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[2003:cb:c734:b800:12c4:65cd:348a:aee6]) by smtp.gmail.com with UTF8SMTPSA id ffacd0b85a97d-38dc2f6aeafsm11910943f8f.20.2025.02.10.11.38.17 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 10 Feb 2025 11:38:19 -0800 (PST) From: David Hildenbrand To: linux-kernel@vger.kernel.org Cc: linux-doc@vger.kernel.org, dri-devel@lists.freedesktop.org, linux-mm@kvack.org, nouveau@lists.freedesktop.org, linux-trace-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org, damon@lists.linux.dev, David Hildenbrand , Andrew Morton , =?utf-8?b?SsOpcsO0bWUgR2xpc3Nl?= , Jonathan Corbet , Alex Shi , Yanteng Si , Karol Herbst , Lyude Paul , Danilo Krummrich , David Airlie , Simona Vetter , Masami Hiramatsu , Oleg Nesterov , Peter Zijlstra , SeongJae Park , "Liam R. Howlett" , Lorenzo Stoakes , Vlastimil Babka , Jann Horn , Pasha Tatashin , Peter Xu , Alistair Popple , Jason Gunthorpe Subject: [PATCH v2 04/17] mm/rmap: implement make_device_exclusive() using folio_walk instead of rmap walk Date: Mon, 10 Feb 2025 20:37:46 +0100 Message-ID: <20250210193801.781278-5-david@redhat.com> X-Mailer: git-send-email 2.48.1 In-Reply-To: <20250210193801.781278-1-david@redhat.com> References: <20250210193801.781278-1-david@redhat.com> MIME-Version: 1.0 X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: Zlfgl219rTp4qosfbLcnQEwE8zCPCkM9Uvu9Q3AMPfk_1739216300 X-Mimecast-Originator: redhat.com content-type: text/plain; charset="US-ASCII"; x-default=true X-Rspam-User: X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: 2242380005 X-Stat-Signature: kfi39b1gkf88jhsqkh4ysnbpzgbs1t58 X-HE-Tag: 1739216302-187591 X-HE-Meta: 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 ty2THpuu r6S6+28VbQOx7NmD3xd0DXmz8sM0IyrTcvc1VYj75/tEL5gmJAQfQsuqg9mMY+nsPgJqU9AVBDeRH9sekk8WGWoY5uXr60sw8xeew3E8tKTh7f8/eRkYQVqJnxIwTH/QTt7Q6oqaaB+mNp5bBjXVBOlAaKpZqIxlfu8yZBsJIijbSCpVbYF3dAjfNGgU7eKHJgVcJ0adNvbJjj6dp5GzCTBVCeN9elIp/yCp/W/0DlCb1fwSXuG2jzet/45xdbExsPgKW472S93Bl/0HsjJrxNkDlLEsglvjeu3Ik/ve5QbjMQ1SvgPxQN5zBqDkixDKvBV7Mk2DrM/DOHnNkNIbkYivTyhHLvpYS6Zd8emfgJkxfzxs4bsmwE3WZoVbw+gaoJw1DODIDlhTNupCUnpT6je+H6SfFB8ZXD2udbhgNuGQUvYJaVOkpARBgTHP2oZ2PFG3cEZIgmGzcy89SFD2Aem8Xyw1Klj9BX1L5Z8cTrY+pM9Ndc5mge6mZCCVe3gqox38voZhHpvL3AShPBSQP8BLAavtiOwWmSxXXnVfGS5cFyPA= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: We require a writable PTE and only support anonymous folio: we can only have exactly one PTE pointing at that page, which we can just lookup using a folio walk, avoiding the rmap walk and the anon VMA lock. So let's stop doing an rmap walk and perform a folio walk instead, so we can easily just modify a single PTE and avoid relying on rmap/mapcounts. We now effectively work on a single PTE instead of multiple PTEs of a large folio, allowing for conversion of individual PTEs from non-exclusive to device-exclusive -- note that the opposite direction always works on single PTEs: restore_exclusive_pte(). With this change, device-exclusive handling is fully compatible with THPs / large folios. We still require PMD-sized THPs to get PTE-mapped, and supporting PMD-mapped THP (without the PTE-remapping) is a different endeavour that might not be worth it at this point: it might even have negative side-effects [1]. This gets rid of the "folio_mapcount()" usage and let's us fix ordinary rmap walks (migration/swapout) next. Spell out that messing with the mapcount is wrong and must be fixed. [1] https://lkml.kernel.org/r/Z5tI-cOSyzdLjoe_@phenom.ffwll.local Signed-off-by: David Hildenbrand --- mm/rmap.c | 200 ++++++++++++++++++------------------------------------ 1 file changed, 67 insertions(+), 133 deletions(-) diff --git a/mm/rmap.c b/mm/rmap.c index 7ccf850565d33..0cd2a2d3de00d 100644 --- a/mm/rmap.c +++ b/mm/rmap.c @@ -2375,131 +2375,6 @@ void try_to_migrate(struct folio *folio, enum ttu_flags flags) } #ifdef CONFIG_DEVICE_PRIVATE -struct make_exclusive_args { - struct mm_struct *mm; - unsigned long address; - void *owner; - bool valid; -}; - -static bool page_make_device_exclusive_one(struct folio *folio, - struct vm_area_struct *vma, unsigned long address, void *priv) -{ - struct mm_struct *mm = vma->vm_mm; - DEFINE_FOLIO_VMA_WALK(pvmw, folio, vma, address, 0); - struct make_exclusive_args *args = priv; - pte_t pteval; - struct page *subpage; - bool ret = true; - struct mmu_notifier_range range; - swp_entry_t entry; - pte_t swp_pte; - pte_t ptent; - - mmu_notifier_range_init_owner(&range, MMU_NOTIFY_EXCLUSIVE, 0, - vma->vm_mm, address, min(vma->vm_end, - address + folio_size(folio)), - args->owner); - mmu_notifier_invalidate_range_start(&range); - - while (page_vma_mapped_walk(&pvmw)) { - /* Unexpected PMD-mapped THP? */ - VM_BUG_ON_FOLIO(!pvmw.pte, folio); - - ptent = ptep_get(pvmw.pte); - if (!pte_present(ptent)) { - ret = false; - page_vma_mapped_walk_done(&pvmw); - break; - } - - subpage = folio_page(folio, - pte_pfn(ptent) - folio_pfn(folio)); - address = pvmw.address; - - /* Nuke the page table entry. */ - flush_cache_page(vma, address, pte_pfn(ptent)); - pteval = ptep_clear_flush(vma, address, pvmw.pte); - - /* Set the dirty flag on the folio now the pte is gone. */ - if (pte_dirty(pteval)) - folio_mark_dirty(folio); - - /* - * Check that our target page is still mapped at the expected - * address. - */ - if (args->mm == mm && args->address == address && - pte_write(pteval)) - args->valid = true; - - /* - * Store the pfn of the page in a special migration - * pte. do_swap_page() will wait until the migration - * pte is removed and then restart fault handling. - */ - if (pte_write(pteval)) - entry = make_writable_device_exclusive_entry( - page_to_pfn(subpage)); - else - entry = make_readable_device_exclusive_entry( - page_to_pfn(subpage)); - swp_pte = swp_entry_to_pte(entry); - if (pte_soft_dirty(pteval)) - swp_pte = pte_swp_mksoft_dirty(swp_pte); - if (pte_uffd_wp(pteval)) - swp_pte = pte_swp_mkuffd_wp(swp_pte); - - set_pte_at(mm, address, pvmw.pte, swp_pte); - - /* - * There is a reference on the page for the swap entry which has - * been removed, so shouldn't take another. - */ - folio_remove_rmap_pte(folio, subpage, vma); - } - - mmu_notifier_invalidate_range_end(&range); - - return ret; -} - -/** - * folio_make_device_exclusive - Mark the folio exclusively owned by a device. - * @folio: The folio to replace page table entries for. - * @mm: The mm_struct where the folio is expected to be mapped. - * @address: Address where the folio is expected to be mapped. - * @owner: passed to MMU_NOTIFY_EXCLUSIVE range notifier callbacks - * - * Tries to remove all the page table entries which are mapping this - * folio and replace them with special device exclusive swap entries to - * grant a device exclusive access to the folio. - * - * Context: Caller must hold the folio lock. - * Return: false if the page is still mapped, or if it could not be unmapped - * from the expected address. Otherwise returns true (success). - */ -static bool folio_make_device_exclusive(struct folio *folio, - struct mm_struct *mm, unsigned long address, void *owner) -{ - struct make_exclusive_args args = { - .mm = mm, - .address = address, - .owner = owner, - .valid = false, - }; - struct rmap_walk_control rwc = { - .rmap_one = page_make_device_exclusive_one, - .done = folio_not_mapped, - .anon_lock = folio_lock_anon_vma_read, - .arg = &args, - }; - - rmap_walk(folio, &rwc); - - return args.valid && !folio_mapcount(folio); -} - /** * make_device_exclusive() - Mark a page for exclusive use by a device * @mm: mm_struct of associated target process @@ -2541,22 +2416,31 @@ static bool folio_make_device_exclusive(struct folio *folio, struct page *make_device_exclusive(struct mm_struct *mm, unsigned long addr, void *owner, struct folio **foliop) { - struct folio *folio; + struct mmu_notifier_range range; + struct folio *folio, *fw_folio; + struct vm_area_struct *vma; + struct folio_walk fw; struct page *page; - long npages; + swp_entry_t entry; + pte_t swp_pte; mmap_assert_locked(mm); + addr = PAGE_ALIGN_DOWN(addr); /* * Fault in the page writable and try to lock it; note that if the * address would already be marked for exclusive use by a device, * the GUP call would undo that first by triggering a fault. + * + * If any other device would already map this page exclusively, the + * fault will trigger a conversion to an ordinary + * (non-device-exclusive) PTE and issue a MMU_NOTIFY_EXCLUSIVE. */ - npages = get_user_pages_remote(mm, addr, 1, - FOLL_GET | FOLL_WRITE | FOLL_SPLIT_PMD, - &page, NULL); - if (npages != 1) - return ERR_PTR(npages); + page = get_user_page_vma_remote(mm, addr, + FOLL_GET | FOLL_WRITE | FOLL_SPLIT_PMD, + &vma); + if (IS_ERR(page)) + return page; folio = page_folio(page); if (!folio_test_anon(folio) || folio_test_hugetlb(folio)) { @@ -2569,11 +2453,61 @@ struct page *make_device_exclusive(struct mm_struct *mm, unsigned long addr, return ERR_PTR(-EBUSY); } - if (!folio_make_device_exclusive(folio, mm, addr, owner)) { + /* + * Inform secondary MMUs that we are going to convert this PTE to + * device-exclusive, such that they unmap it now. Note that the + * caller must filter this event out to prevent livelocks. + */ + mmu_notifier_range_init_owner(&range, MMU_NOTIFY_EXCLUSIVE, 0, + mm, addr, addr + PAGE_SIZE, owner); + mmu_notifier_invalidate_range_start(&range); + + /* + * Let's do a second walk and make sure we still find the same page + * mapped writable. Note that any page of an anonymous folio can + * only be mapped writable using exactly one PTE ("exclusive"), so + * there cannot be other mappings. + */ + fw_folio = folio_walk_start(&fw, vma, addr, 0); + if (fw_folio != folio || fw.page != page || + fw.level != FW_LEVEL_PTE || !pte_write(fw.pte)) { + if (fw_folio) + folio_walk_end(&fw, vma); + mmu_notifier_invalidate_range_end(&range); folio_unlock(folio); folio_put(folio); return ERR_PTR(-EBUSY); } + + /* Nuke the page table entry so we get the uptodate dirty bit. */ + flush_cache_page(vma, addr, page_to_pfn(page)); + fw.pte = ptep_clear_flush(vma, addr, fw.ptep); + + /* Set the dirty flag on the folio now the PTE is gone. */ + if (pte_dirty(fw.pte)) + folio_mark_dirty(folio); + + /* + * Store the pfn of the page in a special device-exclusive PFN swap PTE. + * do_swap_page() will trigger the conversion back while holding the + * folio lock. + */ + entry = make_writable_device_exclusive_entry(page_to_pfn(page)); + swp_pte = swp_entry_to_pte(entry); + if (pte_soft_dirty(fw.pte)) + swp_pte = pte_swp_mksoft_dirty(swp_pte); + /* The pte is writable, uffd-wp does not apply. */ + set_pte_at(mm, addr, fw.ptep, swp_pte); + + /* + * TODO: The device-exclusive PFN swap PTE holds a folio reference but + * does not count as a mapping (mapcount), which is wrong and must be + * fixed, otherwise RMAP walks don't behave as expected. + */ + folio_remove_rmap_pte(folio, page, vma); + + folio_walk_end(&fw, vma); + mmu_notifier_invalidate_range_end(&range); *foliop = folio; return page; }