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Message-ID: <a6d4dff5-15be-48e3-9bb8-00bb44dc5584@redhat.com>
Date: Thu, 20 Nov 2025 19:43:42 +0200
From: Mika Penttilä <mpenttil@...hat.com>
To: Matthew Brost <matthew.brost@...el.com>, linux-kernel@...r.kernel.org,
dri-devel@...ts.freedesktop.org, linux-mm@...ck.org
Cc: Andrew Morton <akpm@...ux-foundation.org>,
David Hildenbrand <david@...hat.com>, Zi Yan <ziy@...dia.com>,
Joshua Hahn <joshua.hahnjy@...il.com>, Rakie Kim <rakie.kim@...com>,
Byungchul Park <byungchul@...com>, Gregory Price <gourry@...rry.net>,
Ying Huang <ying.huang@...ux.alibaba.com>,
Alistair Popple <apopple@...dia.com>, Oscar Salvador <osalvador@...e.de>,
Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
Baolin Wang <baolin.wang@...ux.alibaba.com>,
"Liam R . Howlett" <Liam.Howlett@...cle.com>, Nico Pache
<npache@...hat.com>, Ryan Roberts <ryan.roberts@....com>,
Dev Jain <dev.jain@....com>, Barry Song <baohua@...nel.org>,
Lyude Paul <lyude@...hat.com>, Danilo Krummrich <dakr@...nel.org>,
David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
Ralph Campbell <rcampbell@...dia.com>,
Francois Dugast <francois.dugast@...el.com>,
Balbir Singh <balbirs@...dia.com>
Subject: Re: [PATCH] fixup: mm/migrate_device: handle partially mapped folios
during
Hi,
On 11/20/25 19:05, Matthew Brost wrote:
> Splitting a partially mapped folio caused a regression in the Intel Xe
> SVM test suite in the mremap section, resulting in the following stack
> trace:
>
> NFO: task kworker/u65:2:1642 blocked for more than 30 seconds.
> [ 212.624286] Tainted: G S W 6.18.0-rc6-xe+ #1719
> [ 212.638288] Workqueue: xe_page_fault_work_queue xe_pagefault_queue_work [xe]
> [ 212.638323] Call Trace:
> [ 212.638324] <TASK>
> [ 212.638325] __schedule+0x4b0/0x990
> [ 212.638330] schedule+0x22/0xd0
> [ 212.638331] io_schedule+0x41/0x60
> [ 212.638333] migration_entry_wait_on_locked+0x1d8/0x2d0
> [ 212.638336] ? __pfx_wake_page_function+0x10/0x10
> [ 212.638339] migration_entry_wait+0xd2/0xe0
> [ 212.638341] hmm_vma_walk_pmd+0x7c9/0x8d0
> [ 212.638343] walk_pgd_range+0x51d/0xa40
> [ 212.638345] __walk_page_range+0x75/0x1e0
> [ 212.638347] walk_page_range_mm+0x138/0x1f0
> [ 212.638349] hmm_range_fault+0x59/0xa0
> [ 212.638351] drm_gpusvm_get_pages+0x194/0x7b0 [drm_gpusvm_helper]
> [ 212.638354] drm_gpusvm_range_get_pages+0x2d/0x40 [drm_gpusvm_helper]
> [ 212.638355] __xe_svm_handle_pagefault+0x259/0x900 [xe]
> [ 212.638375] ? update_load_avg+0x7f/0x6c0
> [ 212.638377] ? update_curr+0x13d/0x170
> [ 212.638379] xe_svm_handle_pagefault+0x37/0x90 [xe]
> [ 212.638396] xe_pagefault_queue_work+0x2da/0x3c0 [xe]
> [ 212.638420] process_one_work+0x16e/0x2e0
> [ 212.638422] worker_thread+0x284/0x410
> [ 212.638423] ? __pfx_worker_thread+0x10/0x10
> [ 212.638425] kthread+0xec/0x210
> [ 212.638427] ? __pfx_kthread+0x10/0x10
> [ 212.638428] ? __pfx_kthread+0x10/0x10
> [ 212.638430] ret_from_fork+0xbd/0x100
> [ 212.638433] ? __pfx_kthread+0x10/0x10
> [ 212.638434] ret_from_fork_asm+0x1a/0x30
> [ 212.638436] </TASK>
>
> The issue appears to be that migration PTEs are not properly removed
> after a split.
>
> This change refactors the code to perform the split in a slightly
> different manner while retaining the original patch’s intent. With this
> update, the Intel Xe SVM test suite fully passes.
>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: David Hildenbrand <david@...hat.com>
> Cc: Zi Yan <ziy@...dia.com>
> Cc: Joshua Hahn <joshua.hahnjy@...il.com>
> Cc: Rakie Kim <rakie.kim@...com>
> Cc: Byungchul Park <byungchul@...com>
> Cc: Gregory Price <gourry@...rry.net>
> Cc: Ying Huang <ying.huang@...ux.alibaba.com>
> Cc: Alistair Popple <apopple@...dia.com>
> Cc: Oscar Salvador <osalvador@...e.de>
> Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> Cc: Baolin Wang <baolin.wang@...ux.alibaba.com>
> Cc: Liam R. Howlett <Liam.Howlett@...cle.com>
> Cc: Nico Pache <npache@...hat.com>
> Cc: Ryan Roberts <ryan.roberts@....com>
> Cc: Dev Jain <dev.jain@....com>
> Cc: Barry Song <baohua@...nel.org>
> Cc: Lyude Paul <lyude@...hat.com>
> Cc: Danilo Krummrich <dakr@...nel.org>
> Cc: David Airlie <airlied@...il.com>
> Cc: Simona Vetter <simona@...ll.ch>
> Cc: Ralph Campbell <rcampbell@...dia.com>
> Cc: Mika Penttilä <mpenttil@...hat.com>
> Cc: Francois Dugast <francois.dugast@...el.com>
> Cc: Balbir Singh <balbirs@...dia.com>
> Signed-off-by: Matthew Brost <matthew.brost@...el.com>
>
> ---
> This fixup should be squashed into the patch "mm/migrate_device: handle
> partially mapped folios during" in mm/mm-unstable
>
> I replaced the original patch with a local patch I authored a while back
> that solves the same problem but uses a different code structure. The
> failing test case—only available on an Xe driver—passes with this patch.
> I can attempt to fix up the original patch within its structure if
> that’s preferred.
> ---
> mm/migrate_device.c | 42 ++++++++++++++++++++++++------------------
> 1 file changed, 24 insertions(+), 18 deletions(-)
>
> diff --git a/mm/migrate_device.c b/mm/migrate_device.c
> index fa42d2ebd024..69e88f4a2563 100644
> --- a/mm/migrate_device.c
> +++ b/mm/migrate_device.c
> @@ -254,6 +254,7 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
> spinlock_t *ptl;
> struct folio *fault_folio = migrate->fault_page ?
> page_folio(migrate->fault_page) : NULL;
> + struct folio *split_folio = NULL;
> pte_t *ptep;
>
> again:
> @@ -266,10 +267,11 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
> return 0;
> }
>
> - ptep = pte_offset_map_lock(mm, pmdp, addr, &ptl);
> + ptep = pte_offset_map_lock(mm, pmdp, start, &ptl);
> if (!ptep)
> goto again;
> arch_enter_lazy_mmu_mode();
> + ptep += (addr - start) / PAGE_SIZE;
>
> for (; addr < end; addr += PAGE_SIZE, ptep++) {
> struct dev_pagemap *pgmap;
> @@ -347,22 +349,6 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
> pgmap->owner != migrate->pgmap_owner)
> goto next;
> }
> - folio = page ? page_folio(page) : NULL;
> - if (folio && folio_test_large(folio)) {
> - int ret;
> -
> - pte_unmap_unlock(ptep, ptl);
> - ret = migrate_vma_split_folio(folio,
> - migrate->fault_page);
> -
> - if (ret) {
> - ptep = pte_offset_map_lock(mm, pmdp, addr, &ptl);
> - goto next;
> - }
> -
> - addr = start;
> - goto again;
> - }
> mpfn = migrate_pfn(pfn) | MIGRATE_PFN_MIGRATE;
> mpfn |= pte_write(pte) ? MIGRATE_PFN_WRITE : 0;
> }
> @@ -400,6 +386,11 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
> bool anon_exclusive;
> pte_t swp_pte;
>
> + if (folio_order(folio)) {
> + split_folio = folio;
> + goto split;
> + }
> +
> flush_cache_page(vma, addr, pte_pfn(pte));
> anon_exclusive = folio_test_anon(folio) &&
> PageAnonExclusive(page);
> @@ -478,8 +469,23 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
> if (unmapped)
> flush_tlb_range(walk->vma, start, end);
>
> +split:
> arch_leave_lazy_mmu_mode();
> - pte_unmap_unlock(ptep - 1, ptl);
> + pte_unmap_unlock(ptep - 1 + !!split_folio, ptl);
> +
> + if (split_folio) {
> + int ret;
> +
> + ret = split_folio(split_folio);
> + if (fault_folio != split_folio)
> + folio_unlock(split_folio);
I think wrong folio is left locked in case of fault_folio != NULL. Look how
migrate_vma_split_folio() handles it.
> + folio_put(split_folio);
> + if (ret)
> + return migrate_vma_collect_skip(addr, end, walk);
> +
> + split_folio = NULL;
> + goto again;
> + }
>
> return 0;
> }
How is this making a difference, I suppose it's only the
migrate_vma_collect_skip() after failed split?
Why are you just removing the other migrate_vma_split_folio() call?
--Mika
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