[PATCH v2] drm/pagemap: Fix folio allocation fallback and use-after-put

drm_pagemap_migrate_populate_ram_pfn() had two issues when populating
RAM PFNs with higher-order folios:

1. The higher-order vma_alloc_folio()/folio_alloc() calls did not pass
   __GFP_NOWARN, so a THP allocation failure under memory pressure
   would spam the kernel log, and there was no fallback path despite a
   TODO comment stating one was needed. Add __GFP_NOWARN to the
   higher-order allocation and, on failure, fall back to order-0
   allocations for the entire range originally covered by the failed
   higher-order allocation, leaving MIGRATE_PFN_COMPOUND unset for
   those PFNs.

2. In the free_pages error path, order was computed via
   folio_order(page_folio(page)) *after* put_page(page) had already
   dropped the reference, resulting in a use-after-free/put when that
   was the last reference on the page. Compute order before releasing
   the page.

Fixes: ddeda6136038 ("drm/pagemap: Allocate folios when possible")
Cc: [email protected]
Assisted-by: Claude:claude-sonnet-5
Signed-off-by: Matthew Brost <[email protected]>

---
v2:
 - Bound order-0 by num_pages (Sashiko)
---
 drivers/gpu/drm/drm_pagemap.c | 33 ++++++++++++++++++++++++++++-----
 1 file changed, 28 insertions(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/drm_pagemap.c b/drivers/gpu/drm/drm_pagemap.c
index 892b325fa99b..923f96d5c40e 100644
--- a/drivers/gpu/drm/drm_pagemap.c
+++ b/drivers/gpu/drm/drm_pagemap.c
@@ -891,11 +891,34 @@ static int drm_pagemap_migrate_populate_ram_pfn(struct vm_area_struct *vas,
 
 		order = folio_order(page_folio(src_page));
 
-		/* TODO: Support fallback to single pages if THP allocation fails */
 		if (vas)
-			folio = vma_alloc_folio(GFP_HIGHUSER, order, vas, addr);
+			folio = vma_alloc_folio(GFP_HIGHUSER | __GFP_NOWARN, order, vas, addr);
 		else
-			folio = folio_alloc(GFP_HIGHUSER, order);
+			folio = folio_alloc(GFP_HIGHUSER | __GFP_NOWARN, order);
+
+		if (!folio && order) {
+			/*
+			 * Higher-order allocation failed, fall back to
+			 * order-0 allocations for the entire range covered
+			 * by the original higher-order allocation, without
+			 * setting MIGRATE_PFN_COMPOUND, until we move past
+			 * that range.
+			 */
+			unsigned long nr = NR_PAGES(order);
+			unsigned long j;
+
+			for (j = 0; j < nr && i < npages; j++, i++, addr += PAGE_SIZE) {
+				folio = vas ?
+					vma_alloc_folio(GFP_HIGHUSER, 0, vas, addr) :
+					folio_alloc(GFP_HIGHUSER, 0);
+				if (!folio)
+					goto free_pages;
+
+				page = folio_page(folio, 0);
+				mpfn[i] = migrate_pfn(page_to_pfn(page));
+			}
+			continue;
+		}
 
 		if (!folio)
 			goto free_pages;
@@ -940,11 +963,11 @@ static int drm_pagemap_migrate_populate_ram_pfn(struct vm_area_struct *vas,
 		if (!page)
 			goto next_put;
 
+		order = folio_order(page_folio(page));
+
 		put_page(page);
 		mpfn[i] = 0;
 
-		order = folio_order(page_folio(page));
-
 next_put:
 		i += NR_PAGES(order);
 	}
-- 
2.34.1