Re: [PATCH v5 08/12] mm: enable lazy_mmu sections to nest

[David Hildenbrand (Red Hat)]

On 11/24/25 14:22, Kevin Brodsky wrote:
> Despite recent efforts to prevent lazy_mmu sections from nesting, it
> remains difficult to ensure that it never occurs - and in fact it
> does occur on arm64 in certain situations (CONFIG_DEBUG_PAGEALLOC).
> Commit 1ef3095b1405 ("arm64/mm: Permit lazy_mmu_mode to be nested")
> made nesting tolerable on arm64, but without truly supporting it:
> the inner call to leave() disables the batching optimisation before
> the outer section ends.
>
> This patch actually enables lazy_mmu sections to nest by tracking
> the nesting level in task_struct, in a similar fashion to e.g.
> pagefault_{enable,disable}(). This is fully handled by the generic
> lazy_mmu helpers that were recently introduced.
>
> lazy_mmu sections were not initially intended to nest, so we need to
> clarify the semantics w.r.t. the arch_*_lazy_mmu_mode() callbacks.
> This patch takes the following approach:
>
> * The outermost calls to lazy_mmu_mode_{enable,disable}() trigger
>    calls to arch_{enter,leave}_lazy_mmu_mode() - this is unchanged.
>
> * Nested calls to lazy_mmu_mode_{enable,disable}() are not forwarded
>    to the arch via arch_{enter,leave} - lazy MMU remains enabled so
>    the assumption is that these callbacks are not relevant. However,
>    existing code may rely on a call to disable() to flush any batched
>    state, regardless of nesting. arch_flush_lazy_mmu_mode() is
>    therefore called in that situation.
>
> A separate interface was recently introduced to temporarily pause
> the lazy MMU mode: lazy_mmu_mode_{pause,resume}(). pause() fully
> exits the mode *regardless of the nesting level*, and resume()
> restores the mode at the same nesting level.
>
> pause()/resume() are themselves allowed to nest, so we actually
> store two nesting levels in task_struct: enable_count and
> pause_count. A new helper in_lazy_mmu_mode() is introduced to
> determine whether we are currently in lazy MMU mode; this will be
> used in subsequent patches to replace the various ways arch's
> currently track whether the mode is enabled.
>
> In summary (enable/pause represent the values *after* the call):
>
> lazy_mmu_mode_enable()		-> arch_enter()	    enable=1 pause=0
>      lazy_mmu_mode_enable()	-> ø		    enable=2 pause=0
> 	lazy_mmu_mode_pause()	-> arch_leave()     enable=2 pause=1
> 	lazy_mmu_mode_resume()	-> arch_enter()     enable=2 pause=0
>      lazy_mmu_mode_disable()	-> arch_flush()     enable=1 pause=0
> lazy_mmu_mode_disable()		-> arch_leave()     enable=0 pause=0
>
> Note: in_lazy_mmu_mode() is added to <linux/sched.h> to allow arch
> headers included by <linux/pgtable.h> to use it.
>
> Signed-off-by: Kevin Brodsky <kevin.brodsky@xxxxxxx>

Nothing jumped at me, so

Acked-by: David Hildenbrand (Red Hat) <david@xxxxxxxxxx>

Hoping we can get some more eyes to have a look.

--
Cheers

David