Re: [PATCH -mm] mm, swap: Use page-cluster as max window of VMA based swap readahead

["Huang\, Ying" <ying.huang@intel.com>]

Minchan Kim <minchan@kernel.org> writes:

> On Tue, Oct 10, 2017 at 02:08:55PM +0800, Huang, Ying wrote:
>> From: Huang Ying <ying.huang@intel.com>
>> 
>> When the VMA based swap readahead was introduced, a new knob
>> 
>>   /sys/kernel/mm/swap/vma_ra_max_order
>> 
>> was added as the max window of VMA swap readahead.  This is to make it
>> possible to use different max window for VMA based readahead and
>> original physical readahead.  But Minchan Kim pointed out that this
>> will cause a regression because setting page-cluster sysctl to zero
>> cannot disable swap readahead with the change.
>> 
>> To fix the regression, the page-cluster sysctl is used as the max
>> window of both the VMA based swap readahead and original physical swap
>> readahead.  If more fine grained control is needed in the future, more
>> knobs can be added as the subordinate knobs of the page-cluster
>> sysctl.
>> 
>> The vma_ra_max_order knob is deleted.  Because the knob was
>> introduced in v4.14-rc1, and this patch is targeting being merged
>> before v4.14 releasing, there should be no existing users of this
>> newly added ABI.
>> 
>> Cc: Johannes Weiner <hannes@cmpxchg.org>
>> Cc: Rik van Riel <riel@redhat.com>
>> Cc: Shaohua Li <shli@kernel.org>
>> Cc: Hugh Dickins <hughd@google.com>
>> Cc: Fengguang Wu <fengguang.wu@intel.com>
>> Cc: Tim Chen <tim.c.chen@intel.com>
>> Cc: Dave Hansen <dave.hansen@intel.com>
>> Reported-by: Minchan Kim <minchan@kernel.org>
>
> It seems your script is Ccing only with Cc: tag, not other tags.
> Fix it so any participant of topic can get the mail.

I just used `git send-email`, no other scripts.  We need to fix `git send-email`?

>> Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
>> ---
>>  Documentation/ABI/testing/sysfs-kernel-mm-swap | 10 ---------
>>  mm/swap_state.c                                | 31 ++------------------------
>>  2 files changed, 2 insertions(+), 39 deletions(-)
>> 
>> diff --git a/Documentation/ABI/testing/sysfs-kernel-mm-swap b/Documentation/ABI/testing/sysfs-kernel-mm-swap
>> index 587db52084c7..94672016c268 100644
>> --- a/Documentation/ABI/testing/sysfs-kernel-mm-swap
>> +++ b/Documentation/ABI/testing/sysfs-kernel-mm-swap
>> @@ -14,13 +14,3 @@ Description:	Enable/disable VMA based swap readahead.
>>  		still used for tmpfs etc. other users.  If set to
>>  		false, the global swap readahead algorithm will be
>>  		used for all swappable pages.
>> -
>> -What:		/sys/kernel/mm/swap/vma_ra_max_order
>> -Date:		August 2017
>> -Contact:	Linux memory management mailing list <linux-mm@kvack.org>
>> -Description:	The max readahead size in order for VMA based swap readahead
>> -
>> -		VMA based swap readahead algorithm will readahead at
>> -		most 1 << max_order pages for each readahead.  The
>> -		real readahead size for each readahead will be scaled
>> -		according to the estimation algorithm.
>> diff --git a/mm/swap_state.c b/mm/swap_state.c
>> index ed91091d1e68..4d01ecff62d9 100644
>> --- a/mm/swap_state.c
>> +++ b/mm/swap_state.c
>> @@ -39,10 +39,6 @@ struct address_space *swapper_spaces[MAX_SWAPFILES];
>>  static unsigned int nr_swapper_spaces[MAX_SWAPFILES];
>>  bool swap_vma_readahead = true;
>>  
>> -#define SWAP_RA_MAX_ORDER_DEFAULT	3
>> -
>> -static int swap_ra_max_order = SWAP_RA_MAX_ORDER_DEFAULT;
>> -
>>  #define SWAP_RA_WIN_SHIFT	(PAGE_SHIFT / 2)
>>  #define SWAP_RA_HITS_MASK	((1UL << SWAP_RA_WIN_SHIFT) - 1)
>>  #define SWAP_RA_HITS_MAX	SWAP_RA_HITS_MASK
>> @@ -672,7 +668,8 @@ struct page *swap_readahead_detect(struct vm_fault *vmf,
>>  	if (page)
>>  		return page;
>>  
>> -	max_win = 1 << READ_ONCE(swap_ra_max_order);
>> +	max_win = 1 << min_t(unsigned int, READ_ONCE(page_cluster),
>> +			     SWAP_RA_ORDER_CEILING);
>
>
> Why doesn't you check it in swap_use_vma_readahead in my original patch?
> My purpose in there is to avoid unnecessary function call and
> lookup_swap_cache in there.
> If you really want to do that in swap_readahead_detect itself, please
> move above routine to ahead of faddr = vmf->address to avoid unncessary
> instructions.

Although the avoided instructions is just a few, I can move the max
window checking as the first statements in the function.

Best Regards,
Huang, Ying

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