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X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Thu, Sep 18, 2025 at 01:04:31PM +0800, Lance Yang wrote: > From: Lance Yang > > Guard PTE markers are installed via MADV_GUARD_INSTALL to create > lightweight guard regions. > > Currently, any collapse path (khugepaged or MADV_COLLAPSE) will fail when > encountering such a range. > > MADV_COLLAPSE fails deep inside the collapse logic when trying to swap-in > the special marker in __collapse_huge_page_swapin(). > > hpage_collapse_scan_pmd() > `- collapse_huge_page() > `- __collapse_huge_page_swapin() -> fails! > > khugepaged's behavior is slightly different due to its max_ptes_swap limit > (default 64). 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 SCAN_PTE_NON_PRESENT, > avoiding wasted work. > > Suggested-by: Lorenzo Stoakes > Signed-off-by: Lance Yang LGTM, so: Reviewed-by: Lorenzo Stoakes > --- > mm/khugepaged.c | 10 ++++++++++ > 1 file changed, 10 insertions(+) > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > index 9ed1af2b5c38..70ebfc7c1f3e 100644 > --- a/mm/khugepaged.c > +++ b/mm/khugepaged.c > @@ -1306,6 +1306,16 @@ 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_NON_PRESENT; > + goto out_unmap; > + } > continue; > } else { > result = SCAN_EXCEED_SWAP_PTE; > -- > 2.49.0 >