[PATCH] um: implement flush_cache_vmap/flush_cache_vunmap

[Anton Ivanov]

On 15/03/2021 22:38, Johannes Berg wrote:
> From: Johannes Berg <johannes.berg at intel.com>
> 
> vmalloc() heavy workloads in UML are extremely slow, due to
> flushing the entire kernel VM space (flush_tlb_kernel_vm())
> on the first segfault.
> 
> Implement flush_cache_vmap() to avoid that, and while at it
> also add flush_cache_vunmap() since it's trivial.
> 
> This speeds up my vmalloc() heavy test of copying files out
> from /sys/kernel/debug/gcov/ by 30x (from 30s to 1s.)
> 
> Signed-off-by: Johannes Berg <johannes.berg at intel.com>
> ---
>   arch/um/include/asm/cacheflush.h | 9 +++++++++
>   arch/um/include/asm/tlb.h        | 2 +-
>   2 files changed, 10 insertions(+), 1 deletion(-)
>   create mode 100644 arch/um/include/asm/cacheflush.h
> 
> diff --git a/arch/um/include/asm/cacheflush.h b/arch/um/include/asm/cacheflush.h
> new file mode 100644
> index 000000000000..4c9858cd36ec
> --- /dev/null
> +++ b/arch/um/include/asm/cacheflush.h
> @@ -0,0 +1,9 @@
> +#ifndef __UM_ASM_CACHEFLUSH_H
> +#define __UM_ASM_CACHEFLUSH_H
> +
> +#include <asm/tlbflush.h>
> +#define flush_cache_vmap flush_tlb_kernel_range
> +#define flush_cache_vunmap flush_tlb_kernel_range
> +
> +#include <asm-generic/cacheflush.h>
> +#endif /* __UM_ASM_CACHEFLUSH_H */
> diff --git a/arch/um/include/asm/tlb.h b/arch/um/include/asm/tlb.h
> index ff9c62828962..0422467bda5b 100644
> --- a/arch/um/include/asm/tlb.h
> +++ b/arch/um/include/asm/tlb.h
> @@ -5,7 +5,7 @@
>   #include <linux/mm.h>
>   
>   #include <asm/tlbflush.h>
> -#include <asm-generic/cacheflush.h>
> +#include <asm/cacheflush.h>
>   #include <asm-generic/tlb.h>
>   
>   #endif
> 

Well spotted.

Unless I am mistaken, there may be a slightly better way of doing it.

We can implement arch_sync_kernel_mappings() and sync only where the page modified mask says so by setting ARCH_PAGE_TABLE_SYNC_MASK

This way flush_cache_* can remain nops as in asm-generic

I am going to give that a spin, if it works, I will post it to the list by lunchtime GMT

 >


-- 
Anton R. Ivanov
https://www.kot-begemot.co.uk/