Will Deacon [off-list ref] writes:
On Wed, Jun 07, 2017 at 03:30:37PM +0100, Catalin Marinas wrote:
quoted
On Wed, Jun 07, 2017 at 02:47:32PM +0100, Will Deacon wrote:
quoted
On Wed, May 17, 2017 at 04:23:34PM +0100, Punit Agrawal wrote:
quoted
--- a/arch/arm64/mm/hugetlbpage.c
+++ b/arch/arm64/mm/hugetlbpage.c
@@ -136,36 +136,27 @@ pte_t *huge_pte_offset(struct mm_struct *mm, unsigned long addr)
{
pgd_t *pgd;
pud_t *pud;
- pmd_t *pmd = NULL;
- pte_t *pte = NULL;
+ pmd_t *pmd;
pgd = pgd_offset(mm, addr);
pr_debug("%s: addr:0x%lx pgd:%p\n", __func__, addr, pgd);
if (!pgd_present(*pgd))
return NULL;
+
pud = pud_offset(pgd, addr);
- if (!pud_present(*pud))
+ if (pud_none(*pud))
return NULL;
Do you actually need this special case?
quoted
-
- if (pud_huge(*pud))
+ /* swap or huge page */
+ if (!pud_present(*pud) || pud_huge(*pud))
... couldn't you just add a '|| pud_none(*pud)' in here?
I think an earlier version took this approach but...
quoted
quoted
quoted
return (pte_t *)pud;
But then you no longer return NULL if *pud == 0.
Does that actually matter? The bits of hugetlb code I looked at will
deferenced the returned pud and handle the huge_pte_none case correctly.
For hugetlb fault handling (hugetlb_fault()), returning NULL vs pointer
to the pud/pmd results in different behaviour. If we return the pud when
pud_none(), then we lose the resulting hugepage size check we get from
huge_pte_alloc().
Will
On Wed, Jun 07, 2017 at 04:32:28PM +0100, Punit Agrawal wrote:
Will Deacon [off-list ref] writes:
quoted
On Wed, Jun 07, 2017 at 03:30:37PM +0100, Catalin Marinas wrote:
quoted
On Wed, Jun 07, 2017 at 02:47:32PM +0100, Will Deacon wrote:
quoted
On Wed, May 17, 2017 at 04:23:34PM +0100, Punit Agrawal wrote:
quoted
--- a/arch/arm64/mm/hugetlbpage.c
+++ b/arch/arm64/mm/hugetlbpage.c
@@ -136,36 +136,27 @@ pte_t *huge_pte_offset(struct mm_struct *mm, unsigned long addr)
{
pgd_t *pgd;
pud_t *pud;
- pmd_t *pmd = NULL;
- pte_t *pte = NULL;
+ pmd_t *pmd;
pgd = pgd_offset(mm, addr);
pr_debug("%s: addr:0x%lx pgd:%p\n", __func__, addr, pgd);
if (!pgd_present(*pgd))
return NULL;
+
pud = pud_offset(pgd, addr);
- if (!pud_present(*pud))
+ if (pud_none(*pud))
return NULL;
Do you actually need this special case?
quoted
-
- if (pud_huge(*pud))
+ /* swap or huge page */
+ if (!pud_present(*pud) || pud_huge(*pud))
... couldn't you just add a '|| pud_none(*pud)' in here?
I think an earlier version took this approach but...
quoted
quoted
quoted
quoted
return (pte_t *)pud;
But then you no longer return NULL if *pud == 0.
Does that actually matter? The bits of hugetlb code I looked at will
deferenced the returned pud and handle the huge_pte_none case correctly.
For hugetlb fault handling (hugetlb_fault()), returning NULL vs pointer
to the pud/pmd results in different behaviour. If we return the pud when
pud_none(), then we lose the resulting hugepage size check we get from
huge_pte_alloc().
Ok, so does that mean that many of the huge_pte_none checks in mm/hugetlb.c
that operate on a huge_ptep_get of non-NULL output from huge_pte_offset are
actually redundant?
Will
On Wed, Jun 07, 2017 at 04:32:28PM +0100, Punit Agrawal wrote:
Will Deacon [off-list ref] writes:
quoted
On Wed, Jun 07, 2017 at 03:30:37PM +0100, Catalin Marinas wrote:
quoted
On Wed, Jun 07, 2017 at 02:47:32PM +0100, Will Deacon wrote:
quoted
On Wed, May 17, 2017 at 04:23:34PM +0100, Punit Agrawal wrote:
quoted
--- a/arch/arm64/mm/hugetlbpage.c
+++ b/arch/arm64/mm/hugetlbpage.c
@@ -136,36 +136,27 @@ pte_t *huge_pte_offset(struct mm_struct *mm, unsigned long addr)
{
pgd_t *pgd;
pud_t *pud;
- pmd_t *pmd = NULL;
- pte_t *pte = NULL;
+ pmd_t *pmd;
pgd = pgd_offset(mm, addr);
pr_debug("%s: addr:0x%lx pgd:%p\n", __func__, addr, pgd);
if (!pgd_present(*pgd))
return NULL;
+
pud = pud_offset(pgd, addr);
- if (!pud_present(*pud))
+ if (pud_none(*pud))
return NULL;
Do you actually need this special case?
quoted
-
- if (pud_huge(*pud))
+ /* swap or huge page */
+ if (!pud_present(*pud) || pud_huge(*pud))
... couldn't you just add a '|| pud_none(*pud)' in here?
I think an earlier version took this approach but...
quoted
quoted
quoted
quoted
return (pte_t *)pud;
But then you no longer return NULL if *pud == 0.
Does that actually matter? The bits of hugetlb code I looked at will
deferenced the returned pud and handle the huge_pte_none case correctly.
For hugetlb fault handling (hugetlb_fault()), returning NULL vs pointer
to the pud/pmd results in different behaviour. If we return the pud when
pud_none(), then we lose the resulting hugepage size check we get from
huge_pte_alloc().
At a quick look, there are a few other places where not returning NULL
has some other effects (though I don't think any of them are fatal):
- copy_huge_tlb_page_range() - unnecessary allocation of a destination
pud
- huge_pmd_share() - do we actually need the subsequent get_page()?
- page_vma_mapped_walk() - it even has a comment: "when pud is not
present, pte will be NULL". Now, that's no longer true with swap
entries but we'd never return NULL for a pud_none() case
Current code behaviour is to return NULL when !p*d_present(). We are
slightly relaxing this for swap entries while still returning NULL for
the p*d_none() case but I wouldn't go that far as to never return NULL
here.
--
Catalin