Re: [PATCH v2 2/2] powerpc/mm: call H_BLOCK_REMOVE when supported

[Michael Ellerman]

Hi Laurent,

Few comments ...

Laurent Dufour <ldufour@xxxxxxxxxxxxx> writes:
> Now we do not call _BLOCK_REMOVE all the time when the feature is
> exhibited.

This isn't true until after the patch is applied, ie. the tense is
wrong. The rest of the change log explains things fine, so just drop
that sentence I think.

Can you include the info about the oops in here.

> Depending on the hardware and the hypervisor, the hcall H_BLOCK_REMOVE may
> not be able to process all the page size for a segment base page size, as
                                      ^
                                      sizes
> reported by the TLB Invalidate Characteristics.o
                                                 ^
                                                 stray "o"
>
> For each couple base segment page size and actual page size, this
           ^
           "pair of"
> characteristic is telling the size of the block the hcall is supporting.
                 ^                                          ^
                 "tells us"                                 supports
>
> Due to the involve complexity in do_block_remove() and call_block_remove(),
             ^
             "required" is better I think
> and the fact currently a 8 size block is returned by the hypervisor,  we
              ^          ^
              that       "block of size 8"
> are only supporting 8 size block to the H_BLOCK_REMOVE hcall.
>
> Furthermore a warning message is displayed at boot time in the case of an
> unsupported block size.

I'm not sure we should be doing that? It could be unnecessarily spammy.

> In order to identify this limitation easily in the code,a local define
> HBLKR_SUPPORTED_SIZE defining the currently supported block size, and a
> dedicated checking helper is_supported_hlbkr() are introduced.
>
> For regular pages and hugetlb, the assumption is made that the page size is
> equal to the base page size. For THP the page size is assumed to be 16M.
>
> Signed-off-by: Laurent Dufour <ldufour@xxxxxxxxxxxxx>
> ---
>  arch/powerpc/platforms/pseries/lpar.c | 35 +++++++++++++++++++++++++--
>  1 file changed, 33 insertions(+), 2 deletions(-)
>
> diff --git a/arch/powerpc/platforms/pseries/lpar.c b/arch/powerpc/platforms/pseries/lpar.c
> index 98a5c2ff9a0b..e2ad9b3b1097 100644
> --- a/arch/powerpc/platforms/pseries/lpar.c
> +++ b/arch/powerpc/platforms/pseries/lpar.c
> @@ -65,6 +65,13 @@ EXPORT_SYMBOL(plpar_hcall_norets);
>   */
>  static int hblkr_size[MMU_PAGE_COUNT][MMU_PAGE_COUNT];
>  
> +/*
> + * Due to the involved complexity, and that the current hypervisor is only
> + * returning this value or 0, we are limiting the support of the H_BLOCK_REMOVE
> + * buffer size to 8 size block.
> + */
> +#define HBLKR_SUPPORTED_BLOCK_SIZE 8
> +
>  #ifdef CONFIG_VIRT_CPU_ACCOUNTING_NATIVE
>  static u8 dtl_mask = DTL_LOG_PREEMPT;
>  #else
> @@ -993,6 +1000,15 @@ static void pSeries_lpar_hpte_invalidate(unsigned long slot, unsigned long vpn,
>  #define HBLKR_CTRL_ERRNOTFOUND	0x8800000000000000UL
>  #define HBLKR_CTRL_ERRBUSY	0xa000000000000000UL
>  
> +/*
> + * Returned true if we are supporting this block size for the specified segment
> + * base page size and actual page size.
> + */
> +static inline bool is_supported_hlbkr(int bpsize, int psize)
> +{
> +	return (hblkr_size[bpsize][psize] == HBLKR_SUPPORTED_BLOCK_SIZE);
> +}
> +
>  /**
>   * H_BLOCK_REMOVE caller.
>   * @idx should point to the latest @param entry set with a PTEX.
> @@ -1152,7 +1168,11 @@ static inline void __pSeries_lpar_hugepage_invalidate(unsigned long *slot,
>  	if (lock_tlbie)
>  		spin_lock_irqsave(&pSeries_lpar_tlbie_lock, flags);
>  
> -	if (firmware_has_feature(FW_FEATURE_BLOCK_REMOVE))
> +	/*
> +	 * Assuming THP size is 16M, and we only support 8 bytes size buffer
> +	 * for the momment.
> +	 */
> +	if (is_supported_hlbkr(psize, MMU_PAGE_16M))

It's not very clear that this is correct in all cases. ie. how do we
know we're being called for THP and not regular huge page?

I think we're only called via:
  flush_hash_hugepage()
  -> mmu_hash_ops.hugepage_invalidate()
     pSeries_lpar_hugepage_invalidate()
     -> __pSeries_lpar_hugepage_invalidate()

And flush_hash_hugepage() is called via:
  __hash_page_thp()
  and
  hpte_do_hugepage_flush()

The first is presumably fine, the 2nd is called in a few places:
  __flush_hash_table_range() under if (is_thp)
  hash__pmd_hugepage_update()


But it's a little bit fragile if the code ever evolves. Not sure if
there's a better solution, other than just documenting it.

>  		hugepage_block_invalidate(slot, vpn, count, psize, ssize);
>  	else
>  		hugepage_bulk_invalidate(slot, vpn, count, psize, ssize);
> @@ -1437,6 +1457,14 @@ void __init pseries_lpar_read_hblkr_characteristics(void)
>  
>  		block_size = 1 << block_size;
>  
> +		/*
> +		 * If the block size is not supported by the kernel, report it,
> +		 * but continue reading the values, and the following blocks.
> +		 */
> +		if (block_size != HBLKR_SUPPORTED_BLOCK_SIZE)
> +			pr_warn("Unsupported H_BLOCK_REMOVE block size : %d\n",
> +				block_size);

Does this need a printk? I'm worried it could end up triggering and
scaring people unnecessarily.

> +
>  		for (npsize = local_buffer[idx++];  npsize > 0; npsize--)
>  			check_lp_set_hblk((unsigned int) local_buffer[idx++],
>  					  block_size);
> @@ -1468,7 +1496,10 @@ static void pSeries_lpar_flush_hash_range(unsigned long number, int local)
>  	if (lock_tlbie)
>  		spin_lock_irqsave(&pSeries_lpar_tlbie_lock, flags);
>  
> -	if (firmware_has_feature(FW_FEATURE_BLOCK_REMOVE)) {
> +	/*
> +	 * Currently, we only support 8 bytes size buffer in do_block_remove().
> +	 */
> +	if (is_supported_hlbkr(batch->psize, batch->psize)) {
>  		do_block_remove(number, batch, param);
>  		goto out;
>  	}
> -- 
> 2.23.0

cheers