On Jul 20 10:27, Yang Shi wrote:
> On Wed, Jul 20, 2022 at 7:06 AM Zach O'Keefe <zokeefe@xxxxxxxxxx> 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@xxxxxxxxx>
>
> >
> > Fixes: 9fab4752a181 ("mm/khugepaged: add flag to predicate khugepaged-only behavior")
> > Link: https://lore.kernel.org/linux-mm/Ys2qJm6FaOQcxkha@xxxxxxxxxx/
> > Signed-off-by: Zach O'Keefe <zokeefe@xxxxxxxxxx>
> > ---
> > 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
> >
