linuxkpi: work with numpages > 1 in the set_pages_*() KPIs

These calls are used for buddy pages at least in drm's ttm_pool, which
leads to a panic when we invoke lowmem handlers and drm tries to shrink
the pool.

Cope with numpages > 1 by traversing the contiguous pages and executing
the adjustment there, as well, as suggested by markj@.  Previous
versions have tried to use the corresponding `set_memory_*()` functions,
but it is believed that not updating `md.pat_mode` breaks subsequent
userspace mappings in ways that may result in things like screen tearing
or other artifacts when running i915kms.

This stabilized my amdgpu laptop running two VMs, chromium and a
concurrent buildworld.

Reviewed by:	bz, markj
Differential Revision:	https://reviews.freebsd.org/D57004

2 files changed, 26 insertions, 12 deletions

diff --git a/sys/compat/linuxkpi/common/include/asm/set_memory.h b/sys/compat/linuxkpi/common/include/asm/set_memory.h
index f328fcabd243..f45a51a9710b 100644
--- a/sys/compat/linuxkpi/common/include/asm/set_memory.h
+++ b/sys/compat/linuxkpi/common/include/asm/set_memory.h
@@ -62,35 +62,28 @@ set_memory_wb(unsigned long addr, int numpages)
 	return (-pmap_change_attr((void *)addr, len, VM_MEMATTR_WRITE_BACK));
 }
 
+int lkpi_set_pages_attr(struct page *page, int numpages, vm_memattr_t ma);
+
 static inline int
 set_pages_uc(struct page *page, int numpages)
 {
-	KASSERT(numpages == 1, ("%s: numpages %d", __func__, numpages));
-
-	pmap_page_set_memattr(page, VM_MEMATTR_UNCACHEABLE);
-	return (0);
+	return (lkpi_set_pages_attr(page, numpages, VM_MEMATTR_UNCACHEABLE));
 }
 
 static inline int
 set_pages_wc(struct page *page, int numpages)
 {
-	KASSERT(numpages == 1, ("%s: numpages %d", __func__, numpages));
-
 #ifdef VM_MEMATTR_WRITE_COMBINING
-	pmap_page_set_memattr(page, VM_MEMATTR_WRITE_COMBINING);
+	return (lkpi_set_pages_attr(page, numpages, VM_MEMATTR_WRITE_COMBINING));
 #else
 	return (set_pages_uc(page, numpages));
 #endif
-	return (0);
 }
 
 static inline int
 set_pages_wb(struct page *page, int numpages)
 {
-	KASSERT(numpages == 1, ("%s: numpages %d", __func__, numpages));
-
-	pmap_page_set_memattr(page, VM_MEMATTR_WRITE_BACK);
-	return (0);
+	return (lkpi_set_pages_attr(page, numpages, VM_MEMATTR_WRITE_BACK));
 }
 
 static inline int
diff --git a/sys/compat/linuxkpi/common/src/linux_page.c b/sys/compat/linuxkpi/common/src/linux_page.c
index b91115a5ff16..62bfa07dd297 100644
--- a/sys/compat/linuxkpi/common/src/linux_page.c
+++ b/sys/compat/linuxkpi/common/src/linux_page.c
@@ -710,6 +710,27 @@ lkpi_arch_phys_wc_del(int reg)
 #endif
 }
 
+int
+lkpi_set_pages_attr(struct page *page, int numpages, vm_memattr_t ma)
+{
+	while (numpages-- > 0) {
+		/*
+		 * pmap_page_set_memattr() would only update the DMAP mapping
+		 * if it's a normal page, leaving the kernel map untouched.
+		 */
+		MPASS(page->object != kernel_object);
+
+		/*
+		 * pmap_page_set_memattr() sets page->md.pat_mode, which is
+		 * crucial for future userspace mappings.
+		 */
+		pmap_page_set_memattr(page, ma);
+		page++;
+	}
+
+	return (0);
+}
+
 /*
  * This is a highly simplified version of the Linux page_frag_cache.
  * We only support up-to 1 single page as fragment size and we will