
> 
> I mean I think we can just use mprotect_folio_pte_batch() no? It's not
> abstracting much here, and we can just do all this handling there. Maybe have to
> pass in a bunch more params, but it saves us having to do all this.

In an ideal world we would flatten and just have mprotect_folio_pte_batch()
return the batch size considering all the relevant PTE bits AND the
AnonExclusive bit on the pages. IIRC one of Dev's earlier versions modified the
core folio_pte_batch() function to also look at the AnonExclusive bit, but I
really disliked changing that core function (I think others did too?).

So barring that approach, we are really only left with the batch and sub-batch
approach - although, yes, it could be abstracted more. We could maintain a
context struct that persists across all calls to mprotect_folio_pte_batch() and
it can use that to keep it's state to remember if we are in the middle of a
sub-batch and decide either to call folio_pte_batch() to get a new batch, or
call anon_exclusive_batch() to get the next sub-batch within the current batch.
But that started to feel overly abstracted to me.

This loop approach, as written, felt more straightforward for the reader to
understand (i.e. the least-worst option). Is the context approach what you are
suggesting or do you have something else in mind?

> 
> Alternatively, we could add a new wrapper function, but yeah definitely not
> this.
> 
> Also the C programming language asks... etc etc. ;)
> 
> Overall since you always end up processing folio_nr_pages(folio) you can just
> have the batch function or a wrapper return this and do updates as necessary
> here on that basis, and leave the 'sub' batching to that function.

Sorry I don't understand this statement - could you clarify? Especially the bit
about "always ... processing folio_nr_pages(folio)"; I don't think we do. In
various corner cases the size of the folio has no relationship to the way the
PTEs are mapped.

Thanks,
Ryan

> 
> 
>>  		} else if (is_swap_pte(oldpte)) {
>>  			swp_entry_t entry = pte_to_swp_entry(oldpte);
>>  			pte_t newpte;
>> --
>> 2.30.2
>>