On 26.08.25 17:39, Lorenzo Stoakes wrote:

On Tue, Aug 26, 2025 at 05:24:22PM +0200, David Hildenbrand wrote:

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On 26.08.25 16:32, Lorenzo Stoakes wrote:

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On Tue, Aug 26, 2025 at 04:28:20PM +0200, David Hildenbrand wrote:

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On 26.08.25 16:21, Lorenzo Stoakes wrote:

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On Tue, Aug 26, 2025 at 03:12:08PM +0200, David Hildenbrand wrote:

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On 12.08.25 17:44, Lorenzo Stoakes wrote:

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We now need to account for flag initialisation on fork. We retain the
existing logic as much as we can, but dub the existing flag mask legacy.

These flags are therefore required to fit in the first 32-bits of the flags
field.

However, further flag propagation upon fork can be implemented in mm_init()
on a per-flag basis.

We ensure we clear the entire bitmap prior to setting it, and use
__mm_flags_get_word() and __mm_flags_set_word() to manipulate these legacy
fields efficiently.

Signed-off-by: Lorenzo Stoakes <redacted>
---
     include/linux/mm_types.h | 13 ++++++++++---
     kernel/fork.c            |  7 +++++--
     2 files changed, 15 insertions(+), 5 deletions(-)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 38b3fa927997..25577ab39094 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h

@@ -1820,16 +1820,23 @@ enum {
     #define MMF_TOPDOWN		31	/* mm searches top down by default */
     #define MMF_TOPDOWN_MASK	_BITUL(MMF_TOPDOWN)
-#define MMF_INIT_MASK		(MMF_DUMPABLE_MASK | MMF_DUMP_FILTER_MASK |\
+#define MMF_INIT_LEGACY_MASK	(MMF_DUMPABLE_MASK | MMF_DUMP_FILTER_MASK |\
     				 MMF_DISABLE_THP_MASK | MMF_HAS_MDWE_MASK |\
     				 MMF_VM_MERGE_ANY_MASK | MMF_TOPDOWN_MASK)
-static inline unsigned long mmf_init_flags(unsigned long flags)
+/* Legacy flags must fit within 32 bits. */
+static_assert((u64)MMF_INIT_LEGACY_MASK <= (u64)UINT_MAX);

Why not use the magic number 32 you are mentioning in the comment? :)

Meh I mean UINT_MAX works as a good 'any bit' mask and this will work on
both 32-bit and 64-bit systems.

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static_assert((u32)MMF_INIT_LEGACY_MASK != MMF_INIT_LEGACY_MASK);

On 32-bit that'd not work would it?

On 32bit, BIT(32) would exceed the shift width of unsigned long -> undefined
behavior.

The compiler should naturally complain.

Yeah, I don't love that sorry. Firstly it's a warning, so you may well miss it
(I just tried),

Upstream bots usually complain at you for warnings :P

Fine, but it's not a static assert and they can be delayed.

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and secondly you're making the static assert not have any
meaning except that you expect to trigger a compiler warning, it's a bit
bizarre.

On 64 bit where BIT(32) *makes any sense* it triggers as expected, no?

It's not a static assert.

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My solution works (unless you can see a reason it shouldn't) and I don't find
this approach any simpler.

Please explain to me like I am a 5 yo how your approach works with BIT(32)
on 32bit when the behavior on 32bit is undefined. :P

OK right I see, in both cases BIT(32) is going to cause a warning on 32-bit.

I was wrong in thinking (u64)(1UL << 32) would get fixed up because of the
outer cast I guess.

This was the mistake here, so fine, we could do it this way.

I guess I'll have to respin the series at this point.

Let me think again: assuming someone would mess up the BIT() thing and 
convert back to 1 << NR, your variant would catch it on 32bit I guess.

So given I was primarily confused by the "u64" when talking about 32it, 
no need for a full resend of this series just to clean this up.

-- 
Cheers

David / dhildenb