Re: [PATCH v16 11/25] mm: pagewalk: Add p4d_entry() and pgd_entry()

[Thomas Hellström (VMware) <thomas_os@xxxxxxxxxxxx>]

On 12/12/19 12:23 PM, Thomas Hellström (VMware) wrote:
> On 12/6/19 2:53 PM, Steven Price wrote:
> > pgd_entry() and pud_entry() were removed by commit 0b1fbfe50006c410
> > ("mm/pagewalk: remove pgd_entry() and pud_entry()") because there were
> > no users. We're about to add users so reintroduce them, along with
> > p4d_entry() as we now have 5 levels of tables.
> >
> > Note that commit a00cc7d9dd93d66a ("mm, x86: add support for
> > PUD-sized transparent hugepages") already re-added pud_entry() but with
> > different semantics to the other callbacks. Since there have never
> > been upstream users of this, revert the semantics back to match the
> > other callbacks. This means pud_entry() is called for all entries, not
> > just transparent huge pages.
>
> Actually, there are two users of pud_entry(), in hmm.c and since 
> 5.5rc1 also mapping_dirty_helpers.c. The latter one is unproblematic 
> and requires no attention but the one in hmm.c is probably largely 
> untested, and seems to assume it was called outside of the spinlock.
>
> The problem with the current patch is that the hmm pud_entry will 
> traverse also pmds, so that will be done twice now.
>
> In another thread we were discussing a means of rerunning the level 
> (in case of a race), or continuing after a level, based on the return 
> value after the callback. The change was fairly invasive,
Hmm. Forgot to remove the above text that appears twice. :(. The correct 
one is inline below.

> > Tested-by: Zong Li <zong.li@xxxxxxxxxx>
> > Signed-off-by: Steven Price <steven.price@xxxxxxx>
> > ---
> >   include/linux/pagewalk.h | 19 +++++++++++++------
> >   mm/pagewalk.c            | 27 ++++++++++++++++-----------
> >   2 files changed, 29 insertions(+), 17 deletions(-)
> >
> > diff --git a/include/linux/pagewalk.h b/include/linux/pagewalk.h
> > index 6ec82e92c87f..06790f23957f 100644
> > --- a/include/linux/pagewalk.h
> > +++ b/include/linux/pagewalk.h
> > @@ -8,15 +8,15 @@ struct mm_walk;
> >     /**
> >    * mm_walk_ops - callbacks for walk_page_range
> > - * @pud_entry:        if set, called for each non-empty PUD 
> > (2nd-level) entry
> > - *            this handler should only handle pud_trans_huge() puds.
> > - *            the pmd_entry or pte_entry callbacks will be used for
> > - *            regular PUDs.
> > - * @pmd_entry:        if set, called for each non-empty PMD 
> > (3rd-level) entry
> > + * @pgd_entry:        if set, called for each non-empty PGD 
> > (top-level) entry
> > + * @p4d_entry:        if set, called for each non-empty P4D entry
> > + * @pud_entry:        if set, called for each non-empty PUD entry
> > + * @pmd_entry:        if set, called for each non-empty PMD entry
> >    *            this handler is required to be able to handle
> >    *            pmd_trans_huge() pmds.  They may simply choose to
> >    *            split_huge_page() instead of handling it explicitly.
> > - * @pte_entry:        if set, called for each non-empty PTE 
> > (4th-level) entry
> > + * @pte_entry:        if set, called for each non-empty PTE 
> > (lowest-level)
> > + *            entry
> >    * @pte_hole:        if set, called for each hole at all levels
> >    * @hugetlb_entry:    if set, called for each hugetlb entry
> >    * @test_walk:        caller specific callback function to 
> > determine whether
> > @@ -27,8 +27,15 @@ struct mm_walk;
> >    * @pre_vma:            if set, called before starting walk on a 
> > non-null vma.
> >    * @post_vma:           if set, called after a walk on a non-null 
> > vma, provided
> >    *                      that @pre_vma and the vma walk succeeded.
> > + *
> > + * p?d_entry callbacks are called even if those levels are folded on a
> > + * particular architecture/configuration.
> >    */
> >   struct mm_walk_ops {
> > +    int (*pgd_entry)(pgd_t *pgd, unsigned long addr,
> > +             unsigned long next, struct mm_walk *walk);
> > +    int (*p4d_entry)(p4d_t *p4d, unsigned long addr,
> > +             unsigned long next, struct mm_walk *walk);
> >       int (*pud_entry)(pud_t *pud, unsigned long addr,
> >                unsigned long next, struct mm_walk *walk);
> >       int (*pmd_entry)(pmd_t *pmd, unsigned long addr,
> > diff --git a/mm/pagewalk.c b/mm/pagewalk.c
> > index ea0b9e606ad1..c089786e7a7f 100644
> > --- a/mm/pagewalk.c
> > +++ b/mm/pagewalk.c
> > @@ -94,15 +94,9 @@ static int walk_pud_range(p4d_t *p4d, unsigned 
> > long addr, unsigned long end,
> >           }
> >             if (ops->pud_entry) {
> > -            spinlock_t *ptl = pud_trans_huge_lock(pud, walk->vma);
> > -
> > -            if (ptl) {
> > -                err = ops->pud_entry(pud, addr, next, walk);
> > -                spin_unlock(ptl);
> > -                if (err)
> > -                    break;
> > -                continue;
> > -            }
> > +            err = ops->pud_entry(pud, addr, next, walk);
> > +            if (err)
> > +                break;
>
> Actually, there are two current users of pud_entry(), in hmm.c and 
> since 5.5rc1 also mapping_dirty_helpers.c. The latter one is 
> unproblematic and requires no attention but the one in hmm.c is 
> probably largely untested, and seems to assume it was called outside 
> of the spinlock.
>
> The problem with the current patch is that the hmm pud_entry will 
> traverse also pmds, so that will now be done twice.
>
> /Thomas