[RFC 6/7] mm/vmalloc: Support non-blocking GFP flags in __vmalloc_area_node()

Uladzislau Rezki (Sony)
2025-07-04T15:26:00Z

This patch makes __vmalloc_area_node() to correctly handle non-blocking
allocation requests, such as GFP_ATOMIC and GFP_NOWAIT. Main changes:

- nested_gfp flag follows the same non-blocking constraints
as the primary gfp_mask, ensuring consistency and avoiding
sleeping allocations in atomic contexts.

- if blocking is not allowed, __GFP_NOFAIL is forcibly cleared
and warning is issued if it was set, since __GFP_NOFAIL is
incompatible with non-blocking contexts;

- Add a __GFP_HIGHMEM to gfp_mask only for blocking requests
if there are no DMA constraints.

- in non-blocking mode we use memalloc_noreclaim_save/restore()
to prevent reclaim related operations that may sleep while
setting up page tables or mapping pages.

This is particularly important for page table allocations that
internally use GFP_PGTABLE_KERNEL, which may sleep unless such
scope restrictions are applied. For example:

<snip>
#define GFP_PGTABLE_KERNEL (GFP_KERNEL | __GFP_ZERO)

__pte_alloc_kernel()
pte_alloc_one_kernel(&init_mm);
pagetable_alloc_noprof(GFP_PGTABLE_KERNEL & ~__GFP_HIGHMEM, 0);
<snip>

Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
---
mm/vmalloc.c | 30 +++++++++++++++++++++++++-----
1 file changed, 25 insertions(+), 5 deletions(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 2eaff0575a9e..fe1699e01e02 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -3711,7 +3711,7 @@ static void *__vmalloc_area_node(struct vm_struct *area, gfp_t gfp_mask,
pgprot_t prot, unsigned int page_shift,
int node)
{
- const gfp_t nested_gfp = (gfp_mask & GFP_RECLAIM_MASK) | __GFP_ZERO;
+ gfp_t nested_gfp = (gfp_mask & GFP_RECLAIM_MASK) | __GFP_ZERO;
bool nofail = gfp_mask & __GFP_NOFAIL;
unsigned long addr = (unsigned long)area->addr;
unsigned long size = get_vm_area_size(area);
@@ -3719,12 +3719,28 @@ static void *__vmalloc_area_node(struct vm_struct *area, gfp_t gfp_mask,
unsigned int nr_small_pages = size >> PAGE_SHIFT;
unsigned int page_order;
unsigned int flags;
+ bool noblock;
int ret;

array_size = (unsigned long)nr_small_pages * sizeof(struct page *);
+ noblock = !gfpflags_allow_blocking(gfp_mask);

- if (!(gfp_mask & (GFP_DMA | GFP_DMA32)))
- gfp_mask |= __GFP_HIGHMEM;
+ if (noblock) {
+ /* __GFP_NOFAIL is incompatible with non-blocking contexts. */
+ WARN_ON_ONCE(gfp_mask & __GFP_NOFAIL);
+ gfp_mask &= ~__GFP_NOFAIL;
+
+ /*
+ * In non-sleeping contexts, ensure nested allocations follow
+ * same non-blocking rules.
+ */
+ nested_gfp = gfp_mask | __GFP_ZERO;
+ nofail = false;
+ } else {
+ /* Allow highmem allocations if there are no DMA constraints. */
+ if (!(gfp_mask & (GFP_DMA | GFP_DMA32)))
+ gfp_mask |= __GFP_HIGHMEM;
+ }

/* Please note that the recursion is strictly bounded. */
if (array_size > PAGE_SIZE) {
@@ -3788,7 +3804,9 @@ static void *__vmalloc_area_node(struct vm_struct *area, gfp_t gfp_mask,
* page tables allocations ignore external gfp mask, enforce it
* by the scope API
*/
- if ((gfp_mask & (__GFP_FS | __GFP_IO)) == __GFP_IO)
+ if (noblock)
+ flags = memalloc_noreclaim_save();
+ else if ((gfp_mask & (__GFP_FS | __GFP_IO)) == __GFP_IO)
flags = memalloc_nofs_save();
else if ((gfp_mask & (__GFP_FS | __GFP_IO)) == 0)
flags = memalloc_noio_save();
@@ -3800,7 +3818,9 @@ static void *__vmalloc_area_node(struct vm_struct *area, gfp_t gfp_mask,
schedule_timeout_uninterruptible(1);
} while (nofail && (ret < 0));

- if ((gfp_mask & (__GFP_FS | __GFP_IO)) == __GFP_IO)
+ if (noblock)
+ memalloc_noreclaim_restore(flags);
+ else if ((gfp_mask & (__GFP_FS | __GFP_IO)) == __GFP_IO)
memalloc_nofs_restore(flags);
else if ((gfp_mask & (__GFP_FS | __GFP_IO)) == 0)
memalloc_noio_restore(flags);
--
2.39.5