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U2FsdGVkX18odDMwlKQZm3TUbYxrQnqsF3g+/+RNfi1dNp2xM0CzuDt4iNqpQtbnRtETSlPtz6eXsQfki8pwaACiostkrR7E6OFy2zp3rIfsLK/033Q63t9MTM8JB/qZkezgg9AgP9YY4Q8TuaoWHIUJGHN3d32IDhTATxTqBEMJGOWC3HRiqNe4gvQYwugkDTzO63Uult5At40CMGmoql2qF5/LdMysJCGE90TarQHgAJd6ZnDO+GPhUnj2rse1i3IGlJv6n03NWin0DESJFhDYl4PKc21B9tsdl48BfVak24ZRFXe7BYDyaZ6f+VJBuEfgGWqhpKqPuHv/nTiOJpdVE1sq0mQ2vyg36woqL0GOs/YS+pC4R4xhTz7FRv5chWSeqyu1KRWMrqxlgnvG76yf3fqo2cjTGQ8TrPnUttTfU3ARiBuXe/u4igoeZEQsL0nse8B8HMVE9GSN+c4RlsUNciUIuM7pxtUz0nUwcsZil19fl/flTsnGTuDXZKXQ0Kkkt0QXXK1JjDMxrYgEnRGpfCcrEp4Ax2QC7wmQca01s0/+WkuW0utaU0NRK3gO6O0n1XWJUR1k5tS17wO2u2Rxbc7MSVCf4tFcyCaFL3UcAzDwlbY+P/YWBXzdd3dthss/5XMBnvGwL9HhHEZ09UlCaDfPCLR4ZlH/WhVQMyEkI+pSMgANR6kTKGeBsFgdLKlXOrTMKM83UlEiE+2Qv3Q2xEb47nHpezjC8qjrzGTg1V8+ItN7mM0x56lJZVeqaE8GC7oU8KebVDjUN9D/r3H+bzzvtsPOpsmotQNwDABAWXjIYAhKIxqjxxWIHB8NhtwQOppgd6fuxiIvCcfgdn/tj01BQ7/TKRVWj29xBTGOIXJa5xSe7K1huqpMFIuhVSi3LKZ3VX/aFogLfXz4hLdb1werjl/j75gH2vfzrLLA7tUm/3QVouS2W6eAoL1jxfCu33FmXeW/h/Pi+J1 GjE4ZfxK 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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 ;) No smileys in commit messages please... :) > > 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, I wonder if we really need to have it literally called out though, can we just use: SCAN_PTE_NON_PRESENT Instead? As it is, indeed, non-present :) > }; > > #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; > } 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; > break; > default: > -- > 2.49.0 > Cheers, Lorenzo