[PATCH v2 17/17] mm/rmap: avoid -EBUSY from make_device_exclusive()

[David Hildenbrand <david@redhat.com>]

Failing to obtain the folio lock, for example because the folio is
concurrently getting migrated or swapped out, can easily make the callers
fail: for example, the hmm selftest can sometimes be observed to fail
because of this. Instead of forcing the caller to retry, let's simply
retry in this to-be-expected case.

Similarly, avoid spurious failures simply because we raced with someone
(e.g., swapout) modifying the page table such that our folio_walk fails.

Simply unconditionally lock the folio, and retry GUP if our folio_walk
fails. Note that the folio_walk repeatedly failing is not something we
expect.

Note that we might want to avoid grabbing the folio lock at some point;
for now, keep that as is and only unconditionally lock the folio.

With this change, the hmm selftests don't fail simply because the folio
is already locked. While this fixes the selftests in some cases, it's
likely not something that deserves a "Fixes:".

Signed-off-by: David Hildenbrand <david@redhat.com>
---
 mm/rmap.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/mm/rmap.c b/mm/rmap.c
index e2a543f639ce3..0f760b93fc0a2 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -2435,6 +2435,7 @@ struct page *make_device_exclusive(struct mm_struct *mm, unsigned long addr,
 	struct page *page;
 	swp_entry_t entry;
 	pte_t swp_pte;
+	int ret;
 
 	mmap_assert_locked(mm);
 	addr = PAGE_ALIGN_DOWN(addr);
@@ -2448,6 +2449,7 @@ struct page *make_device_exclusive(struct mm_struct *mm, unsigned long addr,
 	 * fault will trigger a conversion to an ordinary
 	 * (non-device-exclusive) PTE and issue a MMU_NOTIFY_EXCLUSIVE.
 	 */
+retry:
 	page = get_user_page_vma_remote(mm, addr,
 					FOLL_GET | FOLL_WRITE | FOLL_SPLIT_PMD,
 					&vma);
@@ -2460,9 +2462,10 @@ struct page *make_device_exclusive(struct mm_struct *mm, unsigned long addr,
 		return ERR_PTR(-EOPNOTSUPP);
 	}
 
-	if (!folio_trylock(folio)) {
+	ret = folio_lock_killable(folio);
+	if (ret) {
 		folio_put(folio);
-		return ERR_PTR(-EBUSY);
+		return ERR_PTR(ret);
 	}
 
 	/*
@@ -2488,7 +2491,7 @@ struct page *make_device_exclusive(struct mm_struct *mm, unsigned long addr,
 		mmu_notifier_invalidate_range_end(&range);
 		folio_unlock(folio);
 		folio_put(folio);
-		return ERR_PTR(-EBUSY);
+		goto retry;
 	}
 
 	/* Nuke the page table entry so we get the uptodate dirty bit. */
-- 
2.48.1