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Message-ID: <0e116c7e-d276-418d-a8cd-47cdd9f2d00d@redhat.com>
Date: Tue, 12 Aug 2025 09:37:14 +0300
From: Mika Penttilä <mpenttil@...hat.com>
To: Matthew Brost <matthew.brost@...el.com>
Cc: Balbir Singh <balbirs@...dia.com>, dri-devel@...ts.freedesktop.org,
linux-mm@...ck.org, linux-kernel@...r.kernel.org,
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>
Subject: Re: [v3 03/11] mm/migrate_device: THP migration of zone device pages
On 8/12/25 09:33, Matthew Brost wrote:
> On Tue, Aug 12, 2025 at 09:25:29AM +0300, Mika Penttilä wrote:
>> On 8/12/25 08:54, Matthew Brost wrote:
>>
>>> On Tue, Aug 12, 2025 at 08:35:49AM +0300, Mika Penttilä wrote:
>>>> Hi,
>>>>
>>>> On 8/12/25 05:40, Balbir Singh wrote:
>>>>
>>>>> MIGRATE_VMA_SELECT_COMPOUND will be used to select THP pages during
>>>>> migrate_vma_setup() and MIGRATE_PFN_COMPOUND will make migrating
>>>>> device pages as compound pages during device pfn migration.
>>>>>
>>>>> migrate_device code paths go through the collect, setup
>>>>> and finalize phases of migration.
>>>>>
>>>>> The entries in src and dst arrays passed to these functions still
>>>>> remain at a PAGE_SIZE granularity. When a compound page is passed,
>>>>> the first entry has the PFN along with MIGRATE_PFN_COMPOUND
>>>>> and other flags set (MIGRATE_PFN_MIGRATE, MIGRATE_PFN_VALID), the
>>>>> remaining entries (HPAGE_PMD_NR - 1) are filled with 0's. This
>>>>> representation allows for the compound page to be split into smaller
>>>>> page sizes.
>>>>>
>>>>> migrate_vma_collect_hole(), migrate_vma_collect_pmd() are now THP
>>>>> page aware. Two new helper functions migrate_vma_collect_huge_pmd()
>>>>> and migrate_vma_insert_huge_pmd_page() have been added.
>>>>>
>>>>> migrate_vma_collect_huge_pmd() can collect THP pages, but if for
>>>>> some reason this fails, there is fallback support to split the folio
>>>>> and migrate it.
>>>>>
>>>>> migrate_vma_insert_huge_pmd_page() closely follows the logic of
>>>>> migrate_vma_insert_page()
>>>>>
>>>>> Support for splitting pages as needed for migration will follow in
>>>>> later patches in this series.
>>>>>
>>>>> 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: Matthew Brost <matthew.brost@...el.com>
>>>>> Cc: Francois Dugast <francois.dugast@...el.com>
>>>>>
>>>>> Signed-off-by: Balbir Singh <balbirs@...dia.com>
>>>>> ---
>>>>> include/linux/migrate.h | 2 +
>>>>> mm/migrate_device.c | 457 ++++++++++++++++++++++++++++++++++------
>>>>> 2 files changed, 396 insertions(+), 63 deletions(-)
>>>>>
>>>>> diff --git a/include/linux/migrate.h b/include/linux/migrate.h
>>>>> index acadd41e0b5c..d9cef0819f91 100644
>>>>> --- a/include/linux/migrate.h
>>>>> +++ b/include/linux/migrate.h
>>>>> @@ -129,6 +129,7 @@ static inline int migrate_misplaced_folio(struct folio *folio, int node)
>>>>> #define MIGRATE_PFN_VALID (1UL << 0)
>>>>> #define MIGRATE_PFN_MIGRATE (1UL << 1)
>>>>> #define MIGRATE_PFN_WRITE (1UL << 3)
>>>>> +#define MIGRATE_PFN_COMPOUND (1UL << 4)
>>>>> #define MIGRATE_PFN_SHIFT 6
>>>>>
>>>>> static inline struct page *migrate_pfn_to_page(unsigned long mpfn)
>>>>> @@ -147,6 +148,7 @@ enum migrate_vma_direction {
>>>>> MIGRATE_VMA_SELECT_SYSTEM = 1 << 0,
>>>>> MIGRATE_VMA_SELECT_DEVICE_PRIVATE = 1 << 1,
>>>>> MIGRATE_VMA_SELECT_DEVICE_COHERENT = 1 << 2,
>>>>> + MIGRATE_VMA_SELECT_COMPOUND = 1 << 3,
>>>>> };
>>>>>
>>>>> struct migrate_vma {
>>>>> diff --git a/mm/migrate_device.c b/mm/migrate_device.c
>>>>> index 0ed337f94fcd..6621bba62710 100644
>>>>> --- a/mm/migrate_device.c
>>>>> +++ b/mm/migrate_device.c
>>>>> @@ -14,6 +14,7 @@
>>>>> #include <linux/pagewalk.h>
>>>>> #include <linux/rmap.h>
>>>>> #include <linux/swapops.h>
>>>>> +#include <asm/pgalloc.h>
>>>>> #include <asm/tlbflush.h>
>>>>> #include "internal.h"
>>>>>
>>>>> @@ -44,6 +45,23 @@ static int migrate_vma_collect_hole(unsigned long start,
>>>>> if (!vma_is_anonymous(walk->vma))
>>>>> return migrate_vma_collect_skip(start, end, walk);
>>>>>
>>>>> + if (thp_migration_supported() &&
>>>>> + (migrate->flags & MIGRATE_VMA_SELECT_COMPOUND) &&
>>>>> + (IS_ALIGNED(start, HPAGE_PMD_SIZE) &&
>>>>> + IS_ALIGNED(end, HPAGE_PMD_SIZE))) {
>>>>> + migrate->src[migrate->npages] = MIGRATE_PFN_MIGRATE |
>>>>> + MIGRATE_PFN_COMPOUND;
>>>>> + migrate->dst[migrate->npages] = 0;
>>>>> + migrate->npages++;
>>>>> + migrate->cpages++;
>>>>> +
>>>>> + /*
>>>>> + * Collect the remaining entries as holes, in case we
>>>>> + * need to split later
>>>>> + */
>>>>> + return migrate_vma_collect_skip(start + PAGE_SIZE, end, walk);
>>>>> + }
>>>>> +
>>>>> for (addr = start; addr < end; addr += PAGE_SIZE) {
>>>>> migrate->src[migrate->npages] = MIGRATE_PFN_MIGRATE;
>>>>> migrate->dst[migrate->npages] = 0;
>>>>> @@ -54,57 +72,151 @@ static int migrate_vma_collect_hole(unsigned long start,
>>>>> return 0;
>>>>> }
>>>>>
>>>>> -static int migrate_vma_collect_pmd(pmd_t *pmdp,
>>>>> - unsigned long start,
>>>>> - unsigned long end,
>>>>> - struct mm_walk *walk)
>>>>> +/**
>>>>> + * migrate_vma_collect_huge_pmd - collect THP pages without splitting the
>>>>> + * folio for device private pages.
>>>>> + * @pmdp: pointer to pmd entry
>>>>> + * @start: start address of the range for migration
>>>>> + * @end: end address of the range for migration
>>>>> + * @walk: mm_walk callback structure
>>>>> + *
>>>>> + * Collect the huge pmd entry at @pmdp for migration and set the
>>>>> + * MIGRATE_PFN_COMPOUND flag in the migrate src entry to indicate that
>>>>> + * migration will occur at HPAGE_PMD granularity
>>>>> + */
>>>>> +static int migrate_vma_collect_huge_pmd(pmd_t *pmdp, unsigned long start,
>>>>> + unsigned long end, struct mm_walk *walk,
>>>>> + struct folio *fault_folio)
>>>>> {
>>>>> + struct mm_struct *mm = walk->mm;
>>>>> + struct folio *folio;
>>>>> struct migrate_vma *migrate = walk->private;
>>>>> - struct folio *fault_folio = migrate->fault_page ?
>>>>> - page_folio(migrate->fault_page) : NULL;
>>>>> - struct vm_area_struct *vma = walk->vma;
>>>>> - struct mm_struct *mm = vma->vm_mm;
>>>>> - unsigned long addr = start, unmapped = 0;
>>>>> spinlock_t *ptl;
>>>>> - pte_t *ptep;
>>>>> + swp_entry_t entry;
>>>>> + int ret;
>>>>> + unsigned long write = 0;
>>>>>
>>>>> -again:
>>>>> - if (pmd_none(*pmdp))
>>>>> + ptl = pmd_lock(mm, pmdp);
>>>>> + if (pmd_none(*pmdp)) {
>>>>> + spin_unlock(ptl);
>>>>> return migrate_vma_collect_hole(start, end, -1, walk);
>>>>> + }
>>>>>
>>>>> if (pmd_trans_huge(*pmdp)) {
>>>>> - struct folio *folio;
>>>>> -
>>>>> - ptl = pmd_lock(mm, pmdp);
>>>>> - if (unlikely(!pmd_trans_huge(*pmdp))) {
>>>>> + if (!(migrate->flags & MIGRATE_VMA_SELECT_SYSTEM)) {
>>>>> spin_unlock(ptl);
>>>>> - goto again;
>>>>> + return migrate_vma_collect_skip(start, end, walk);
>>>>> }
>>>>>
>>>>> folio = pmd_folio(*pmdp);
>>>>> if (is_huge_zero_folio(folio)) {
>>>>> spin_unlock(ptl);
>>>>> - split_huge_pmd(vma, pmdp, addr);
>>>>> - } else {
>>>>> - int ret;
>>>>> + return migrate_vma_collect_hole(start, end, -1, walk);
>>>>> + }
>>>>> + if (pmd_write(*pmdp))
>>>>> + write = MIGRATE_PFN_WRITE;
>>>>> + } else if (!pmd_present(*pmdp)) {
>>>>> + entry = pmd_to_swp_entry(*pmdp);
>>>>> + folio = pfn_swap_entry_folio(entry);
>>>>> +
>>>>> + if (!is_device_private_entry(entry) ||
>>>>> + !(migrate->flags & MIGRATE_VMA_SELECT_DEVICE_PRIVATE) ||
>>>>> + (folio->pgmap->owner != migrate->pgmap_owner)) {
>>>>> + spin_unlock(ptl);
>>>>> + return migrate_vma_collect_skip(start, end, walk);
>>>>> + }
>>>>>
>>>>> - folio_get(folio);
>>>>> + if (is_migration_entry(entry)) {
>>>>> + migration_entry_wait_on_locked(entry, ptl);
>>>>> spin_unlock(ptl);
>>>>> - /* FIXME: we don't expect THP for fault_folio */
>>>>> - if (WARN_ON_ONCE(fault_folio == folio))
>>>>> - return migrate_vma_collect_skip(start, end,
>>>>> - walk);
>>>>> - if (unlikely(!folio_trylock(folio)))
>>>>> - return migrate_vma_collect_skip(start, end,
>>>>> - walk);
>>>>> - ret = split_folio(folio);
>>>>> - if (fault_folio != folio)
>>>>> - folio_unlock(folio);
>>>>> - folio_put(folio);
>>>>> - if (ret)
>>>>> - return migrate_vma_collect_skip(start, end,
>>>>> - walk);
>>>>> + return -EAGAIN;
>>>>> }
>>>>> +
>>>>> + if (is_writable_device_private_entry(entry))
>>>>> + write = MIGRATE_PFN_WRITE;
>>>>> + } else {
>>>>> + spin_unlock(ptl);
>>>>> + return -EAGAIN;
>>>>> + }
>>>>> +
>>>>> + folio_get(folio);
>>>>> + if (folio != fault_folio && unlikely(!folio_trylock(folio))) {
>>>>> + spin_unlock(ptl);
>>>>> + folio_put(folio);
>>>>> + return migrate_vma_collect_skip(start, end, walk);
>>>>> + }
>>>>> +
>>>>> + if (thp_migration_supported() &&
>>>>> + (migrate->flags & MIGRATE_VMA_SELECT_COMPOUND) &&
>>>>> + (IS_ALIGNED(start, HPAGE_PMD_SIZE) &&
>>>>> + IS_ALIGNED(end, HPAGE_PMD_SIZE))) {
>>>>> +
>>>>> + struct page_vma_mapped_walk pvmw = {
>>>>> + .ptl = ptl,
>>>>> + .address = start,
>>>>> + .pmd = pmdp,
>>>>> + .vma = walk->vma,
>>>>> + };
>>>>> +
>>>>> + unsigned long pfn = page_to_pfn(folio_page(folio, 0));
>>>>> +
>>>>> + migrate->src[migrate->npages] = migrate_pfn(pfn) | write
>>>>> + | MIGRATE_PFN_MIGRATE
>>>>> + | MIGRATE_PFN_COMPOUND;
>>>>> + migrate->dst[migrate->npages++] = 0;
>>>>> + migrate->cpages++;
>>>>> + ret = set_pmd_migration_entry(&pvmw, folio_page(folio, 0));
>>>>> + if (ret) {
>>>>> + migrate->npages--;
>>>>> + migrate->cpages--;
>>>>> + migrate->src[migrate->npages] = 0;
>>>>> + migrate->dst[migrate->npages] = 0;
>>>>> + goto fallback;
>>>>> + }
>>>>> + migrate_vma_collect_skip(start + PAGE_SIZE, end, walk);
>>>>> + spin_unlock(ptl);
>>>>> + return 0;
>>>>> + }
>>>>> +
>>>>> +fallback:
>>>>> + spin_unlock(ptl);
>>>>> + if (!folio_test_large(folio))
>>>>> + goto done;
>>>>> + ret = split_folio(folio);
>>>>> + if (fault_folio != folio)
>>>>> + folio_unlock(folio);
>>>>> + folio_put(folio);
>>>>> + if (ret)
>>>>> + return migrate_vma_collect_skip(start, end, walk);
>>>>> + if (pmd_none(pmdp_get_lockless(pmdp)))
>>>>> + return migrate_vma_collect_hole(start, end, -1, walk);
>>>>> +
>>>>> +done:
>>>>> + return -ENOENT;
>>>>> +}
>>>>> +
>>>>> +static int migrate_vma_collect_pmd(pmd_t *pmdp,
>>>>> + unsigned long start,
>>>>> + unsigned long end,
>>>>> + struct mm_walk *walk)
>>>>> +{
>>>>> + struct migrate_vma *migrate = walk->private;
>>>>> + struct vm_area_struct *vma = walk->vma;
>>>>> + struct mm_struct *mm = vma->vm_mm;
>>>>> + unsigned long addr = start, unmapped = 0;
>>>>> + spinlock_t *ptl;
>>>>> + struct folio *fault_folio = migrate->fault_page ?
>>>>> + page_folio(migrate->fault_page) : NULL;
>>>>> + pte_t *ptep;
>>>>> +
>>>>> +again:
>>>>> + if (pmd_trans_huge(*pmdp) || !pmd_present(*pmdp)) {
>>>>> + int ret = migrate_vma_collect_huge_pmd(pmdp, start, end, walk, fault_folio);
>>>>> +
>>>>> + if (ret == -EAGAIN)
>>>>> + goto again;
>>>>> + if (ret == 0)
>>>>> + return 0;
>>>>> }
>>>>>
>>>>> ptep = pte_offset_map_lock(mm, pmdp, addr, &ptl);
>>>>> @@ -222,8 +334,7 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
>>>>> mpfn |= pte_write(pte) ? MIGRATE_PFN_WRITE : 0;
>>>>> }
>>>>>
>>>>> - /* FIXME support THP */
>>>>> - if (!page || !page->mapping || PageTransCompound(page)) {
>>>>> + if (!page || !page->mapping) {
>>>>> mpfn = 0;
>>>>> goto next;
>>>>> }
>>>>> @@ -394,14 +505,6 @@ static bool migrate_vma_check_page(struct page *page, struct page *fault_page)
>>>>> */
>>>>> int extra = 1 + (page == fault_page);
>>>>>
>>>>> - /*
>>>>> - * FIXME support THP (transparent huge page), it is bit more complex to
>>>>> - * check them than regular pages, because they can be mapped with a pmd
>>>>> - * or with a pte (split pte mapping).
>>>>> - */
>>>>> - if (folio_test_large(folio))
>>>>> - return false;
>>>>> -
>>>> You cannot remove this check unless support normal mTHP folios migrate to device,
>>>> which I think this series doesn't do, but maybe should?
>>>>
>>> Currently, mTHP should be split upon collection, right? The only way a
>>> THP should be collected is if it directly maps to a PMD. If a THP or
>>> mTHP is found in PTEs (i.e., in migrate_vma_collect_pmd outside of
>>> migrate_vma_collect_huge_pmd), it should be split there. I sent this
>>> logic to Balbir privately, but it appears to have been omitted.
>> I think currently if mTHP is found byte PTEs folio just isn't migrated.
> If this is the fault page, you'd just spin forever. IIRC this how it
> popped in my testing. I'll try to follow up with a fixes patch as I have
> bandwidth.
Uh yes indeed that's a bug!
>
>> Yes maybe they should be just split while collected now. Best would of course
> +1 for now.
>
>> to migrate (like as order-0 pages for device) for not to split all mTHPs.
>> And yes maybe this all controlled by different flag.
>>
> +1 for different flag eventually.
>
> Matt
>
>>> I’m quite sure this missing split is actually an upstream bug, but it
>>> has been suppressed by PMDs being split upon device fault. I have a test
>>> that performs a ton of complete mremap—nonsense no one would normally
>>> do, but which should work—that exposed this. I can rebase on this series
>>> and see if the bug appears, or try the same nonsense without the device
>>> faulting first and splitting the pages, to trigger the bug.
>>>
>>> Matt
>>>
>>>> --Mika
>>>>
>> --Mika
>>
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