Re: [PATCH v2 2/7] mm: introduce local state for lazy_mmu sections

[Kevin Brodsky <kevin.brodsky@xxxxxxx>]

On 09/09/2025 07:40, Andrew Morton wrote:
> On Mon,  8 Sep 2025 08:39:26 +0100 Kevin Brodsky <kevin.brodsky@xxxxxxx> 
> wrote:
>
>> arch_{enter,leave}_lazy_mmu_mode() currently have a stateless API
>> (taking and returning no value). This is proving problematic in
>> situations where leave() needs to restore some context back to its
>> original state (before enter() was called). In particular, this
>> makes it difficult to support the nesting of lazy_mmu sections -
>> leave() does not know whether the matching enter() call occurred
>> while lazy_mmu was already enabled, and whether to disable it or
>> not.
>>
>> This patch gives all architectures the chance to store local state
>> while inside a lazy_mmu section by making enter() return some value,
>> storing it in a local variable, and having leave() take that value.
>> That value is typed lazy_mmu_state_t - each architecture defining
>> __HAVE_ARCH_ENTER_LAZY_MMU_MODE is free to define it as it sees fit.
>> For now we define it as int everywhere, which is sufficient to
>> support nesting.
>>
>> The diff is unfortunately rather large as all the API changes need
>> to be done atomically. Main parts:
> This has a build error:
>
>   CC      arch/x86/kernel/asm-offsets.s
> In file included from ./arch/x86/include/asm/irqflags.h:102,
>                  from ./include/linux/irqflags.h:18,
>                  from ./include/linux/spinlock.h:59,
>                  from ./include/linux/swait.h:7,
>                  from ./include/linux/completion.h:12,
>                  from ./include/linux/crypto.h:15,
>                  from arch/x86/kernel/asm-offsets.c:9:
> ./arch/x86/include/asm/paravirt.h: In function 'arch_enter_lazy_mmu_mode':
> ./arch/x86/include/asm/paravirt.h:534:16: error: 'LAZY_MMU_DEFAULT' 
> undeclared (first use in this function)
>   534 |         return LAZY_MMU_DEFAULT;
>       |                ^~~~~~~~~~~~~~~~
> ./arch/x86/include/asm/paravirt.h:534:16: note: each undeclared identifier is 
> re
>
> which gets fixed up later in the series.

Oh indeed good catch! I don't think there's an easy way to fix this
cleanly due to the header soup. Since it's just a temporary change, I
suggest:

diff --git a/arch/x86/include/asm/paravirt.h
b/arch/x86/include/asm/paravirt.h
index 65a0d394fba1..67b9549b4255 100644
--- a/arch/x86/include/asm/paravirt.h
+++ b/arch/x86/include/asm/paravirt.h
@@ -531,7 +531,7 @@ static inline lazy_mmu_state_t
arch_enter_lazy_mmu_mode(void)
 {
     PVOP_VCALL0(mmu.lazy_mode.enter);
 
-    return LAZY_MMU_DEFAULT;
+    return 0; /* LAZY_MMU_DEFAULT */
 }
 
 static inline void arch_leave_lazy_mmu_mode(lazy_mmu_state_t state)


That will generate a trivial conflict with patch 4, naturally.

Should I send a v3 with that change?

- Kevin