Peter Xu peterx@redhat.com writes:
On Wed, Aug 24, 2022 at 01:03:38PM +1000, Alistair Popple wrote:
migrate_vma_setup() has a fast path in migrate_vma_collect_pmd() that installs migration entries directly if it can lock the migrating page. When removing a dirty pte the dirty bit is supposed to be carried over to the underlying page to prevent it being lost.
Currently migrate_vma_*() can only be used for private anonymous mappings. That means loss of the dirty bit usually doesn't result in data loss because these pages are typically not file-backed. However pages may be backed by swap storage which can result in data loss if an attempt is made to migrate a dirty page that doesn't yet have the PageDirty flag set.
In this case migration will fail due to unexpected references but the dirty pte bit will be lost. If the page is subsequently reclaimed data won't be written back to swap storage as it is considered uptodate, resulting in data loss if the page is subsequently accessed.
Prevent this by copying the dirty bit to the page when removing the pte to match what try_to_migrate_one() does.
Signed-off-by: Alistair Popple apopple@nvidia.com Acked-by: Peter Xu peterx@redhat.com Reported-by: Huang Ying ying.huang@intel.com Fixes: 8c3328f1f36a ("mm/migrate: migrate_vma() unmap page from vma while collecting pages") Cc: stable@vger.kernel.org
Changes for v3:
- Defer TLB flushing
- Split a TLB flushing fix into a separate change.
Changes for v2:
- Fixed up Reported-by tag.
- Added Peter's Acked-by.
- Atomically read and clear the pte to prevent the dirty bit getting set after reading it.
- Added fixes tag
mm/migrate_device.c | 9 +++++++-- 1 file changed, 7 insertions(+), 2 deletions(-)
diff --git a/mm/migrate_device.c b/mm/migrate_device.c index 6a5ef9f..51d9afa 100644 --- a/mm/migrate_device.c +++ b/mm/migrate_device.c @@ -7,6 +7,7 @@ #include <linux/export.h> #include <linux/memremap.h> #include <linux/migrate.h> +#include <linux/mm.h> #include <linux/mm_inline.h> #include <linux/mmu_notifier.h> #include <linux/oom.h> @@ -196,7 +197,7 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp, anon_exclusive = PageAnon(page) && PageAnonExclusive(page); if (anon_exclusive) { flush_cache_page(vma, addr, pte_pfn(*ptep));
ptep_clear_flush(vma, addr, ptep);
pte = ptep_clear_flush(vma, addr, ptep); if (page_try_share_anon_rmap(page)) { set_pte_at(mm, addr, ptep, pte);@@ -206,11 +207,15 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp, goto next; } } else {
ptep_get_and_clear(mm, addr, ptep);
pte = ptep_get_and_clear(mm, addr, ptep); }I remember that in v2 both flush_cache_page() and ptep_get_and_clear() are moved above the condition check so they're called unconditionally. Could you explain the rational on why it's changed back (since I think v2 was the correct approach)?
Mainly because I agree with your original comments, that it would be better to keep the batching of TLB flushing if possible. After the discussion I don't think there is any issues with HW pte dirty bits here. There are already other cases where HW needs to get that right anyway (eg. zap_pte_range).
The other question is if we want to split the patch, would it be better to move the tlb changes to patch 1, and leave the dirty bit fix in patch 2?
Isn't that already the case? Patch 1 moves the TLB flush before the PTL as suggested, patch 2 atomically copies the dirty bit without changing any TLB flushing.
migrate->cpages++;
/* Set the dirty flag on the folio now the pte is gone. */if (pte_dirty(pte))folio_mark_dirty(page_folio(page));/* Setup special migration page table entry */ if (mpfn & MIGRATE_PFN_WRITE) entry = make_writable_migration_entry(-- git-series 0.9.1