On Mon, Aug 10, 2026 at 10:26:27AM +0800, Huang, Ying wrote: > Hi, Matthew, > > Matthew Brost <[email protected]> writes: > > > When a CPU faults on a device private PMD and the device driver can only > > allocate order-0 destination folios, __migrate_device_pages() has to > > split the source THP via migrate_vma_split_unmapped_folio(). That path > > is broken in two independent ways when the fault is what triggered the > > migration. > > > > First, the split never succeeds. At the point folio_split_unmapped() is > > called the folio carries two references beyond the ones it is > > entitled to: > > > > 1 - taken by do_huge_pmd_device_private() for the duration of the > > ->migrate_to_ram() callback > > 2 - taken by migrate_vma_collect_huge_pmd() when the folio was > > collected > > > > (the mapping reference having been dropped by set_pmd_migration_entry()). > > > > folio_split_unmapped() requires folio_expected_ref_count(folio) == > > folio_ref_count(folio) - 1, i.e. it tolerates exactly one caller > > reference. With both of the above held the check sees 2 against an > > expected 0 and returns -EAGAIN, so the migration is abandoned and the > > CPU fault makes no progress. > > > > The PTE-based split path does not have this problem: > > migrate_vma_split_folio() is called before any collect reference is > > taken and explicitly skips folio_get() for the fault folio, so the fault > > reference is the single caller reference the split expects. > > > > Fix it by dropping the fault reference across the split and re-taking it > > afterwards. do_huge_pmd_device_private() derives the fault page from the > > PMD entry, so it is always the head page of the folio and always ends up > > in the head folio of an uniform split to order 0; re-taking the > > reference on the folio therefore puts it back exactly where > > do_huge_pmd_device_private() will release it. The folio cannot be freed > > while the reference is dropped because the collect reference is still > > held. > > > > Second, the folio is split globally but the page tables were demoted > > only locally: > > > > split_huge_pmd_address(migrate->vma, addr, true); > > ret = folio_split_unmapped(folio, 0); > > > > migrate_device_unmap() unmaps via try_to_migrate(folio, 0), deliberately > > without TTU_SPLIT_HUGE_PMD, so every VMA that PMD maps the folio is left > > holding a PMD sized migration entry. A folio that was PMD mapped in more > > than one VMA -- after fork(), for example -- therefore keeps huge > > migration entries in all the other VMAs while only migrate->vma is > > demoted. > > > > folio_split_unmapped() does not notice: the folio is fully unmapped, so > > it only looks at the refcount and happily splits to order 0. The other > > VMAs are then left pointing a huge PMD at an order-0 folio, and > > migrate_vma_finalize() -> remove_migration_ptes() walks into it: > > > > page dumped because: VM_BUG_ON_FOLIO(folio_test_hugetlb(folio) || > > !folio_test_pmd_mappable(folio)) > > kernel BUG at mm/migrate.c:368! > > RIP: 0010:remove_migration_pte+0x56a/0x9b0 > > Call Trace: > > rmap_walk_anon+0xfc/0x260 > > remove_migration_ptes+0x79/0xb0 > > __migrate_device_finalize+0x113/0x290 > > __drm_pagemap_migrate_to_ram+0x278/0x360 [drm_gpusvm_helper] > > drm_pagemap_migrate_to_ram+0x5c/0x80 [drm_gpusvm_helper] > > do_huge_pmd_device_private+0x160/0x280 > > Which is the branch your patchset based on? I found that > drm_pagemap_migrate_populate_ram_pfn() in mm-everything-2026-08-08-07-08 > still don't support fallback to single pages if THP allocation fails as > in the following comments, >
This entire series, on drm-tip (i.e., the 6 patches posted here [1]). [1] https://patchwork.freedesktop.org/series/171651/ > /* TODO: Support fallback to single pages if THP allocation fails */ > > > > Without CONFIG_DEBUG_VM the VM_BUG_ON_FOLIO() is compiled out and > > remove_migration_pmd() installs a huge PMD pointing at an order-0 page > > instead, along with add_mm_counter(mm, MM_ANONPAGES, HPAGE_PMD_NR). The > > victim mm then maps 2MB of address space onto a single 4K page, which > > shows up later as bad rss-counter state, leaked page tables and page > > allocator freelist corruption in unrelated processes. > > > > Note this second problem was latent before the refcount fix above: the > > split always failed, and the failed attempt left migrate->vma demoted, > > so the retried fault took the PTE path, where __folio_split() unmaps > > with TTU_SPLIT_HUGE_PMD and demotes every VMA. > > > > Fix it by walking the rmap and demoting every PMD sized migration entry > > mapping the folio before splitting it. Demote with freeze = false: entry > > creation in __split_huge_pmd_locked() is dispatched on > > pmd_is_migration_entry(), not on freeze, so a migration PMD becomes PTE > > sized migration entries either way, and freeze only controls a trailing > > put_page(). With freeze = false there is no refcount change at all, > > which makes the demotion idempotent across N VMAs. > > > > rmap_walk_control.anon_lock is deliberately left unset: > > folio_lock_anon_vma_read() depends on folio_mapped(), and the folio is > > already fully unmapped here. This mirrors remove_migration_ptes(). > > > > Finally, refuse the split for a folio that is not anonymous. The rmap > > walk would otherwise reach a file backed VMA, where > > split_huge_pmd_address() zaps the PMD instead of demoting it. > > > > Fixes: 4265d67e405a ("mm/migrate_device: add THP splitting during > > migration") > > Cc: Andrew Morton <[email protected]> > > Cc: David Hildenbrand <[email protected]> > > Cc: Lorenzo Stoakes <[email protected]> > > Cc: Zi Yan <[email protected]> > > Cc: Baolin Wang <[email protected]> > > Cc: Liam R. Howlett <[email protected]> > > Cc: Nico Pache <[email protected]> > > Cc: Ryan Roberts <[email protected]> > > Cc: Dev Jain <[email protected]> > > Cc: Barry Song <[email protected]> > > Cc: Lance Yang <[email protected]> > > Cc: Usama Arif <[email protected]> > > Cc: Joshua Hahn <[email protected]> > > Cc: Rakie Kim <[email protected]> > > Cc: Byungchul Park <[email protected]> > > Cc: Gregory Price <[email protected]> > > Cc: Ying Huang <[email protected]> > > Cc: Alistair Popple <[email protected]> > > Cc: Balbir Singh <[email protected]> > > Cc: Maarten Lankhorst <[email protected]> > > Cc: Maxime Ripard <[email protected]> > > Cc: Thomas Zimmermann <[email protected]> > > Cc: David Airlie <[email protected]> > > Cc: Simona Vetter <[email protected]> > > Cc: Thomas Hellström <[email protected]> > > Cc: Francois Dugast <[email protected]> > > Cc: [email protected] > > Cc: [email protected] > > Cc: [email protected] > > Cc: [email protected] > > Assisted-by: GitHub_Copilot:claude-opus-5 > > Signed-off-by: Matthew Brost <[email protected]> > > --- > > mm/migrate_device.c | 98 ++++++++++++++++++++++++++++++++++++++++----- > > 1 file changed, 89 insertions(+), 9 deletions(-) > > > > diff --git a/mm/migrate_device.c b/mm/migrate_device.c > > index ae9027421b80..ae17bd516d24 100644 > > --- a/mm/migrate_device.c > > +++ b/mm/migrate_device.c > > @@ -899,22 +899,104 @@ static int migrate_vma_insert_huge_pmd_page(struct > > migrate_vma *migrate, > > return 0; > > } > > > > +static bool migrate_vma_split_pmd_one(struct folio *folio, > > + struct vm_area_struct *vma, > > + unsigned long addr, void *arg) > > +{ > > + DEFINE_FOLIO_VMA_WALK(pvmw, folio, vma, addr, PVMW_SYNC | > > PVMW_MIGRATION); > > + > > + while (page_vma_mapped_walk(&pvmw)) { > > + if (pvmw.pte) > > + continue; > > + > > + addr = pvmw.address; > > + page_vma_mapped_walk_done(&pvmw); > > + > > + /* > > + * Demote with freeze = false: the PMD already holds a > > + * migration entry, so __split_huge_pmd_locked() creates PTE > > + * sized migration entries from it and leaves the refcount > > + * alone. There is at most one PMD mapping @folio per VMA, so > > + * stop the walk here. > > + */ > > + split_huge_pmd_address(vma, addr, false); > > + break; > > + } > > + > > + return true; > > +} > > + > > +/* > > + * Demote every PMD sized migration entry that maps @folio to PTE sized > > ones. > > + * > > + * migrate_device_unmap() unmaps with try_to_migrate(folio, 0), i.e. > > without > > + * TTU_SPLIT_HUGE_PMD, so a folio that was PMD mapped in several VMAs -- > > after > > + * fork(), for instance -- ends up with a PMD sized migration entry in > > every one > > + * of them. folio_split_unmapped() below does not care, it only looks at > > the > > + * refcount, so splitting the folio without demoting all of those first > > would > > + * leave the other VMAs pointing a huge PMD at what is now an order-0 > > folio. > > + * remove_migration_ptes() trips over that in migrate_vma_finalize(). > > + */ > > +static void migrate_vma_split_pmd_mappings(struct folio *folio) > > +{ > > + struct rmap_walk_control rwc = { > > + .rmap_one = migrate_vma_split_pmd_one, > > + }; > > + > > + /* > > + * Do not pass .anon_lock: folio_lock_anon_vma_read() requires > > + * folio_mapped(), and @folio is already fully unmapped here. > > + */ > > + rmap_walk(folio, &rwc); > > +} > > + > > static int migrate_vma_split_unmapped_folio(struct migrate_vma *migrate, > > - unsigned long idx, unsigned long > > addr, > > + unsigned long idx, > > struct folio *folio) > > { > > unsigned long i; > > unsigned long pfn; > > unsigned long flags; > > + bool fault_folio; > > int ret = 0; > > > > /* > > - * take a reference, since split_huge_pmd_address() with freeze = true > > - * drops a reference at the end. > > + * migrate_vma_split_pmd_mappings() walks the rmap, and > > + * split_huge_pmd_address() zaps rather than demotes a PMD in a VMA that > > + * is not anonymous. migrate_vma_collect_huge_pmd() does not check the > > + * VMA type, so a file THP can reach here; the rest of the migrate_vma() > > + * machinery only supports anonymous memory anyway. > > */ > > - folio_get(folio); > > - split_huge_pmd_address(migrate->vma, addr, true); > > + if (!folio_test_anon(folio)) > > + return -EINVAL; > > + > > + /* > > + * A CPU fault on a device private PMD holds an extra reference on the > > + * folio, taken by do_huge_pmd_device_private(). folio_split_unmapped() > > + * only tolerates a single caller reference, so the split would always > > + * fail with -EAGAIN while this fault reference is held. > > + * > > + * do_huge_pmd_device_private() derives the fault page from the PMD > > + * entry, so it is always the head page of @folio, and therefore always > > + * ends up in the head folio after an uniform split to order 0. Drop > > + * the reference across the split and re-take it on the head folio > > + * afterwards, leaving the reference exactly where it is expected to be > > + * released. > > + * > > + * The folio cannot go away while the reference is dropped: the > > + * reference taken by migrate_vma_collect_huge_pmd() is still held. > > + */ > > + fault_folio = migrate->fault_page && > > + page_folio(migrate->fault_page) == folio; > > + > > + migrate_vma_split_pmd_mappings(folio); > > + > > + if (fault_folio) > > + folio_put(folio); > > ret = folio_split_unmapped(folio, 0); > > + if (fault_folio) > > + folio_get(folio); > > + > > Is it better to pass "extra_cnt" to folio_split_unmapped()? This > follows the coding style of the other migrate functions better, like > that in __migrate_device_pages(). > That is an option. To be minimally invasive, I went this route. I also didn't know offhand what would happen if our head page had an extra reference and we then called folio_split_unmapped() with "extra_cnt", or how that would affect the reference counts of the newly split pages (i.e., whether we would need to adjust the reference counts of all split pages after folio_split_unmapped() returns). However, I could quickly reason that dropping the reference and then reacquiring it was functionally correct and safe. Matt > > if (ret) > > return ret; > > migrate->src[idx] &= ~MIGRATE_PFN_COMPOUND; > > @@ -935,7 +1017,7 @@ static int migrate_vma_insert_huge_pmd_page(struct > > migrate_vma *migrate, > > } > > > > static int migrate_vma_split_unmapped_folio(struct migrate_vma *migrate, > > - unsigned long idx, unsigned long > > addr, > > + unsigned long idx, > > struct folio *folio) > > { > > return 0; > > @@ -1103,7 +1185,6 @@ static void __migrate_device_pages(unsigned long > > *src_pfns, > > struct mmu_notifier_range range; > > unsigned long i, j; > > bool notified = false; > > - unsigned long addr; > > > > for (i = 0; i < npages; ) { > > struct page *newpage = migrate_pfn_to_page(dst_pfns[i]); > > @@ -1177,8 +1258,7 @@ static void __migrate_device_pages(unsigned long > > *src_pfns, > > goto next; > > } > > nr = 1 << folio_order(folio); > > - addr = migrate->start + i * PAGE_SIZE; > > - if (migrate_vma_split_unmapped_folio(migrate, > > i, addr, folio)) { > > + if (migrate_vma_split_unmapped_folio(migrate, > > i, folio)) { > > src_pfns[i] &= ~(MIGRATE_PFN_MIGRATE | > > MIGRATE_PFN_COMPOUND); > > goto next; > > --- > Best Regards, > Huang, Ying
