Re: [PATCH v1 04/12] mm/rmap: implement make_device_exclusive() using folio_walk instead of rmap walk

[Alistair Popple <apopple@nvidia.com>]

On Wed, Jan 29, 2025 at 12:54:02PM +0100, David Hildenbrand wrote:
> 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 other way always
> worked on single PTEs.
> 
> We can drop the MMU_NOTIFY_EXCLUSIVE MMU notifier call and document why
> that is not required: GUP will already take care of the
> MMU_NOTIFY_EXCLUSIVE call if required (there is already a device-exclusive
> entry) when not finding a present PTE and having to trigger a fault and
> ending up in remove_device_exclusive_entry().

I will have to look at this a bit more closely tomorrow but this doesn't seem
right to me. We may be transitioning from a present PTE (ie. a writable
anonymous mapping) to a non-present PTE (ie. a device-exclusive entry) and
therefore any secondary processors (eg. other GPUs, iommus, etc.) will need to
update their copies of the PTE. So I think the notifier call is needed.

> Note that the PTE is
> always writable, and we can always create a writable-device-exclusive
> entry.
> 
> With this change, device-exclusive 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.
> 
> 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.
> 
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
>  mm/rmap.c | 188 ++++++++++++++++--------------------------------------
>  1 file changed, 55 insertions(+), 133 deletions(-)
> 
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 676df4fba5b0..49ffac6d27f8 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 an address for exclusive use by a device
>   * @mm: mm_struct of associated target process
> @@ -2530,9 +2405,12 @@ 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 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);
>  
> @@ -2540,12 +2418,16 @@ struct page *make_device_exclusive(struct mm_struct *mm, unsigned long addr,
>  	 * Fault in the page writable and try to lock it; note that if the
>  	 * address would already be marked for exclusive use by the 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)) {
> @@ -2558,11 +2440,51 @@ 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)) {
> +	/*
> +	 * Let's do a second walk and make sure we still find the same page
> +	 * mapped writable. If we don't find what we expect, we will trigger
> +	 * GUP again to fix it up. Note that a page of an anonymous folio can
> +	 * only be mapped writable using exactly one page table mapping
> +	 * ("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);
>  		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 non-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 non-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);
>  	*foliop = folio;
>  	return page;
>  }
> -- 
> 2.48.1
>