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

["Uladzislau Rezki (Sony)" <urezki@gmail.com>]

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