linux-kernel - Re: [PATCH v3 3/6] mm: introduce pte_move_swp_offset() helper which can move offset bidirectionally

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Date: Sat, 4 May 2024 07:07:35 +0800
From: Barry Song <21cnbao@...il.com>
To: Chris Li <chrisl@...nel.org>
Cc: akpm@...ux-foundation.org, linux-mm@...ck.org, 
	baolin.wang@...ux.alibaba.com, david@...hat.com, hanchuanhua@...o.com, 
	hannes@...xchg.org, hughd@...gle.com, kasong@...cent.com, 
	linux-kernel@...r.kernel.org, ryan.roberts@....com, surenb@...gle.com, 
	v-songbaohua@...o.com, willy@...radead.org, xiang@...nel.org, 
	ying.huang@...el.com, yosryahmed@...gle.com, yuzhao@...gle.com, 
	ziy@...dia.com
Subject: Re: [PATCH v3 3/6] mm: introduce pte_move_swp_offset() helper which
 can move offset bidirectionally

On Sat, May 4, 2024 at 4:51 AM Chris Li <chrisl@...nel.org> wrote:
>
> On Thu, May 2, 2024 at 5:51 PM Barry Song <21cnbao@...il.com> wrote:
> >
> > From: Barry Song <v-songbaohua@...o.com>
> >
> > There could arise a necessity to obtain the first pte_t from a swap
> > pte_t located in the middle. For instance, this may occur within the
> > context of do_swap_page(), where a page fault can potentially occur in
> > any PTE of a large folio. To address this, the following patch introduces
> > pte_move_swp_offset(), a function capable of bidirectional movement by
> > a specified delta argument. Consequently, pte_increment_swp_offset()
> > will directly invoke it with delta = 1.
>
> BTW, this patch has conflict with the latest mm-unstable. You might
> need to refresh it.
>
> I do not see the delta = -1 usage case here. Maybe merge the patch
> with the follow up patch that uses the delta = -1?
> Does delta only make sense as 1 and -1?

nop. delta can be any value from -1 to -(nr_pages - 1). This is used to
get the first pte from vmf->pte in patdh6.

it might be better to be a separate patch as it is introducing a new
helper? do_swap_page() might not be the only user in the future?

>
> This patch doesn't seem straightly necessary to me.
>
> Chris
>
>
> Chris
>
> >
> > Suggested-by: "Huang, Ying" <ying.huang@...el.com>
> > Signed-off-by: Barry Song <v-songbaohua@...o.com>
> > ---
> >  mm/internal.h | 25 +++++++++++++++++++++----
> >  1 file changed, 21 insertions(+), 4 deletions(-)
> >
> > diff --git a/mm/internal.h b/mm/internal.h
> > index c5552d35d995..cfe4aed66a5c 100644
> > --- a/mm/internal.h
> > +++ b/mm/internal.h
> > @@ -211,18 +211,21 @@ static inline int folio_pte_batch(struct folio *folio, unsigned long addr,
> >  }
> >
> >  /**
> > - * pte_next_swp_offset - Increment the swap entry offset field of a swap pte.
> > + * pte_move_swp_offset - Move the swap entry offset field of a swap pte
> > + *      forward or backward by delta
> >   * @pte: The initial pte state; is_swap_pte(pte) must be true and
> >   *      non_swap_entry() must be false.
> > + * @delta: The direction and the offset we are moving; forward if delta
> > + *      is positive; backward if delta is negative
> >   *
> > - * Increments the swap offset, while maintaining all other fields, including
> > + * Moves the swap offset, while maintaining all other fields, including
> >   * swap type, and any swp pte bits. The resulting pte is returned.
> >   */
> > -static inline pte_t pte_next_swp_offset(pte_t pte)
> > +static inline pte_t pte_move_swp_offset(pte_t pte, long delta)
> >  {
> >         swp_entry_t entry = pte_to_swp_entry(pte);
> >         pte_t new = __swp_entry_to_pte(__swp_entry(swp_type(entry),
> > -                                                  (swp_offset(entry) + 1)));
> > +                                                  (swp_offset(entry) + delta)));
> >
> >         if (pte_swp_soft_dirty(pte))
> >                 new = pte_swp_mksoft_dirty(new);
> > @@ -234,6 +237,20 @@ static inline pte_t pte_next_swp_offset(pte_t pte)
> >         return new;
> >  }
> >
> > +
> > +/**
> > + * pte_next_swp_offset - Increment the swap entry offset field of a swap pte.
> > + * @pte: The initial pte state; is_swap_pte(pte) must be true and
> > + *      non_swap_entry() must be false.
> > + *
> > + * Increments the swap offset, while maintaining all other fields, including
> > + * swap type, and any swp pte bits. The resulting pte is returned.
> > + */
> > +static inline pte_t pte_next_swp_offset(pte_t pte)
> > +{
> > +       return pte_move_swp_offset(pte, 1);
> > +}
> > +
> >  /**
> >   * swap_pte_batch - detect a PTE batch for a set of contiguous swap entries
> >   * @start_ptep: Page table pointer for the first entry.
> > --
> > 2.34.1

Thanks
Barry