[PATCH v3] mm/vmalloc: clarify why vmap_range_noflush() might sleep

[Brendan Jackman <jackmanb@google.com>]

The only reason vmap_range_noflush() can sleep is because of pagetable
allocations.

The actual allocation mechanism is arch-specific so might_alloc()
doesn't work here (what GFP flags would be used?). Hence, just add a
comment.

Also note that this might do a TLB shootdown. This is not actually
sleeping but it requires IRQs on for x86, and might_sleep() incidentally
serves to detect violations of that too.

Signed-off-by: Brendan Jackman <jackmanb@google.com>
---
Applies to mm-new.

Changes in v3:
- Added note about TLB shootdowns and x86.
- Since the comment no longer fit on one line anyway, also be a bit more
  verbose in general.
- Link to v2: https://lore.kernel.org/r/20251209-b4-vmalloc-might_alloc-v2-1-2462ba26ead1@google.com

Changes in v2:
- Switched from might_alloc() to keeping might_sleep() + comment
- Link to v1: https://lore.kernel.org/r/20251208-b4-vmalloc-might_alloc-v1-1-94a9bb8ecb08@google.com
---
 mm/vmalloc.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 41dd01e8430c548167d6123f6ec66bf798f20eec..be4e5f2566a4490d45de29568df538b7c8515b01 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -305,6 +305,11 @@ static int vmap_range_noflush(unsigned long addr, unsigned long end,
 	int err;
 	pgtbl_mod_mask mask = 0;
 
+	/*
+	 * Might allocate pagetables (for most archs a more precise annotation
+	 * would be might_alloc(GFP_PGTABLE_KERNEL)). Also might shootdown TLB
+	 * (requires IRQs enabled on x86).
+	 */
 	might_sleep();
 	BUG_ON(addr >= end);
 

---
base-commit: d0a24447990a9d8212bfb3a692d59efa74ce9f86
change-id: 20251208-b4-vmalloc-might_alloc-754a791e4e10

Best regards,
-- 
Brendan Jackman <jackmanb@google.com>