[PATCH v7 2/6] mm/memory-failure: surface unhandlable kernel pages as -ENOTRECOVERABLE

[Breno Leitao <leitao@debian.org>]

get_any_page() collapses three different failure modes into a single
-EIO return:

  * the put_page race in the !count_increased path;
  * the HWPoisonHandlable() rejection that bounces out of
    __get_hwpoison_page() with -EBUSY and exhausts shake_page() retries;
  * the HWPoisonHandlable() rejection that goes through the
    count_increased / put_page / shake_page retry loop.

The first is transient (the page is racing with the allocator).  The
second can be either transient (a userspace folio briefly off LRU
during migration/compaction) or stable (slab/vmalloc/page-table/
kernel-stack pages).  The third describes a stable kernel-owned page
that the count_increased=true caller already held a reference on.

Distinguish them on the return path: keep -EIO for both the put_page
race and the -EBUSY-after-retries branch (shake_page() cannot drag a
folio back from active migration, so we cannot prove the page is
permanently kernel-owned from there), keep -EBUSY for the allocation
race (unchanged), and return -ENOTRECOVERABLE only from the
count_increased-true HWPoisonHandlable() rejection that exhausts its
retries -- the caller's reference is structural evidence that the
page is owned by the kernel.

Extend the unhandlable-page pr_err() to fire for either errno and
update the get_hwpoison_page() kerneldoc.

memory_failure() still folds every negative return into
MF_MSG_GET_HWPOISON via its existing "else if (res < 0)" branch, so
this patch is a no-op for users of memory_failure() and only changes
the errno that soft_offline_page() can propagate to its callers.  A
follow-up wires the new return code through memory_failure() and
reports MF_MSG_KERNEL for the unrecoverable cases.

Suggested-by: David Hildenbrand <david@kernel.org>
Signed-off-by: Breno Leitao <leitao@debian.org>
---
 mm/memory-failure.c | 18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

diff --git a/mm/memory-failure.c b/mm/memory-failure.c
index 49bcfbd04d213..bae883df3ccb2 100644
--- a/mm/memory-failure.c
+++ b/mm/memory-failure.c
@@ -1408,6 +1408,15 @@ static int get_any_page(struct page *p, unsigned long flags)
 				shake_page(p);
 				goto try_again;
 			}
+			/*
+			 * Return -EIO rather than -ENOTRECOVERABLE: this
+			 * branch is also reached for pages that are merely
+			 * off-LRU transiently (e.g. a folio in the middle
+			 * of migration or compaction), which shake_page()
+			 * cannot drag back.  The caller cannot prove the
+			 * page is permanently kernel-owned from here, so
+			 * keep it on the recoverable errno.
+			 */
 			ret = -EIO;
 			goto out;
 		}
@@ -1427,10 +1436,10 @@ static int get_any_page(struct page *p, unsigned long flags)
 			goto try_again;
 		}
 		put_page(p);
-		ret = -EIO;
+		ret = -ENOTRECOVERABLE;
 	}
 out:
-	if (ret == -EIO)
+	if (ret == -EIO || ret == -ENOTRECOVERABLE)
 		pr_err("%#lx: unhandlable page.\n", page_to_pfn(p));
 
 	return ret;
@@ -1487,7 +1496,10 @@ static int __get_unpoison_page(struct page *page)
  *         -EIO for pages on which we can not handle memory errors,
  *         -EBUSY when get_hwpoison_page() has raced with page lifecycle
  *         operations like allocation and free,
- *         -EHWPOISON when the page is hwpoisoned and taken off from buddy.
+ *         -EHWPOISON when the page is hwpoisoned and taken off from buddy,
+ *         -ENOTRECOVERABLE for stable kernel-owned pages the handler
+ *         cannot recover (PG_reserved, slab, vmalloc, page tables,
+ *         kernel stacks, and similar non-LRU/non-buddy pages).
  */
 static int get_hwpoison_page(struct page *p, unsigned long flags)
 {

-- 
2.53.0-Meta