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It won't fail as deep, but it will still needlessly scan up > to 64 swap entries before bailing out. > > IMHO, we can and should detect this much earlier ;) > > This patch adds a check directly inside the PTE scan loop. If a guard > marker is found, the scan is aborted immediately with a new SCAN_PTE_GUARD > status, avoiding wasted work. > > Signed-off-by: Lance Yang > --- > mm/khugepaged.c | 12 ++++++++++++ > 1 file changed, 12 insertions(+) > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > index e54f99bb0b57..910a6f2ec8a9 100644 > --- a/mm/khugepaged.c > +++ b/mm/khugepaged.c > @@ -59,6 +59,7 @@ enum scan_result { > SCAN_STORE_FAILED, > SCAN_COPY_MC, > SCAN_PAGE_FILLED, > + SCAN_PTE_GUARD, > }; > > #define CREATE_TRACE_POINTS > @@ -1317,6 +1318,16 @@ static int hpage_collapse_scan_pmd(struct mm_struct *mm, > result = SCAN_PTE_UFFD_WP; > goto out_unmap; > } > + /* > + * Guard PTE markers are installed by > + * MADV_GUARD_INSTALL. Any collapse path must > + * not touch them, so abort the scan immediately > + * if one is found. > + */ > + if (is_guard_pte_marker(pteval)) { > + result = SCAN_PTE_GUARD; > + goto out_unmap; > + } > continue; This looks good, but see below. > } else { > result = SCAN_EXCEED_SWAP_PTE; > @@ -2860,6 +2871,7 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start, > case SCAN_PAGE_COMPOUND: > case SCAN_PAGE_LRU: > case SCAN_DEL_PAGE_LRU: > + case SCAN_PTE_GUARD: > last_fail = result; Should we not do this, and just send this case over to the default case. That would mean immediate exit with -EINVAL, instead of iterating over the complete range, potentially collapsing a non-guard range, and returning -EINVAL. I do not think we should spend a significant time in the kernel when the user is literally invoking madvise(MADV_GUARD_INSTALL) and madvise(MADV_COLLAPSE) on overlapping regions. > break; > default: