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

[Andrew Morton <akpm@linux-foundation.org>]

On Mon,  8 Sep 2025 08:39:26 +0100 Kevin Brodsky <kevin.brodsky@arm.com> 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.