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(). 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@xxxxxxxxxx>
---
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
