mm/khugepaged: return -EAGAIN for SCAN_PAGE_HAS_PRIVATE in MADV_COLLAPSE

MADV_COLLAPSE uses errno values to provide actionable feedback to
userspace.  Temporary resource constraints are mapped to -EAGAIN so the
caller may retry, while intrinsic failures of the specified range are
mapped to -EINVAL.

collapse_file() returns SCAN_PAGE_HAS_PRIVATE when filemap_release_folio()
fails while isolating file-backed folios for collapse.  This currently
falls through the default case in madvise_collapse_errno() and is reported
to userspace as -EINVAL.

However, filemap_release_folio() failure commonly reflects temporary folio
state rather than a permanently uncollapsible range.

For example, ext4 returns false when a folio still has dirty journalled
data, btrfs returns false for dirty or writeback folios before extent
state release, and NFS may return false while reclaiming
filesystem-private folio state.

In such cases, retrying MADV_COLLAPSE after writeback, reclaim or journal
progress may succeed.  This matches the existing -EAGAIN handling for
SCAN_PAGE_DIRTY_OR_WRITEBACK and other transient collapse failures more
closely than -EINVAL.

Therefore, map SCAN_PAGE_HAS_PRIVATE to -EAGAIN so userspace receives
retryable feedback for this temporary failure path.

Link: https://lore.kernel.org/20260429140434.439456-1-agarwal.vineet2006@gmail.com
Signed-off-by: Vineet Agarwal <agarwal.vineet2006@gmail.com>
Reviewed-by: Dev Jain <dev.jain@arm.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Liam Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Nico Pache <npache@redhat.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/khugepaged.c

@@ -2808,6 +2808,7 @@ static int madvise_collapse_errno(enum scan_result r)
 	case SCAN_PAGE_LRU:
 	case SCAN_DEL_PAGE_LRU:
 	case SCAN_PAGE_FILLED:
+	case SCAN_PAGE_HAS_PRIVATE:
 	case SCAN_PAGE_DIRTY_OR_WRITEBACK:
 		return -EAGAIN;
 	/*