[PATCH v2] drm/ttm: Fix compile error when CONFIG_SHMEM is not set

[Steven Rostedt]

From: Steven Rostedt <rostedt at goodmis.org>

When CONFIG_SHMEM is not set, the following compiler error occurs:

ld: vmlinux.o: in function `ttm_backup_backup_page':
(.text+0x10363bc): undefined reference to `shmem_writeout'
make[3]: *** [/work/build/trace/nobackup/linux.git/scripts/Makefile.vmlinux:91: vmlinux.unstripped] Error 1
make[2]: *** [/work/build/trace/nobackup/linux.git/Makefile:1241: vmlinux] Error 2
make[1]: *** [/work/build/trace/nobackup/linux.git/Makefile:248: __sub-make] Error 2
make[1]: Leaving directory '/work/build/nobackup/tracetest'
make: *** [Makefile:248: __sub-make] Error 2

This is due to the replacement of writepage and calling swap_writeout()
and shmem_writeout() directly. The issue is that when CONFIG_SHMEM is not
defined, shmem_writeout() is also not defined.

The function ttm_backup_backup_page() called mapping->a_ops->writepage()
which was then changed to call shmem_writeout() directly.

Even before commit 84798514db50 ("mm: Remove swap_writepage() and
shmem_writepage()"), it didn't make sense to call anything other than
shmem_writeout() as the ttm_backup deals only with shmem folios.

Have DRM_TTM config option select SHMEM to guarantee that shmem_writeout()
is available.

Link: https://lore.kernel.org/all/20250602170500.48713a2b@gandalf.local.home/

Suggested-by: Hugh Dickins <hughd at google.com>
Fixes: 84798514db50 ("mm: Remove swap_writepage() and shmem_writepage()")
Signed-off-by: Steven Rostedt (Google) <rostedt at goodmis.org>
---
Changes since v1: https://lore.kernel.org/all/20250602170500.48713a2b@gandalf.local.home/

- Instead of adding a shmem_writeout() stub, just make CONFIG_DRM_TTM
  select CONFIG_SHMEM (Hugh Dickins)

 drivers/gpu/drm/Kconfig | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/gpu/drm/Kconfig b/drivers/gpu/drm/Kconfig
index f094797f3b2b..ded28c71d89c 100644
--- a/drivers/gpu/drm/Kconfig
+++ b/drivers/gpu/drm/Kconfig
@@ -188,6 +188,7 @@ source "drivers/gpu/drm/display/Kconfig"
 config DRM_TTM
 	tristate
 	depends on DRM && MMU
+	select SHMEM
 	help
 	  GPU memory management subsystem for devices with multiple
 	  GPU memory types. Will be enabled automatically if a device driver
-- 
2.47.2