Re: [PATCH mm-unstable 2/4] mm/khugepaged: consistently order cc->is_khugepaged and pte_* checks

[Zach O'Keefe <zokeefe@google.com>]

On Jul 20 10:27, Yang Shi wrote:
> On Wed, Jul 20, 2022 at 7:06 AM Zach O'Keefe <zokeefe@google.com> wrote:
> >
> > cc->is_khugepaged is used to predicate the khugepaged-only behavior
> > of enforcing khugepaged heuristics limited by the sysfs knobs
> > khugepaged_max_ptes_[none|swap|shared].
> >
> > In branches where khugepaged_max_ptes_* is checked, consistently check
> > cc->is_khugepaged first.  Also, local counters (for comparison vs
> > khugepaged_max_ptes_* limits) were previously incremented in the
> > comparison expression.  Some of these counters (unmapped) are
> > additionally used outside of khugepaged_max_ptes_* enforcement, and
> > all counters are communicated in tracepoints.  Move the correct
> > accounting of these counters before branching statements to avoid future
> > errors due to C's short-circuiting evaluation.
> 
> Yeah, it is safer to not depend on the order of branch statements to
> inc the counter.
> 

Only cost me a couple hours when I got bit by this after naively moving checks
around :) Hopefully I can save the next person.

Also, thanks for the reviews, Yang!

> Reviewed-by: Yang Shi <shy828301@gmail.com>
> 
> >
> > Fixes: 9fab4752a181 ("mm/khugepaged: add flag to predicate khugepaged-only behavior")
> > Link: https://lore.kernel.org/linux-mm/Ys2qJm6FaOQcxkha@google.com/
> > Signed-off-by: Zach O'Keefe <zokeefe@google.com>
> > ---
> >  mm/khugepaged.c | 49 +++++++++++++++++++++++++++++--------------------
> >  1 file changed, 29 insertions(+), 20 deletions(-)
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index ecd28bfeab60..290422577172 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -574,9 +574,10 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
> >                 pte_t pteval = *_pte;
> >                 if (pte_none(pteval) || (pte_present(pteval) &&
> >                                 is_zero_pfn(pte_pfn(pteval)))) {
> > +                       ++none_or_zero;
> >                         if (!userfaultfd_armed(vma) &&
> > -                           (++none_or_zero <= khugepaged_max_ptes_none ||
> > -                            !cc->is_khugepaged)) {
> > +                           (!cc->is_khugepaged ||
> > +                            none_or_zero <= khugepaged_max_ptes_none)) {
> >                                 continue;
> >                         } else {
> >                                 result = SCAN_EXCEED_NONE_PTE;
> > @@ -596,11 +597,14 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
> >
> >                 VM_BUG_ON_PAGE(!PageAnon(page), page);
> >
> > -               if (cc->is_khugepaged && page_mapcount(page) > 1 &&
> > -                   ++shared > khugepaged_max_ptes_shared) {
> > -                       result = SCAN_EXCEED_SHARED_PTE;
> > -                       count_vm_event(THP_SCAN_EXCEED_SHARED_PTE);
> > -                       goto out;
> > +               if (page_mapcount(page) > 1) {
> > +                       ++shared;
> > +                       if (cc->is_khugepaged &&
> > +                           shared > khugepaged_max_ptes_shared) {
> > +                               result = SCAN_EXCEED_SHARED_PTE;
> > +                               count_vm_event(THP_SCAN_EXCEED_SHARED_PTE);
> > +                               goto out;
> > +                       }
> >                 }
> >
> >                 if (PageCompound(page)) {
> > @@ -1170,8 +1174,9 @@ static int hpage_collapse_scan_pmd(struct mm_struct *mm,
> >              _pte++, _address += PAGE_SIZE) {
> >                 pte_t pteval = *_pte;
> >                 if (is_swap_pte(pteval)) {
> > -                       if (++unmapped <= khugepaged_max_ptes_swap ||
> > -                           !cc->is_khugepaged) {
> > +                       ++unmapped;
> > +                       if (!cc->is_khugepaged ||
> > +                           unmapped <= khugepaged_max_ptes_swap) {
> >                                 /*
> >                                  * Always be strict with uffd-wp
> >                                  * enabled swap entries.  Please see
> > @@ -1189,9 +1194,10 @@ static int hpage_collapse_scan_pmd(struct mm_struct *mm,
> >                         }
> >                 }
> >                 if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
> > +                       ++none_or_zero;
> >                         if (!userfaultfd_armed(vma) &&
> > -                           (++none_or_zero <= khugepaged_max_ptes_none ||
> > -                            !cc->is_khugepaged)) {
> > +                           (!cc->is_khugepaged ||
> > +                            none_or_zero <= khugepaged_max_ptes_none)) {
> >                                 continue;
> >                         } else {
> >                                 result = SCAN_EXCEED_NONE_PTE;
> > @@ -1221,12 +1227,14 @@ static int hpage_collapse_scan_pmd(struct mm_struct *mm,
> >                         goto out_unmap;
> >                 }
> >
> > -               if (cc->is_khugepaged &&
> > -                   page_mapcount(page) > 1 &&
> > -                   ++shared > khugepaged_max_ptes_shared) {
> > -                       result = SCAN_EXCEED_SHARED_PTE;
> > -                       count_vm_event(THP_SCAN_EXCEED_SHARED_PTE);
> > -                       goto out_unmap;
> > +               if (page_mapcount(page) > 1) {
> > +                       ++shared;
> > +                       if (cc->is_khugepaged &&
> > +                           shared > khugepaged_max_ptes_shared) {
> > +                               result = SCAN_EXCEED_SHARED_PTE;
> > +                               count_vm_event(THP_SCAN_EXCEED_SHARED_PTE);
> > +                               goto out_unmap;
> > +                       }
> >                 }
> >
> >                 page = compound_head(page);
> > @@ -1961,8 +1969,9 @@ static int khugepaged_scan_file(struct mm_struct *mm, struct file *file,
> >                         continue;
> >
> >                 if (xa_is_value(page)) {
> > +                       ++swap;
> >                         if (cc->is_khugepaged &&
> > -                           ++swap > khugepaged_max_ptes_swap) {
> > +                           swap > khugepaged_max_ptes_swap) {
> >                                 result = SCAN_EXCEED_SWAP_PTE;
> >                                 count_vm_event(THP_SCAN_EXCEED_SWAP_PTE);
> >                                 break;
> > @@ -2013,8 +2022,8 @@ static int khugepaged_scan_file(struct mm_struct *mm, struct file *file,
> >         rcu_read_unlock();
> >
> >         if (result == SCAN_SUCCEED) {
> > -               if (present < HPAGE_PMD_NR - khugepaged_max_ptes_none &&
> > -                   cc->is_khugepaged) {
> > +               if (cc->is_khugepaged &&
> > +                   present < HPAGE_PMD_NR - khugepaged_max_ptes_none) {
> >                         result = SCAN_EXCEED_NONE_PTE;
> >                         count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
> >                 } else {
> > --
> > 2.37.0.170.g444d1eabd0-goog
> >