Re: [PATCH 2/3] vrange: Add purged page detection on setting memory non-volatile

[John Stultz]

On 03/17/2014 02:39 AM, Jan Kara wrote:
> On Fri 14-03-14 11:33:32, John Stultz wrote:
>> Users of volatile ranges will need to know if memory was discarded.
>> This patch adds the purged state tracking required to inform userland
>> when it marks memory as non-volatile that some memory in that range
>> was purged and needs to be regenerated.
>>
>> This simplified implementation which uses some of the logic from
>> Minchan's earlier efforts, so credit to Minchan for his work.
>   Some minor comments below...
>
>> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
>> Cc: Android Kernel Team <kernel-team@xxxxxxxxxxx>
>> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
>> Cc: Robert Love <rlove@xxxxxxxxxx>
>> Cc: Mel Gorman <mel@xxxxxxxxx>
>> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
>> Cc: Dave Hansen <dave@xxxxxxxx>
>> Cc: Rik van Riel <riel@xxxxxxxxxx>
>> Cc: Dmitry Adamushko <dmitry.adamushko@xxxxxxxxx>
>> Cc: Neil Brown <neilb@xxxxxxx>
>> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
>> Cc: Mike Hommey <mh@xxxxxxxxxxxx>
>> Cc: Taras Glek <tglek@xxxxxxxxxxx>
>> Cc: Dhaval Giani <dgiani@xxxxxxxxxxx>
>> Cc: Jan Kara <jack@xxxxxxx>
>> Cc: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxx>
>> Cc: Michel Lespinasse <walken@xxxxxxxxxx>
>> Cc: Minchan Kim <minchan@xxxxxxxxxx>
>> Cc: linux-mm@xxxxxxxxx <linux-mm@xxxxxxxxx>
>> Signed-off-by: John Stultz <john.stultz@xxxxxxxxxx>
>> ---
>>  include/linux/swap.h   | 15 +++++++++++--
>>  include/linux/vrange.h | 13 ++++++++++++
>>  mm/vrange.c            | 57 ++++++++++++++++++++++++++++++++++++++++++++++++++
>>  3 files changed, 83 insertions(+), 2 deletions(-)
>>
>> diff --git a/include/linux/swap.h b/include/linux/swap.h
>> index 46ba0c6..18c12f9 100644
>> --- a/include/linux/swap.h
>> +++ b/include/linux/swap.h
>> @@ -70,8 +70,19 @@ static inline int current_is_kswapd(void)
>>  #define SWP_HWPOISON_NUM 0
>>  #endif
>>  
>> -#define MAX_SWAPFILES \
>> -	((1 << MAX_SWAPFILES_SHIFT) - SWP_MIGRATION_NUM - SWP_HWPOISON_NUM)
>> +
>> +/*
>> + * Purged volatile range pages
>> + */
>> +#define SWP_VRANGE_PURGED_NUM 1
>> +#define SWP_VRANGE_PURGED (MAX_SWAPFILES + SWP_HWPOISON_NUM + SWP_MIGRATION_NUM)
>> +
>> +
>> +#define MAX_SWAPFILES ((1 << MAX_SWAPFILES_SHIFT)	\
>> +				- SWP_MIGRATION_NUM	\
>> +				- SWP_HWPOISON_NUM	\
>> +				- SWP_VRANGE_PURGED_NUM	\
>> +			)
>>  
>>  /*
>>   * Magic header for a swap area. The first part of the union is
>> diff --git a/include/linux/vrange.h b/include/linux/vrange.h
>> index 652396b..c4a1616 100644
>> --- a/include/linux/vrange.h
>> +++ b/include/linux/vrange.h
>> @@ -1,7 +1,20 @@
>>  #ifndef _LINUX_VRANGE_H
>>  #define _LINUX_VRANGE_H
>>  
>> +#include <linux/swap.h>
>> +#include <linux/swapops.h>
>> +
>>  #define VRANGE_NONVOLATILE 0
>>  #define VRANGE_VOLATILE 1
>>  
>> +static inline swp_entry_t swp_entry_mk_vrange_purged(void)
>> +{
>> +	return swp_entry(SWP_VRANGE_PURGED, 0);
>> +}
>> +
>> +static inline int entry_is_vrange_purged(swp_entry_t entry)
>> +{
>> +	return swp_type(entry) == SWP_VRANGE_PURGED;
>> +}
>> +
>>  #endif /* _LINUX_VRANGE_H */
>> diff --git a/mm/vrange.c b/mm/vrange.c
>> index acb4356..844571b 100644
>> --- a/mm/vrange.c
>> +++ b/mm/vrange.c
>> @@ -8,6 +8,60 @@
>>  #include <linux/mm_inline.h>
>>  #include "internal.h"
>>  
>> +struct vrange_walker {
>> +	struct vm_area_struct *vma;
>> +	int pages_purged;
>   Maybe call this 'was_page_purged'? To better suggest the value is bool
> and not a number of pages... Or make that 'bool' instead of 'int'?

Yea, page_was_purged sounds good to me. Thanks for pointing out the
ambiguity.  Similarly the

entry_is_vrange_purged/swp_entry_mk_vrange_purged functions are a little inconsistently named.



>
>> +};
>> +
>> +static int vrange_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
>> +				struct mm_walk *walk)
>> +{
>> +	struct vrange_walker *vw = walk->private;
>> +	pte_t *pte;
>> +	spinlock_t *ptl;
>> +
>> +	if (pmd_trans_huge(*pmd))
>> +		return 0;
>> +	if (pmd_trans_unstable(pmd))
>> +		return 0;
>> +
>> +	pte = pte_offset_map_lock(walk->mm, pmd, addr, &ptl);
>> +	for (; addr != end; pte++, addr += PAGE_SIZE) {
>> +		if (!pte_present(*pte)) {
>> +			swp_entry_t vrange_entry = pte_to_swp_entry(*pte);
>> +
>> +			if (unlikely(entry_is_vrange_purged(vrange_entry))) {
>> +				vw->pages_purged = 1;
>> +				break;
>> +			}
>> +		}
>> +	}
>> +	pte_unmap_unlock(pte - 1, ptl);
>> +	cond_resched();
>> +
>> +	return 0;
>> +}
>> +
>> +static unsigned long vrange_check_purged(struct mm_struct *mm,
>   What's the point of this function returning ulong when everything else
> expects 'int'?

Thanks, I'll fix that.

>> +					 struct vm_area_struct *vma,
>> +					 unsigned long start,
>> +					 unsigned long end)
>> +{
>> +	struct vrange_walker vw;
>> +	struct mm_walk vrange_walk = {
>> +		.pmd_entry = vrange_pte_range,
>> +		.mm = vma->vm_mm,
>> +		.private = &vw,
>> +	};
>> +	vw.pages_purged = 0;
>> +	vw.vma = vma;
>> +
>> +	walk_page_range(start, end, &vrange_walk);
>> +
>> +	return vw.pages_purged;
>> +
>> +}
>> +
>>  static ssize_t do_vrange(struct mm_struct *mm, unsigned long start,
>>  				unsigned long end, int mode, int *purged)
>>  {
>> @@ -57,6 +111,9 @@ static ssize_t do_vrange(struct mm_struct *mm, unsigned long start,
>>  			break;
>>  		case VRANGE_NONVOLATILE:
>>  			new_flags &= ~VM_VOLATILE;
>> +			lpurged |= vrange_check_purged(mm, vma,
>> +							vma->vm_start,
>> +							vma->vm_end);
>   Hum, why don't you actually just call vrange_check_purge() once for the
> whole syscall range? walk_page_range() seems to handle multiple vmas just
> fine...
>

Another good suggestion!

Thanks!
-john

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