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 GNITiL7l tYnzPeYcEsEVqJg6m5Xwh1OPJ05qG9VhrkHsLhVUArDyIrgVZo2tUqhbox8S3MBjgJlQ6FM6Kwz4aE2QJaogsOMaG3Eox9N6faTkHd+F/SKu0V1LqtlDhSL95GvQjkbZQF18xFuWAGCCQKgGnfySlmXl+3klPC/6jnG91beUBKjEXSWP+Pxlg1Rn4Oo+ULWy7Kkx94NcE7amcOjuyIXjUVyXinpm5AbSPqyd9rCsR8g0xOmEdpueCv3aKML99yCI16v2wRqppzxWCLrRybxfPMld92qiqjLRfX+x2/Z9Q0b/ewYh+SS/kLoHzkJxGEkfSK+O3EJ6i1MWRkSNulVYCyY+eG/dCwpacl+wOuWO7GgeAVLvHycL1hTQUjw== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed Aug 12, 2026 at 5:51 AM EDT, David Hildenbrand (Arm) wrote: >> Huh, that looks odd. >>=20 >> That should just be a VM_WARN_ON_FOLIO(true, ..) or sth like that. >>=20 >> But in collapse_scan_pmd() that warning never existed? So this raises ey= ebrows. >>=20 >> [...] >>=20 >> I'll play with it to see if we can do better and will reply here later. > > > Okay, I think below is what we should do. > > There is one behavioral change: we now longer trace the last folio, which= is the right > thing to do IMHO. > > And I think there is one fix we should pull out and evaluate first: A zer= opage with a > uffd-wp marker is not checked properly, IIUC. > > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > index 5a06e3942e889..8a223659fce2d 100644 > --- a/mm/khugepaged.c > +++ b/mm/khugepaged.c > @@ -119,6 +119,18 @@ struct collapse_control { > DECLARE_BITMAP(mthp_present_ptes, MAX_PTRS_PER_PTE); > }; > =20 > +struct collapse_anon_pte_check_ctx { > + struct collapse_control *cc; > + struct vm_area_struct *vma; > + unsigned int order; > + int none_or_zero_ptes; > + int nonpresent_ptes; > + int shared_ptes; > + unsigned int max_ptes_none; > + int max_ptes_swap; > + unsigned int max_ptes_shared; > +}; > + > /** > * struct khugepaged_scan - cursor for scanning > * @mm_head: the head of the mm list to scan > @@ -696,74 +708,104 @@ static void count_collapse_event(unsigned int orde= r, enum vm_event_item vm_event > count_mthp_stat(order, mthp_event); > } > =20 > +static enum scan_result collapse_anon_pte_check(pte_t pteval, > + unsigned long addr, struct collapse_anon_pte_check_ctx *ctx, > + struct folio **foliop) > +{ > + *foliop =3D NULL; > + > + /* > + * Don't collapse if any of the small PTEs are armed with uffd > + * write protection. Marking the new huge pmd as write protected > + * could bring userfault messages that fall outside of the > + * registered range. > + */ > + if ((pte_present(pteval) && pte_uffd(pteval)) || > + (!pte_present(pteval) && pte_swp_uffd_any(pteval))) > + return SCAN_PTE_UFFD; > + > + if (pte_none_or_zero(pteval)) { > + if (++ctx->none_or_zero_ptes > ctx->max_ptes_none) { > + count_collapse_event(ctx->order, THP_SCAN_EXCEED_NONE_PTE, > + MTHP_STAT_COLLAPSE_EXCEED_NONE); > + return SCAN_EXCEED_NONE_PTE; > + } > + return SCAN_SUCCEED; > + } > + if (!pte_present(pteval)) { > + if (ctx->max_ptes_swap < 0) > + return SCAN_PTE_NON_PRESENT; > + if (++ctx->nonpresent_ptes > ctx->max_ptes_swap) { So max_ptes_swap is actually max_ptes_nonpresent. But due to khugepaged's max_ptes_swap config name, we just keep the variable and related function names that way? > + count_collapse_event(ctx->order, THP_SCAN_EXCEED_SWAP_PTE, > + MTHP_STAT_COLLAPSE_EXCEED_SWAP); > + return SCAN_EXCEED_SWAP_PTE; > + } > + return SCAN_SUCCEED; > + } > + > + *foliop =3D vm_normal_folio(ctx->vma, addr, pteval); > + if (unlikely(!*foliop) || unlikely(folio_is_zone_device(*foliop))) > + return SCAN_PAGE_NULL; > + > + /* > + * If the vma has the VM_DROPPABLE flag, the collapse will > + * preserve the lazyfree property without needing to skip. > + */ > + if (ctx->cc->is_khugepaged && !(ctx->vma->vm_flags & VM_DROPPABLE) && > + folio_test_lazyfree(*foliop) && !pte_dirty(pteval)) > + return SCAN_PAGE_LAZYFREE; > + > + if (!folio_test_anon(*foliop)) > + return SCAN_PAGE_ANON; > + > + if (folio_maybe_mapped_shared(*foliop)) { > + /* > + * TODO: Support shared pages without leading to further > + * mTHP collapses. Currently bringing in new pages via > + * shared may cause a future higher order collapse on a > + * rescan of the same range. > + */ > + if (++ctx->shared_ptes > ctx->max_ptes_shared) { > + count_collapse_event(ctx->order, THP_SCAN_EXCEED_SHARED_PTE, > + MTHP_STAT_COLLAPSE_EXCEED_SHARED); > + return SCAN_EXCEED_SHARED_PTE; > + } > + } > + > + return SCAN_SUCCEED; > +} > + > static enum scan_result __collapse_huge_page_isolate(struct vm_area_stru= ct *vma, > unsigned long start_addr, pte_t *pte, struct collapse_control *cc, > unsigned int order, struct list_head *compound_pagelist) > { > + result =3D collapse_anon_pte_check(pteval, addr, &ctx, &folio); > + if (result !=3D SCAN_SUCCEED) { > + VM_WARN_ON_ONCE(result =3D=3D SCAN_PAGE_ANON); > goto out; > } > + if (!folio) > + continue; SCAN_SUCCEED + folio !=3D NULL means to proceed with the pte, while SCAN_SUCCEED + folio =3D=3D NULL means to skip the pte. collapse_anon_pte_check() probably needs to document this? Yes, it is straightforward at the call sites, since without a folio the following code cannot be executed. Or you think that is self-documented. --=20 Best Regards, Yan, Zi