Re: [PATCH v3] lib/stackdepot: allow optional init and stack_table allocation by kvmalloc()

[Vlastimil Babka]

On 10/13/21 09:30, Vlastimil Babka wrote:
> Currently, enabling CONFIG_STACKDEPOT means its stack_table will be allocated
> from memblock, even if stack depot ends up not actually used. The default size
> of stack_table is 4MB on 32-bit, 8MB on 64-bit.
> 
> This is fine for use-cases such as KASAN which is also a config option and
> has overhead on its own. But it's an issue for functionality that has to be
> actually enabled on boot (page_owner) or depends on hardware (GPU drivers)
> and thus the memory might be wasted. This was raised as an issue [1] when
> attempting to add stackdepot support for SLUB's debug object tracking
> functionality. It's common to build kernels with CONFIG_SLUB_DEBUG and enable
> slub_debug on boot only when needed, or create only specific kmem caches with
> debugging for testing purposes.
> 
> It would thus be more efficient if stackdepot's table was allocated only when
> actually going to be used. This patch thus makes the allocation (and whole
> stack_depot_init() call) optional:
> 
> - Add a CONFIG_STACKDEPOT_ALWAYS_INIT flag to keep using the current
>   well-defined point of allocation as part of mem_init(). Make CONFIG_KASAN
>   select this flag.
> - Other users have to call stack_depot_init() as part of their own init when
>   it's determined that stack depot will actually be used. This may depend on
>   both config and runtime conditions. Convert current users which are
>   page_owner and several in the DRM subsystem. Same will be done for SLUB
>   later.
> - Because the init might now be called after the boot-time memblock allocation
>   has given all memory to the buddy allocator, change stack_depot_init() to
>   allocate stack_table with kvmalloc() when memblock is no longer available.
>   Also handle allocation failure by disabling stackdepot (could have
>   theoretically happened even with memblock allocation previously), and don't
>   unnecessarily align the memblock allocation to its own size anymore.
> 
> [1] https://lore.kernel.org/all/CAMuHMdW=eoVzM1Re5FVoEN87nKfiLmM2+Ah7eNu2KXEhCvbZyA@xxxxxxxxxxxxxx/
...
> ---
> Changes in v3:
> - stack_depot_init_mutex made static and moved inside stack_depot_init()
>   Reported-by: kernel test robot <lkp@xxxxxxxxx>
> - use !stack_table condition instead of stack_table == NULL
>   reported by checkpatch on freedesktop.org patchwork

The last change above was missing because I forgot git commit --amend before
git format-patch. More importantly there was a bot report for FLATMEM. Please
add this fixup. Thanks.

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