mm/khugepaged: map dirty/writeback pages failures to EAGAIN

mm/khugepaged: map dirty/writeback pages failures to EAGAIN

Patch series "mm/khugepaged: fix dirty page handling for MADV_COLLAPSE",
v5.

MADV_COLLAPSE on file-backed mappings fails with -EINVAL when TEXT pages
are dirty. This affects scenarios like package/container updates or
executing binaries immediately after writing them, etc.

The issue is that collapse_file() triggers async writeback and returns
SCAN_FAIL (maps to -EINVAL), expecting khugepaged to revisit later. But
MADV_COLLAPSE is synchronous and userspace expects immediate success or
a clear retry signal.

Reproduction:
 - Compile or copy 2MB-aligned executable to XFS/ext4 FS
 - Call MADV_COLLAPSE on .text section
 - First call fails with -EINVAL (text pages dirty from copy)
 - Second call succeeds (async writeback completed)

Issue Report:
https://lore.kernel.org/all/4e26fe5e-7374-467c-a333-9dd48f85d7cc@amd.com

This patch (of 2):

When collapse_file encounters dirty or writeback pages in file-backed
mappings, it currently returns SCAN_FAIL which maps to -EINVAL.  This is
misleading as EINVAL suggests invalid arguments, whereas dirty/writeback
pages represent transient conditions that may resolve on retry.

Introduce SCAN_PAGE_DIRTY_OR_WRITEBACK to cover both dirty and writeback
states, mapping it to -EAGAIN.  For MADV_COLLAPSE, this provides userspace
with a clear signal that retry may succeed after writeback completes.  For
khugepaged, this is harmless as it will naturally revisit the range during
periodic scans after async writeback completes.

Link: https://lkml.kernel.org/r/20260118190939.8986-2-shivankg@amd.com
Link: https://lkml.kernel.org/r/20260118190939.8986-4-shivankg@amd.com
Fixes: 34488399fa08 ("mm/madvise: add file and shmem support to MADV_COLLAPSE")
Signed-off-by: Shivank Garg <shivankg@amd.com>
Reported-by: Branden Moore <Branden.Moore@amd.com>
Closes: https://lore.kernel.org/all/4e26fe5e-7374-467c-a333-9dd48f85d7cc@amd.com
Reviewed-by: Dev Jain <dev.jain@arm.com>
Reviewed-by: Lance Yang <lance.yang@linux.dev>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Reviewed-by: wang lian <lianux.mm@gmail.com>
Acked-by: David Hildenbrand (Red Hat) <david@kernel.org>
Cc: Barry Song <baohua@kernel.org>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Nico Pache <npache@redhat.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Zach O'Keefe <zokeefe@google.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>