Re: [PATCH v2 3/5] mm: swap_pte_batch: add an output argument to reture if all swap entries are exclusive

[David Hildenbrand]

On 11.04.24 16:54, Ryan Roberts wrote:
> On 09/04/2024 09:26, Barry Song wrote:
> > From: Barry Song <v-songbaohua@xxxxxxxx>
> >
> > Add a boolean argument named any_shared. If any of the swap entries are
> > non-exclusive, set any_shared to true. The function do_swap_page() can
> > then utilize this information to determine whether the entire large
> > folio can be reused.
> >
> > Signed-off-by: Barry Song <v-songbaohua@xxxxxxxx>
> > ---
> >   mm/internal.h | 9 ++++++++-
> >   mm/madvise.c  | 2 +-
> >   mm/memory.c   | 2 +-
> >   3 files changed, 10 insertions(+), 3 deletions(-)
> >
> > diff --git a/mm/internal.h b/mm/internal.h
> > index 9d3250b4a08a..cae39c372bfc 100644
> > --- a/mm/internal.h
> > +++ b/mm/internal.h
> > @@ -238,7 +238,8 @@ static inline pte_t pte_next_swp_offset(pte_t pte)
> >    *
> >    * Return: the number of table entries in the batch.
> >    */
> > -static inline int swap_pte_batch(pte_t *start_ptep, int max_nr, pte_t pte)
> > +static inline int swap_pte_batch(pte_t *start_ptep, int max_nr, pte_t pte,
> > +				bool *any_shared)
>
> Please update the docs in the comment above this for the new param; follow
> folio_pte_batch()'s docs as a template.
>
> >   {
> >   	pte_t expected_pte = pte_next_swp_offset(pte);
> >   	const pte_t *end_ptep = start_ptep + max_nr;
> > @@ -248,12 +249,18 @@ static inline int swap_pte_batch(pte_t *start_ptep, int max_nr, pte_t pte)
> >   	VM_WARN_ON(!is_swap_pte(pte));
> >   	VM_WARN_ON(non_swap_entry(pte_to_swp_entry(pte)));
> >   
> > +	if (any_shared)
> > +		*any_shared |= !pte_swp_exclusive(pte);
>
> This is different from the approach in folio_pte_batch(). It inits *any_shared
> to false and does NOT include the value of the first pte. I think that's odd,
> personally and I prefer your approach. I'm not sure if there was a good reason
> that David chose the other approach?

Because in my case calling code does

nr = folio_pte_batch(folio, addr, src_pte, pte, max_nr, flags,
		     &any_writable);

..

if (any_writable)
	pte = pte_mkwrite(pte, src_vma);

..

and later checks in another function pte_write().

So if the common pattern is that the original PTE will be used for 
checks, then it doesn't make sense to unnecessary checks+setting for the 
first PTE.

--
Cheers,

David / dhildenb