Re: [PATCH v5 10/19] mm: pagewalk: Add p4d_entry() and pgd_entry()

[Mike Rapoport]

On Fri, Mar 22, 2019 at 10:11:59AM +0000, Steven Price wrote:
> On 21/03/2019 21:15, Mike Rapoport wrote:
> > On Thu, Mar 21, 2019 at 02:19:44PM +0000, 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.
> >>
> >> Signed-off-by: Steven Price <steven.price@xxxxxxx>
> >> ---
> >>  include/linux/mm.h |  9 ++++++---
> >>  mm/pagewalk.c      | 27 ++++++++++++++++-----------
> >>  2 files changed, 22 insertions(+), 14 deletions(-)
> >>
> >> diff --git a/include/linux/mm.h b/include/linux/mm.h
> >> index 76769749b5a5..2983f2396a72 100644
> >> --- a/include/linux/mm.h
> >> +++ b/include/linux/mm.h
> >> @@ -1367,10 +1367,9 @@ void unmap_vmas(struct mmu_gather *tlb, struct vm_area_struct *start_vma,
> >>
> >>  /**
> >>   * mm_walk - callbacks for walk_page_range
> >> + * @pgd_entry: if set, called for each non-empty PGD (top-level) entry
> >> + * @p4d_entry: if set, called for each non-empty P4D (1st-level) entry
> > 
> > IMHO, p4d implies the 4th level :)
> 
> You have a good point there... I was simply working back from the
> existing definitions (below) of PTE:4th, PMD:3rd, PUD:2nd. But it's
> already somewhat broken by PGD:0th and my cop-out was calling it "top".
> 
> > I think it would make more sense to start counting from PTE rather than
> > from PGD. Then it would be consistent across architectures with fewer
> > levels.
> 
> It would also be the opposite way round to architectures such as Arm
> which number their levels, for example [1] refers to levels 0-3 (with 3
> being PTE in Linux terms).

By consistent I meant that for architectures with fewer levels we won't be
describing PTE as level 4 when the architecture only has 2 levels.
 
> [1]
> https://developer.arm.com/docs/100940/latest/translation-tables-in-armv8-a
> 
> Probably the least confusing thing is to drop the level numbers in
> brackets since I don't believe they directly match any architecture, and
> hopefully any user of the page walking code is already familiar with the
> P?D terms used by the kernel.

That's a fair assumption :)
Still, maybe we keep your (top-level) for PGD and use (lowest level) for
PTE and drop those in the middle?

> Steve
> 
> >>   * @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
> >>   *	       this handler is required to be able to handle
> >>   *	       pmd_trans_huge() pmds.  They may simply choose to
> >> @@ -1390,6 +1389,10 @@ void unmap_vmas(struct mmu_gather *tlb, struct vm_area_struct *start_vma,
> >>   * (see the comment on walk_page_range() for more details)
> >>   */
> >>  struct mm_walk {
> >> +	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 c3084ff2569d..98373a9f88b8 100644
> >> --- a/mm/pagewalk.c
> >> +++ b/mm/pagewalk.c
> >> @@ -90,15 +90,9 @@ static int walk_pud_range(p4d_t *p4d, unsigned long addr, unsigned long end,
> >>  		}
> >>
> >>  		if (walk->pud_entry) {
> >> -			spinlock_t *ptl = pud_trans_huge_lock(pud, walk->vma);
> >> -
> >> -			if (ptl) {
> >> -				err = walk->pud_entry(pud, addr, next, walk);
> >> -				spin_unlock(ptl);
> >> -				if (err)
> >> -					break;
> >> -				continue;
> >> -			}
> >> +			err = walk->pud_entry(pud, addr, next, walk);
> >> +			if (err)
> >> +				break;
> >>  		}
> >>
> >>  		split_huge_pud(walk->vma, pud, addr);
> >> @@ -131,7 +125,12 @@ static int walk_p4d_range(pgd_t *pgd, unsigned long addr, unsigned long end,
> >>  				break;
> >>  			continue;
> >>  		}
> >> -		if (walk->pmd_entry || walk->pte_entry)
> >> +		if (walk->p4d_entry) {
> >> +			err = walk->p4d_entry(p4d, addr, next, walk);
> >> +			if (err)
> >> +				break;
> >> +		}
> >> +		if (walk->pud_entry || walk->pmd_entry || walk->pte_entry)
> >>  			err = walk_pud_range(p4d, addr, next, walk);
> >>  		if (err)
> >>  			break;
> >> @@ -157,7 +156,13 @@ static int walk_pgd_range(unsigned long addr, unsigned long end,
> >>  				break;
> >>  			continue;
> >>  		}
> >> -		if (walk->pmd_entry || walk->pte_entry)
> >> +		if (walk->pgd_entry) {
> >> +			err = walk->pgd_entry(pgd, addr, next, walk);
> >> +			if (err)
> >> +				break;
> >> +		}
> >> +		if (walk->p4d_entry || walk->pud_entry || walk->pmd_entry ||
> >> +				walk->pte_entry)
> >>  			err = walk_p4d_range(pgd, addr, next, walk);
> >>  		if (err)
> >>  			break;
> >> -- 
> >> 2.20.1
> >>
> > 
> 

-- 
Sincerely yours,
Mike.