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[85.23.48.6]) by smtp.gmail.com with ESMTPSA id 2adb3069b0e04-55b8898be9bsm5046839e87.12.2025.08.12.17.07.28 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 12 Aug 2025 17:07:29 -0700 (PDT) Message-ID: <23a3e9b9-ad35-4469-884d-279aac0868de@redhat.com> Date: Wed, 13 Aug 2025 03:07:27 +0300 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [v3 03/11] mm/migrate_device: THP migration of zone device pages To: Balbir Singh , dri-devel@lists.freedesktop.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Cc: Andrew Morton , David Hildenbrand , Zi Yan , Joshua Hahn , Rakie Kim , Byungchul Park , Gregory Price , Ying Huang , Alistair Popple , Oscar Salvador , Lorenzo Stoakes , Baolin Wang , "Liam R. Howlett" , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lyude Paul , Danilo Krummrich , David Airlie , Simona Vetter , Ralph Campbell , Matthew Brost , Francois Dugast References: <20250812024036.690064-1-balbirs@nvidia.com> <20250812024036.690064-4-balbirs@nvidia.com> <81ca37d5-b1ff-46de-8dcc-b222af350c77@redhat.com> <9207451b-ebd1-46d5-b277-2adf3028f361@nvidia.com> Content-Language: en-US From: =?UTF-8?Q?Mika_Penttil=C3=A4?= In-Reply-To: <9207451b-ebd1-46d5-b277-2adf3028f361@nvidia.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 8/13/25 02:36, Balbir Singh wrote: > On 8/12/25 15:35, 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 >>> Cc: David Hildenbrand >>> Cc: Zi Yan >>> Cc: Joshua Hahn >>> Cc: Rakie Kim >>> Cc: Byungchul Park >>> Cc: Gregory Price >>> Cc: Ying Huang >>> Cc: Alistair Popple >>> Cc: Oscar Salvador >>> Cc: Lorenzo Stoakes >>> Cc: Baolin Wang >>> Cc: "Liam R. Howlett" >>> Cc: Nico Pache >>> Cc: Ryan Roberts >>> Cc: Dev Jain >>> Cc: Barry Song >>> Cc: Lyude Paul >>> Cc: Danilo Krummrich >>> Cc: David Airlie >>> Cc: Simona Vetter >>> Cc: Ralph Campbell >>> Cc: Mika Penttilä >>> Cc: Matthew Brost >>> Cc: Francois Dugast >>> >>> Signed-off-by: Balbir Singh >>> --- >>> 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 >>> #include >>> #include >>> +#include >>> #include >>> #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? >> > mTHP needs to be a follow up series, there are comments in the cover letter. I had an > assert earlier to prevent other folio sizes, but I've removed it and the interface > to drivers does not allow for mTHP device private folios. > > Balbir > pte mapped device private THPs with other sizes also can be created as a result of pmd and folio splits. Your should handle them in one place consistently not by different drivers. As pointed by Matthew, there's the problem with the fault_page if not split and just ignored. --Mika