[PATCH 1/2] mm: make faultaround produce old ptes

[Vinayak Menon]

On 11/28/2017 2:42 PM, Jan Kara wrote:
> On Tue 28-11-17 10:37:49, Vinayak Menon wrote:
>> Based on Kirill's patch [1].
>>
>> Currently, faultaround code produces young pte.  This can screw up
>> vmscan behaviour[2], as it makes vmscan think that these pages are hot
>> and not push them out on first round.
>>
>> During sparse file access faultaround gets more pages mapped and all of
>> them are young.  Under memory pressure, this makes vmscan swap out anon
>> pages instead, or to drop other page cache pages which otherwise stay
>> resident.
>>
>> Modify faultaround to produce old ptes, so they can easily be reclaimed
>> under memory pressure.
>>
>> This can to some extend defeat the purpose of faultaround on machines
>> without hardware accessed bit as it will not help us with reducing the
>> number of minor page faults.
>>
>> Making the faultaround ptes old results in a unixbench regression for some
>> architectures [3][4]. But on some architectures it is not found to cause
>> any regression. So by default produce young ptes and provide an option for
>> architectures to make the ptes old.
>>
>> [1] http://lkml.kernel.org/r/1463488366-47723-1-git-send-email-kirill.shutemov@linux.intel.com
>> [2] https://lkml.kernel.org/r/1460992636-711-1-git-send-email-vinmenon@codeaurora.org
>> [3] https://marc.info/?l=linux-kernel&m=146582237922378&w=2
>> [4] https://marc.info/?l=linux-mm&m=146589376909424&w=2
>>
>> Signed-off-by: Vinayak Menon <vinmenon at codeaurora.org>
>> ---
>>  include/linux/mm-arch-hooks.h | 7 +++++++
>>  include/linux/mm.h            | 2 ++
>>  mm/filemap.c                  | 4 ++++
>>  mm/memory.c                   | 5 +++++
>>  4 files changed, 18 insertions(+)
>>
>> diff --git a/include/linux/mm-arch-hooks.h b/include/linux/mm-arch-hooks.h
>> index 4efc3f56..0322b98 100644
>> --- a/include/linux/mm-arch-hooks.h
>> +++ b/include/linux/mm-arch-hooks.h
>> @@ -22,4 +22,11 @@ static inline void arch_remap(struct mm_struct *mm,
>>  #define arch_remap arch_remap
>>  #endif
>>  
>> +#ifndef arch_faultaround_pte_mkold
>> +static inline void arch_faultaround_pte_mkold(struct vm_fault *vmf)
>> +{
>> +}
>> +#define arch_faultaround_pte_mkold arch_faultaround_pte_mkold
>> +#endif
>> +
>>  #endif /* _LINUX_MM_ARCH_HOOKS_H */
>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>> index 7661156..be689a0 100644
>> --- a/include/linux/mm.h
>> +++ b/include/linux/mm.h
>> @@ -302,6 +302,7 @@ extern int overcommit_kbytes_handler(struct ctl_table *, int, void __user *,
>>  #define FAULT_FLAG_USER		0x40	/* The fault originated in userspace */
>>  #define FAULT_FLAG_REMOTE	0x80	/* faulting for non current tsk/mm */
>>  #define FAULT_FLAG_INSTRUCTION  0x100	/* The fault was during an instruction fetch */
>> +#define FAULT_FLAG_MKOLD	0x200	/* Make faultaround ptes old */
> Nit: Can we make this FAULT_FLAG_PREFAULT_OLD or something like that so
> that it is clear from the flag name that this is about prefaulting of
> pages?
Okay, will change the name.

>>  #define FAULT_FLAG_TRACE \
>>  	{ FAULT_FLAG_WRITE,		"WRITE" }, \
>> @@ -330,6 +331,7 @@ struct vm_fault {
>>  	gfp_t gfp_mask;			/* gfp mask to be used for allocations */
>>  	pgoff_t pgoff;			/* Logical page offset based on vma */
>>  	unsigned long address;		/* Faulting virtual address */
>> +	unsigned long fault_address;    /* Saved faulting virtual address */
> Ugh, so I dislike how you hide the decision about whether the *particular*
> PTE should be old or young in the arch code. Sure the arch wants to decide
> whether the prefaulted PTEs should be old or young and that it has to tell
> us but the arch code has no business in checking whether this is prefault
> or a normal fault - that decision belongs to filemap_map_pages(). So I'd do
> in filemap_map_pages() something like:
>
> 	if (iter.index > start_pgoff && arch_wants_old_faultaround_pte())
> 		vmf->flags |= FAULT_FLAG_PREFAULT_OLD;
Okay, I will fix it.

Thanks,
Vinayak