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[89.27.86.246]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-48e254f02f1sm3413741f8f.3.2026.10.20.23.05.37 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 20 Oct 2026 23:05:38 -0700 (PDT) From: mpenttil@redhat.com To: linux-mm@kvack.org Cc: dri-devel@lists.freedesktop.org, intel-xe@lists.freedesktop.org, linux-kernel@vger.kernel.org, =?UTF-8?q?Mika=20Penttil=C3=A4?= , David Hildenbrand , Jason Gunthorpe , Leon Romanovsky , Alistair Popple , Balbir Singh , Zi Yan , Matthew Brost , Andrew Morton , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Lyude Paul , Danilo Krummrich Subject: [PATCH v16 06/12] mm/hmm: migrate collection in HMM pagewalk - pmd level Date: Wed, 21 Oct 2026 09:05:00 +0300 Message-ID: <20261021060506.2377545-7-mpenttil@redhat.com> X-Mailer: git-send-email 2.55.0 In-Reply-To: <20261021060506.2377545-1-mpenttil@redhat.com> References: <20261021060506.2377545-1-mpenttil@redhat.com> X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit From: Mika Penttilä Implement the needed hmm_vma_handle_migrate_prepare_pmd() function which is mostly carried over from migrate_device.c's migrate_vma_collect_huge_pmd() function. With HMM pagewalk based migration, the idea is that hmm_vma_handle_*() are responsible for faulting, and the pfn collecting part. hmm_vma_handle_migrate_prepare*() do the migration decisions (with HMM_PFN_MIGRATE), possibly split folios, and insert migration ptes/pmds. HMM pagewalk based migration is enabled in later commit, for now now hmm_select_migrate() returns 0. Also avoid the problems present in the migrate_vma_collect_huge_pmd() split fallback path this HMM path supersedes: - extra refcount for fault_folio makes split_folio() fail - wrong folio unlocked after split_folio() Cc: David Hildenbrand Cc: Jason Gunthorpe Cc: Leon Romanovsky Cc: Alistair Popple Cc: Balbir Singh Cc: Zi Yan Cc: Matthew Brost Suggested-by: Alistair Popple Signed-off-by: Mika Penttilä --- mm/hmm.c | 172 ++++++++++++++++++++++++++++++++++++++++++++++++------- 1 file changed, 151 insertions(+), 21 deletions(-) diff --git a/mm/hmm.c b/mm/hmm.c index 0ce947d0e572..5891efee3e18 100644 --- a/mm/hmm.c +++ b/mm/hmm.c @@ -492,31 +492,19 @@ static int hmm_vma_handle_absent_pmd(struct mm_walk *walk, unsigned long start, #ifdef CONFIG_DEVICE_MIGRATION /** - * migrate_vma_split_folio() - Helper function to split a THP folio + * __migrate_vma_split_folio() - split a folio and move the lock/ref to the + * order-0 folio backing @fault_page after the split * @folio: the folio to split - * @fault_page: struct page associated with the fault if any - * @hmm_vma_walk: walk in progress - * @ptep: pte_t * for unmap and unlock ptl + * @fault_page: fault page if any * - * Returns 0 on success + * Returns 0 on success. */ -static int migrate_vma_split_folio(struct folio *folio, - struct page *fault_page, - struct hmm_vma_walk *hmm_vma_walk, - pte_t *ptep) +static int __migrate_vma_split_folio(struct folio *folio, + struct page *fault_page) { - int ret; struct folio *fault_folio = fault_page ? page_folio(fault_page) : NULL; struct folio *new_fault_folio = NULL; - - if (folio != fault_folio) - folio_get(folio); - - pte_unmap_unlock(ptep, hmm_vma_walk->ptl); - hmm_vma_walk->ptelocked = false; - - if (folio != fault_folio) - folio_lock(folio); + int ret; ret = split_folio(folio); if (ret) { @@ -548,14 +536,145 @@ static int migrate_vma_split_folio(struct folio *folio, return 0; } +/** + * migrate_vma_split_folio() - drop the pte lock and split a THP folio + * @folio: the folio to split + * @fault_page: struct page associated with the fault if any + * @hmm_vma_walk: walk in progress + * @ptep: pte_t * for unmap and unlock ptl + * + * Returns 0 on success + */ +static int migrate_vma_split_folio(struct folio *folio, + struct page *fault_page, + struct hmm_vma_walk *hmm_vma_walk, + pte_t *ptep) +{ + struct folio *fault_folio = fault_page ? page_folio(fault_page) : NULL; + + if (folio != fault_folio) + folio_get(folio); + + pte_unmap_unlock(ptep, hmm_vma_walk->ptl); + hmm_vma_walk->ptelocked = false; + + if (folio != fault_folio) + folio_lock(folio); + + return __migrate_vma_split_folio(folio, fault_page); +} + static int hmm_vma_handle_migrate_prepare_pmd(const struct mm_walk *walk, pmd_t *pmdp, unsigned long start, unsigned long end, unsigned long *hmm_pfn) { - // TODO: implement migration entry insertion - return 0; + struct hmm_vma_walk *hmm_vma_walk = walk->private; + struct hmm_range *range = hmm_vma_walk->range; + struct migrate_vma *migrate = range->migrate; + struct folio *fault_folio = NULL; + enum migrate_vma_info minfo; + struct folio *folio; + unsigned long i; + int r = 0; + + // Do we want to migrate at all? + minfo = hmm_select_migrate(range); + if (!minfo) + return r; + + WARN_ON_ONCE(!migrate); + HMM_ASSERT_PMD_LOCKED(hmm_vma_walk, true); + + fault_folio = migrate->fault_page ? + page_folio(migrate->fault_page) : NULL; + + if (pmd_none(*pmdp)) + return hmm_pfns_fill(start, end, hmm_vma_walk, 0); + + if (!(hmm_pfn[0] & HMM_PFN_VALID)) + goto out; + + if (pmd_trans_huge(*pmdp)) { + if (!(minfo & MIGRATE_VMA_SELECT_SYSTEM)) + goto out; + + folio = pmd_folio(*pmdp); + if (is_huge_zero_folio(folio)) + return hmm_pfns_fill(start, end, hmm_vma_walk, 0); + + } else if (!pmd_present(*pmdp)) { + const softleaf_t entry = softleaf_from_pmd(*pmdp); + + if (!softleaf_is_device_private(entry)) + goto out; + + if (!(minfo & MIGRATE_VMA_SELECT_DEVICE_PRIVATE)) + goto out; + + folio = softleaf_to_folio(entry); + if (folio->pgmap->owner != migrate->pgmap_owner) + goto out; + } else { + hmm_vma_walk->last = start; + return -EBUSY; + } + + folio_get(folio); + + if (folio != fault_folio && unlikely(!folio_trylock(folio))) { + folio_put(folio); + hmm_pfns_fill(start, end, hmm_vma_walk, HMM_PFN_ERROR); + return 0; + } + + 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 = hmm_vma_walk->ptl, + .address = start, + .pmd = pmdp, + .vma = walk->vma, + }; + + hmm_pfn[0] |= HMM_PFN_MIGRATE | HMM_PFN_COMPOUND; + + r = set_pmd_migration_entry(&pvmw, folio_page(folio, 0)); + if (r) { + hmm_pfn[0] &= ~(HMM_PFN_MIGRATE | HMM_PFN_COMPOUND); + goto split; /* fall back to splitting the pmd */ + } + for (i = 1, start += PAGE_SIZE; start < end; start += PAGE_SIZE, i++) + hmm_pfn[i] &= HMM_PFN_INOUT_FLAGS; + + } else { + goto split; /* fall back to splitting the pmd */ + } + +out: + return r; + +split: + spin_unlock(hmm_vma_walk->ptl); + hmm_vma_walk->pmdlocked = false; + + /* + * folio_get() above took an extra reference. For the fault folio the + * caller still holds a reference and the lock, which is the + * precondition of __migrate_vma_split_folio(), so drop the extra one. + */ + if (folio == fault_folio) + folio_put(folio); + + r = __migrate_vma_split_folio(folio, migrate->fault_page); + if (r) + return -EFAULT; + + hmm_vma_walk->last = start; + return -EBUSY; } /* @@ -958,6 +1077,17 @@ static int hmm_vma_walk_pmd(pmd_t *pmdp, hmm_vma_walk->pmdlocked = false; } + /* + * hmm_vma_handle_migrate_prepare_pmd() splits the huge pmd in + * place when needed and returns -EBUSY to re-walk the range as + * PTEs; any other error means the split failed. + */ + if (r == -EBUSY) + return -EBUSY; + if (r) { + /* Split not successful, skip */ + return hmm_pfns_fill(start, end, hmm_vma_walk, HMM_PFN_ERROR); + } return r; } -- 2.55.0