Date:	Wed, 10 Jun 2009 15:39:52 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Andi Kleen <andi@...stfloor.org>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Minchan Kim <minchan.kim@...il.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [patch v3] swap: virtual swap readahead

On Tue, 9 Jun 2009 21:37:02 +0200
Johannes Weiner <hannes@...xchg.org> wrote:

> On Tue, Jun 09, 2009 at 09:01:28PM +0200, Johannes Weiner wrote:
> > [resend with lists cc'd, sorry]
> 
> [and fixed Hugh's email.  crap]
> 
> > Hi,
> > 
> > here is a new iteration of the virtual swap readahead.  Per Hugh's
> > suggestion, I moved the pte collecting to the callsite and thus out
> > ouf swap code.  Unfortunately, I had to bound page_cluster due to an
> > array of that many swap entries on the stack, but I think it is better
> > to limit the cluster size to a sane maximum than using dynamic
> > allocation for this purpose.
> > 
> > Thanks all for the helpful suggestions.  KAMEZAWA-san and Minchan, I
> > didn't incorporate your ideas in this patch as I think they belong in
> > a different one with their own justifications.  I didn't ignore them.
> > 
> >        Hannes
> > 
> > ---
> > The current swap readahead implementation reads a physically
> > contiguous group of swap slots around the faulting page to take
> > advantage of the disk head's position and in the hope that the
> > surrounding pages will be needed soon as well.
> > 
> > This works as long as the physical swap slot order approximates the
> > LRU order decently, otherwise it wastes memory and IO bandwidth to
> > read in pages that are unlikely to be needed soon.
> > 
> > However, the physical swap slot layout diverges from the LRU order
> > with increasing swap activity, i.e. high memory pressure situations,
> > and this is exactly the situation where swapin should not waste any
> > memory or IO bandwidth as both are the most contended resources at
> > this point.
> > 
> > Another approximation for LRU-relation is the VMA order as groups of
> > VMA-related pages are usually used together.
> > 
> > This patch combines both the physical and the virtual hint to get a
> > good approximation of pages that are sensible to read ahead.
> > 
> > When both diverge, we either read unrelated data, seek heavily for
> > related data, or, what this patch does, just decrease the readahead
> > efforts.
> > 
> > To achieve this, we have essentially two readahead windows of the same
> > size: one spans the virtual, the other one the physical neighborhood
> > of the faulting page.  We only read where both areas overlap.
> > 
> > Signed-off-by: Johannes Weiner <hannes@...xchg.org>
> > Reviewed-by: Rik van Riel <riel@...hat.com>
> > Cc: Hugh Dickins <hugh.dickins@...cali.co.uk>
> > Cc: Andi Kleen <andi@...stfloor.org>
> > Cc: Wu Fengguang <fengguang.wu@...el.com>
> > Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> > Cc: Minchan Kim <minchan.kim@...il.com>
> > ---
> >  include/linux/swap.h |    4 ++-
> >  kernel/sysctl.c      |    7 ++++-
> >  mm/memory.c          |   55 +++++++++++++++++++++++++++++++++++++++++
> >  mm/shmem.c           |    4 +--
> >  mm/swap_state.c      |   67 ++++++++++++++++++++++++++++++++++++++-------------
> >  5 files changed, 116 insertions(+), 21 deletions(-)
> > 
> > version 3:
> >   o move pte selection to callee (per Hugh)
> >   o limit ra ptes to one pmd entry to avoid multiple
> >     locking/mapping of highptes (per Hugh)
> > 
> > version 2:
> >   o fall back to physical ra window for shmem
> >   o add documentation to the new ra algorithm (per Andrew)
> > 
> > --- a/mm/swap_state.c
> > +++ b/mm/swap_state.c
> > @@ -327,27 +327,14 @@ struct page *read_swap_cache_async(swp_e
> >  	return found_page;
> >  }
> >  
> > -/**
> > - * swapin_readahead - swap in pages in hope we need them soon
> > - * @entry: swap entry of this memory
> > - * @gfp_mask: memory allocation flags
> > - * @vma: user vma this address belongs to
> > - * @addr: target address for mempolicy
> > - *
> > - * Returns the struct page for entry and addr, after queueing swapin.
> > - *
> > +/*
> >   * Primitive swap readahead code. We simply read an aligned block of
> >   * (1 << page_cluster) entries in the swap area. This method is chosen
> >   * because it doesn't cost us any seek time.  We also make sure to queue
> >   * the 'original' request together with the readahead ones...
> > - *
> > - * This has been extended to use the NUMA policies from the mm triggering
> > - * the readahead.
> > - *
> > - * Caller must hold down_read on the vma->vm_mm if vma is not NULL.
> >   */
> > -struct page *swapin_readahead(swp_entry_t entry, gfp_t gfp_mask,
> > -			struct vm_area_struct *vma, unsigned long addr)
> > +static struct page *swapin_readahead_phys(swp_entry_t entry, gfp_t gfp_mask,
> > +				struct vm_area_struct *vma, unsigned long addr)
> >  {
> >  	int nr_pages;
> >  	struct page *page;
> > @@ -373,3 +360,51 @@ struct page *swapin_readahead(swp_entry_
> >  	lru_add_drain();	/* Push any new pages onto the LRU now */
> >  	return read_swap_cache_async(entry, gfp_mask, vma, addr);
> >  }
> > +
> > +/**
> > + * swapin_readahead - swap in pages in hope we need them soon
> > + * @entry: swap entry of this memory
> > + * @gfp_mask: memory allocation flags
> > + * @vma: user vma this address belongs to
> > + * @addr: target address for mempolicy
> > + * @entries: swap slots to consider reading
> > + * @nr_entries: number of @entries
> > + * @cluster: readahead window size in swap slots
> > + *
> > + * Returns the struct page for entry and addr, after queueing swapin.
> > + *
> > + * This has been extended to use the NUMA policies from the mm
> > + * triggering the readahead.
> > + *
> > + * Caller must hold down_read on the vma->vm_mm if vma is not NULL.
> > + */
> > +struct page *swapin_readahead(swp_entry_t entry, gfp_t gfp_mask,
> > +			struct vm_area_struct *vma, unsigned long addr,
> > +			swp_entry_t *entries, int nr_entries,
> > +			unsigned long cluster)
> > +{
> > +	unsigned long pmin, pmax;
> > +	int i;
> > +
> > +	if (!entries)	/* XXX: shmem case */
> > +		return swapin_readahead_phys(entry, gfp_mask, vma, addr);
> > +	pmin = swp_offset(entry) & ~(cluster - 1);
> > +	pmax = pmin + cluster;
> > +	for (i = 0; i < nr_entries; i++) {
> > +		swp_entry_t swp = entries[i];
> > +		struct page *page;
> > +
> > +		if (swp_type(swp) != swp_type(entry))
> > +			continue;
> > +		if (swp_offset(swp) > pmax)
> > +			continue;
> > +		if (swp_offset(swp) < pmin)
> > +			continue;
> > +		page = read_swap_cache_async(swp, gfp_mask, vma, addr);
> > +		if (!page)
> > +			break;
> > +		page_cache_release(page);
> > +	}
> > +	lru_add_drain();	/* Push any new pages onto the LRU now */
> > +	return read_swap_cache_async(entry, gfp_mask, vma, addr);
> > +}
> > --- a/include/linux/swap.h
> > +++ b/include/linux/swap.h
> > @@ -292,7 +292,9 @@ extern struct page *lookup_swap_cache(sw
> >  extern struct page *read_swap_cache_async(swp_entry_t, gfp_t,
> >  			struct vm_area_struct *vma, unsigned long addr);
> >  extern struct page *swapin_readahead(swp_entry_t, gfp_t,
> > -			struct vm_area_struct *vma, unsigned long addr);
> > +			struct vm_area_struct *vma, unsigned long addr,
> > +			swp_entry_t *entries, int nr_entries,
> > +			unsigned long cluster);
> >  
> >  /* linux/mm/swapfile.c */
> >  extern long nr_swap_pages;
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -2440,6 +2440,54 @@ int vmtruncate_range(struct inode *inode
> >  }
> >  
> >  /*
> > + * The readahead window is the virtual area around the faulting page,
> > + * where the physical proximity of the swap slots is taken into
> > + * account as well in swapin_readahead().
> > + *
> > + * While the swap allocation algorithm tries to keep LRU-related pages
> > + * together on the swap backing, it is not reliable on heavy thrashing
> > + * systems where concurrent reclaimers allocate swap slots and/or most
> > + * anonymous memory pages are already in swap cache.
> > + *
> > + * On the virtual side, subgroups of VMA-related pages are usually
> > + * used together, which gives another hint to LRU relationship.
> > + *
> > + * By taking both aspects into account, we get a good approximation of
> > + * which pages are sensible to read together with the faulting one.
> > + */
> > +static int swap_readahead_ptes(struct mm_struct *mm,
> > +			unsigned long addr, pmd_t *pmd,
> > +			swp_entry_t *entries,
> > +			unsigned long cluster)
> > +{
> > +	unsigned long window, min, max, limit;
> > +	spinlock_t *ptl;
> > +	pte_t *ptep;
> > +	int i, nr;
> > +
> > +	window = cluster << PAGE_SHIFT;
> > +	min = addr & ~(window - 1);
> > +	max = min + cluster;

Hmm, max = min + window ?

Thanks,
-Kame

> > +	/*
> > +	 * To keep the locking/highpte mapping simple, stay
> > +	 * within the PTE range of one PMD entry.
> > +	 */
> > +	limit = addr & PMD_MASK;
> > +	if (limit > min)
> > +		min = limit;
> > +	limit = pmd_addr_end(addr, max);
> > +	if (limit < max)
> > +		max = limit;
> > +	limit = max - min;
> > +	ptep = pte_offset_map_lock(mm, pmd, min, &ptl);
> > +	for (i = nr = 0; i < limit; i++)
> > +		if (is_swap_pte(ptep[i]))
> > +			entries[nr++] = pte_to_swp_entry(ptep[i]);
> > +	pte_unmap_unlock(ptep, ptl);
> > +	return nr;
> > +}
> > +
> > +/*
> >   * We enter with non-exclusive mmap_sem (to exclude vma changes,
> >   * but allow concurrent faults), and pte mapped but not yet locked.
> >   * We return with mmap_sem still held, but pte unmapped and unlocked.
> > @@ -2466,9 +2514,14 @@ static int do_swap_page(struct mm_struct
> >  	delayacct_set_flag(DELAYACCT_PF_SWAPIN);
> >  	page = lookup_swap_cache(entry);
> >  	if (!page) {
> > +		int nr, cluster = 1 << page_cluster;
> > +		swp_entry_t entries[cluster];
> > +
> >  		grab_swap_token(); /* Contend for token _before_ read-in */
> > +		nr = swap_readahead_ptes(mm, address, pmd, entries, cluster);
> >  		page = swapin_readahead(entry,
> > -					GFP_HIGHUSER_MOVABLE, vma, address);
> > +					GFP_HIGHUSER_MOVABLE, vma, address,
> > +					entries, nr, cluster);
> >  		if (!page) {
> >  			/*
> >  			 * Back out if somebody else faulted in this pte
> > --- a/mm/shmem.c
> > +++ b/mm/shmem.c
> > @@ -1148,7 +1148,7 @@ static struct page *shmem_swapin(swp_ent
> >  	pvma.vm_pgoff = idx;
> >  	pvma.vm_ops = NULL;
> >  	pvma.vm_policy = spol;
> > -	page = swapin_readahead(entry, gfp, &pvma, 0);
> > +	page = swapin_readahead(entry, gfp, &pvma, 0, NULL, 0, 0);
> >  	return page;
> >  }
> >  
> > @@ -1178,7 +1178,7 @@ static inline void shmem_show_mpol(struc
> >  static inline struct page *shmem_swapin(swp_entry_t entry, gfp_t gfp,
> >  			struct shmem_inode_info *info, unsigned long idx)
> >  {
> > -	return swapin_readahead(entry, gfp, NULL, 0);
> > +	return swapin_readahead(entry, gfp, NULL, 0, NULL, 0, 0);
> >  }
> >  
> >  static inline struct page *shmem_alloc_page(gfp_t gfp,
> > --- a/kernel/sysctl.c
> > +++ b/kernel/sysctl.c
> > @@ -112,6 +112,8 @@ static int min_percpu_pagelist_fract = 8
> >  
> >  static int ngroups_max = NGROUPS_MAX;
> >  
> > +static int page_cluster_max = 5;
> > +
> >  #ifdef CONFIG_MODULES
> >  extern char modprobe_path[];
> >  #endif
> > @@ -966,7 +968,10 @@ static struct ctl_table vm_table[] = {
> >  		.data		= &page_cluster,
> >  		.maxlen		= sizeof(int),
> >  		.mode		= 0644,
> > -		.proc_handler	= &proc_dointvec,
> > +		.proc_handler	= &proc_dointvec_minmax,
> > +		.strategy	= &sysctl_intvec,
> > +		.extra1		= &zero,
> > +		.extra2		= &page_cluster_max,
> >  	},
> >  	{
> >  		.ctl_name	= VM_DIRTY_BACKGROUND,
> > 
> > --
> > To unsubscribe, send a message with 'unsubscribe linux-mm' in
> > the body to majordomo@...ck.org.  For more info on Linux MM,
> > see: http://www.linux-mm.org/ .
> > Don't email: <a href=mailto:"dont@...ck.org"> email@...ck.org </a>
> 

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