Re: [PATCH 1/7] mm: allocate mm_cpumask dynamically based on nr_cpu_ids

[Mike Galbraith]

On Sat, 2018-07-07 at 17:25 -0400, Rik van Riel wrote:
> 
> > ./include/linux/bitmap.h:208:3: warning: âmemsetâ writing 64 bytes
> > into a region of size 0 overflows the destination [-Wstringop-
> > overflow=]
> >    memset(dst, 0, len);
> >    ^~~~~~~~~~~~~~~~~~~
> 
> I don't understand this one.
> 
> Inside init_mm we have this line:
>         .cpu_bitmap     = { [BITS_TO_LONGS(NR_CPUS)] = 0},
> 
> which is the way the documentation suggests statically
> allocated variable size arrays should be allocated 
> and initialized.
> 
> How does that result in a memset of the same size,
> on the same array, to throw an error like above?

Compiler knows that ->cpu_bitmap is 64 bits of storage, and with
!CPUMASK_OFFSTACK, nr_cpumask_bits = NR_CPUS.  With NR_CPUS > 64,
compiler gripes, with NR_CPUS <= 64 it's a happy camper.

> What am I doing wrong?

Below is what I did to get box to both STHU, and to boot with the
openSUSE master branch config I sent.  Without the efi_mm hunk, boot
hangs early with or without the other hunk.

I build and boot tested the openSUSE config, a NOPREEMPT+MAXSMP config,
my local config w. NR_CPUS=8, and master-rt w. NR_CPUS=256, which is
the only one that got any real exercise (building the others).

---
 drivers/firmware/efi/efi.c |    1 +
 include/linux/mm_types.h   |    5 ++++-
 2 files changed, 5 insertions(+), 1 deletion(-)

--- a/drivers/firmware/efi/efi.c
+++ b/drivers/firmware/efi/efi.c
@@ -82,6 +82,7 @@ struct mm_struct efi_mm = {
 	.mmap_sem		= __RWSEM_INITIALIZER(efi_mm.mmap_sem),
 	.page_table_lock	= __SPIN_LOCK_UNLOCKED(efi_mm.page_table_lock),
 	.mmlist			= LIST_HEAD_INIT(efi_mm.mmlist),
+	.cpu_bitmap		= { [BITS_TO_LONGS(NR_CPUS)] = 0},
 };
 
 static bool disable_runtime;
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -501,7 +501,10 @@ extern struct mm_struct init_mm;
 
 static inline void mm_init_cpumask(struct mm_struct *mm)
 {
-	cpumask_clear((struct cpumask *)&mm->cpu_bitmap);
+	unsigned long cpu_bitmap = (unsigned long)mm;
+
+	cpu_bitmap += offsetof(struct mm_struct, cpu_bitmap);
+	cpumask_clear((struct cpumask *)cpu_bitmap);
 }
 
 /* Future-safe accessor for struct mm_struct's cpu_vm_mask. */