[PATCH 3/3] drm/ttm: allocate pool pages as compound (__GFP_COMP)
From: Matthew Brost
Date: Wed Jul 22 2026 - 00:42:47 EST
Historically ttm_pool_alloc_page() deliberately avoided __GFP_COMP for
higher-order allocations and instead stashed the allocation order in
page->private. The stated reason was that mapping a TTM page into a
userspace process and having that process call put_page() on it would be
illegal on a compound page, because the stray reference would fold into
the compound head and could free the whole block behind TTM's back.
That hazard no longer applies to the non-DMA path. TTM faults its pages
into userspace via VM_PFNMAP (see ttm_bo_vm_fault_reserved() /
vmf_insert_pfn_prot()), so the core mm never takes a struct page
reference on them and GUP rejects the range; a stray userspace put_page()
cannot reach these pages at all.
Convert the !ttm_pool_uses_dma_alloc() path to allocate compound pages
with __GFP_COMP and recover the order via folio_order() instead of
page->private:
- ttm_pool_alloc_page(): add __GFP_COMP, drop the page->private write.
- ttm_pool_page_order() / ttm_pool_unmap_and_free(): read the order
from folio_order() for the non-DMA case.
- ttm_pool_split_for_swap(): for compound folios, split via the new
folio_split_driver_managed() helper rather than split_page(), which
rejects compound pages. Each resulting order-0 folio is then freed
individually as it is backed up.
- Drop the now-dead page->private = 0 clears on the purge and
full-backup free paths.
The DMA path is intentionally left unchanged: dma_alloc_attrs() does not
produce compound pages, so it keeps split_page() and the page->private
order stash.
folio_split_driver_managed() lives in the THP split machinery
(mm/huge_memory.c), which only builds when CONFIG_TRANSPARENT_HUGEPAGE
is enabled. Drivers that drive the TTM shrinker and therefore reach the
split path must select TRANSPARENT_HUGEPAGE.
Cc: Maarten Lankhorst <maarten.lankhorst@xxxxxxxxxxxxxxx>
Cc: Maxime Ripard <mripard@xxxxxxxxxx>
Cc: Thomas Zimmermann <tzimmermann@xxxxxxx>
Cc: David Airlie <airlied@xxxxxxxxx>
Cc: Simona Vetter <simona@xxxxxxxx>
Cc: Christian Koenig <christian.koenig@xxxxxxx>
Cc: Huang Rui <ray.huang@xxxxxxx>
Cc: Matthew Auld <matthew.auld@xxxxxxxxx>
Cc: Matthew Brost <matthew.brost@xxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: David Hildenbrand <david@xxxxxxxxxx>
Cc: Lorenzo Stoakes <ljs@xxxxxxxxxx>
Cc: Zi Yan <ziy@xxxxxxxxxx>
Cc: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
Cc: "Liam R. Howlett" <liam@xxxxxxxxxxxxx>
Cc: Nico Pache <npache@xxxxxxxxxx>
Cc: Ryan Roberts <ryan.roberts@xxxxxxx>
Cc: Dev Jain <dev.jain@xxxxxxx>
Cc: Barry Song <baohua@xxxxxxxxxx>
Cc: Lance Yang <lance.yang@xxxxxxxxx>
Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxx>
Cc: Dave Airlie <airlied@xxxxxxxxxx>
Cc: dri-devel@xxxxxxxxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: linux-mm@xxxxxxxxx
Suggested-by: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Signed-off-by: Matthew Brost <matthew.brost@xxxxxxxxx>
Assisted-by: GitHub-Copilot:claude-opus-4.8
---
drivers/gpu/drm/ttm/tests/ttm_pool_test.c | 23 +++++++++---
drivers/gpu/drm/ttm/ttm_backup.c | 8 ++--
drivers/gpu/drm/ttm/ttm_pool.c | 46 +++++++++++++++--------
3 files changed, 53 insertions(+), 24 deletions(-)
diff --git a/drivers/gpu/drm/ttm/tests/ttm_pool_test.c b/drivers/gpu/drm/ttm/tests/ttm_pool_test.c
index be75c8abf388..771ed257778c 100644
--- a/drivers/gpu/drm/ttm/tests/ttm_pool_test.c
+++ b/drivers/gpu/drm/ttm/tests/ttm_pool_test.c
@@ -169,7 +169,14 @@ static void ttm_pool_alloc_basic(struct kunit *test)
KUNIT_ASSERT_NOT_NULL(test, (void *)fst_page->private);
KUNIT_ASSERT_NOT_NULL(test, (void *)last_page->private);
} else {
- KUNIT_ASSERT_EQ(test, fst_page->private, params->order);
+ /*
+ * The non-DMA path allocates compound pages, so the
+ * order is recovered from the folio rather than from
+ * page->private.
+ */
+ KUNIT_ASSERT_EQ(test,
+ folio_order(page_folio(fst_page)),
+ params->order);
}
} else {
if (ttm_pool_uses_dma_alloc(pool)) {
@@ -177,13 +184,19 @@ static void ttm_pool_alloc_basic(struct kunit *test)
KUNIT_ASSERT_NULL(test, (void *)last_page->private);
} else {
/*
- * We expect to alloc one big block, followed by
- * order 0 blocks
+ * We expect to alloc one or more max-order compound
+ * blocks. page_folio() on any subpage resolves to the
+ * compound head, so both the first and last pages
+ * report the max block order.
*/
- KUNIT_ASSERT_EQ(test, fst_page->private,
+ KUNIT_ASSERT_EQ(test,
+ folio_order(page_folio(fst_page)),
+ min_t(unsigned int, MAX_PAGE_ORDER,
+ params->order));
+ KUNIT_ASSERT_EQ(test,
+ folio_order(page_folio(last_page)),
min_t(unsigned int, MAX_PAGE_ORDER,
params->order));
- KUNIT_ASSERT_EQ(test, last_page->private, 0);
}
}
diff --git a/drivers/gpu/drm/ttm/ttm_backup.c b/drivers/gpu/drm/ttm/ttm_backup.c
index 3c067aadc52d..9194747a1dff 100644
--- a/drivers/gpu/drm/ttm/ttm_backup.c
+++ b/drivers/gpu/drm/ttm/ttm_backup.c
@@ -72,9 +72,11 @@ int ttm_backup_copy_page(struct file *backup, struct page *dst,
* ttm_backup_backup_folio() - Backup a folio
* @backup: The struct backup pointer to use.
* @folio: The folio to back up.
- * @order: The allocation order of @folio. Since TTM allocates higher-order
- * pages without __GFP_COMP, folio_nr_pages(@folio) would always
- * return 1; the caller must pass the true order explicitly.
+ * @order: The allocation order of @folio, passed explicitly. For the DMA
+ * path TTM allocates higher-order pages without __GFP_COMP, so
+ * folio_order(@folio) would return 0 rather than the true order;
+ * the caller therefore passes the order explicitly. (The non-DMA
+ * path allocates compound pages, for which the two agree.)
* @writeback: Whether to perform immediate writeback of the folio's pages.
* This may have performance implications.
* @idx: A unique integer for the first page of the folio and each struct backup.
diff --git a/drivers/gpu/drm/ttm/ttm_pool.c b/drivers/gpu/drm/ttm/ttm_pool.c
index 1bf37023fed6..364cc7ec7469 100644
--- a/drivers/gpu/drm/ttm/ttm_pool.c
+++ b/drivers/gpu/drm/ttm/ttm_pool.c
@@ -168,9 +168,11 @@ static struct page *ttm_pool_alloc_page(struct ttm_pool *pool, gfp_t gfp_flags,
struct page *p;
void *vaddr;
- /* Don't set the __GFP_COMP flag for higher order allocations.
- * Mapping pages directly into an userspace process and calling
- * put_page() on a TTM allocated page is illegal.
+ /*
+ * For higher-order allocations be a good citizen: don't dip into
+ * memory reserves, don't retry hard, don't warn on failure and stay
+ * on the local node. The non-DMA path additionally sets __GFP_COMP
+ * below; the DMA path allocates via dma_alloc_attrs().
*/
if (order)
gfp_flags |= __GFP_NOMEMALLOC | __GFP_NORETRY | __GFP_NOWARN |
@@ -189,11 +191,9 @@ static struct page *ttm_pool_alloc_page(struct ttm_pool *pool, gfp_t gfp_flags,
}
if (!ttm_pool_uses_dma_alloc(pool)) {
- p = alloc_pages_node(pool->nid, gfp_flags, order);
- if (p) {
- p->private = order;
+ p = alloc_pages_node(pool->nid, gfp_flags | __GFP_COMP, order);
+ if (p)
mod_lruvec_page_state(p, NR_GPU_ACTIVE, 1 << order);
- }
return p;
}
@@ -482,7 +482,7 @@ static unsigned int ttm_pool_page_order(struct ttm_pool *pool, struct page *p)
return dma->vaddr & ~PAGE_MASK;
}
- return p->private;
+ return folio_order(page_folio(p));
}
/*
@@ -493,15 +493,31 @@ static unsigned int ttm_pool_page_order(struct ttm_pool *pool, struct page *p)
static void ttm_pool_split_for_swap(struct ttm_pool *pool, struct page *p)
{
unsigned int order = ttm_pool_page_order(pool, p);
- pgoff_t nr;
if (!order)
return;
- split_page(p, order);
- nr = 1UL << order;
- while (nr--)
- (p++)->private = 0;
+ if (ttm_pool_uses_dma_alloc(pool)) {
+ pgoff_t nr;
+
+ /*
+ * DMA-alloc pages are not compound; split the plain
+ * higher-order allocation and clear the per-page private
+ * (which held the order for the non-compound case).
+ */
+ split_page(p, order);
+ nr = 1UL << order;
+ while (nr--)
+ (p++)->private = 0;
+ return;
+ }
+
+ /*
+ * The non-DMA path allocates compound folios (__GFP_COMP). Split the
+ * driver-owned, off-LRU, unmapped folio into order-0 folios so each
+ * page can be freed as soon as it has been backed up.
+ */
+ folio_split_driver_managed(page_folio(p), 0);
}
/**
@@ -548,7 +564,7 @@ static pgoff_t ttm_pool_unmap_and_free(struct ttm_pool *pool, struct page *page,
pt = ttm_pool_select_type(pool, caching, order);
} else {
- order = page->private;
+ order = folio_order(page_folio(page));
nr = (1UL << order);
}
@@ -1124,7 +1140,6 @@ long ttm_pool_backup(struct ttm_pool *pool, struct ttm_tt *tt,
num_pages);
if (flags->purge) {
shrunken += num_pages;
- page->private = 0;
__free_pages_gpu_account(page, order, false);
memset(tt->pages + i, 0,
num_pages * sizeof(*tt->pages));
@@ -1214,7 +1229,6 @@ long ttm_pool_backup(struct ttm_pool *pool, struct ttm_tt *tt,
}
/* Fully backed up: free at native order. */
- page->private = 0;
__free_pages_gpu_account(page, order, false);
}
--
2.34.1