Date:	Tue, 19 Feb 2013 18:42:40 -0800
From:	Nitin Gupta <ngupta@...are.org>
To:	Seth Jennings <sjenning@...ux.vnet.ibm.com>
CC:	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Minchan Kim <minchan@...nel.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Dan Magenheimer <dan.magenheimer@...cle.com>,
	Robert Jennings <rcj@...ux.vnet.ibm.com>,
	Jenifer Hopper <jhopper@...ibm.com>,
	Mel Gorman <mgorman@...e.de>,
	Johannes Weiner <jweiner@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Larry Woodman <lwoodman@...hat.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Dave Hansen <dave@...ux.vnet.ibm.com>,
	Joe Perches <joe@...ches.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, devel@...verdev.osuosl.org
Subject: Re: [PATCHv5 1/8] zsmalloc: add to mm/

On 02/19/2013 09:54 AM, Seth Jennings wrote:
> On 02/19/2013 03:18 AM, Joonsoo Kim wrote:
>> Hello, Seth.
>> I'm not sure that this is right time to review, because I already have
>> seen many effort of various people to promote zxxx series. I don't want to
>> be a stopper to promote these. :)
> 
> Any time is good review time :)  Thanks for your review!
> 
>>
>> But, I read the code, now, and then some comments below.
>>
>> On Wed, Feb 13, 2013 at 12:38:44PM -0600, Seth Jennings wrote:
>>> =========
>>> DO NOT MERGE, FOR REVIEW ONLY
>>> This patch introduces zsmalloc as new code, however, it already
>>> exists in drivers/staging.  In order to build successfully, you
>>> must select EITHER to driver/staging version OR this version.
>>> Once zsmalloc is reviewed in this format (and hopefully accepted),
>>> I will create a new patchset that properly promotes zsmalloc from
>>> staging.
>>> =========
>>>
>>> This patchset introduces a new slab-based memory allocator,
>>> zsmalloc, for storing compressed pages.  It is designed for
>>> low fragmentation and high allocation success rate on
>>> large object, but <= PAGE_SIZE allocations.
>>>
>>> zsmalloc differs from the kernel slab allocator in two primary
>>> ways to achieve these design goals.
>>>
>>> zsmalloc never requires high order page allocations to back
>>> slabs, or "size classes" in zsmalloc terms. Instead it allows
>>> multiple single-order pages to be stitched together into a
>>> "zspage" which backs the slab.  This allows for higher allocation
>>> success rate under memory pressure.
>>>
>>> Also, zsmalloc allows objects to span page boundaries within the
>>> zspage.  This allows for lower fragmentation than could be had
>>> with the kernel slab allocator for objects between PAGE_SIZE/2
>>> and PAGE_SIZE.  With the kernel slab allocator, if a page compresses
>>> to 60% of it original size, the memory savings gained through
>>> compression is lost in fragmentation because another object of
>>> the same size can't be stored in the leftover space.
>>>
>>> This ability to span pages results in zsmalloc allocations not being
>>> directly addressable by the user.  The user is given an
>>> non-dereferencable handle in response to an allocation request.
>>> That handle must be mapped, using zs_map_object(), which returns
>>> a pointer to the mapped region that can be used.  The mapping is
>>> necessary since the object data may reside in two different
>>> noncontigious pages.
>>>
>>> zsmalloc fulfills the allocation needs for zram and zswap.
>>>
>>> Acked-by: Nitin Gupta <ngupta@...are.org>
>>> Acked-by: Minchan Kim <minchan@...nel.org>
>>> Signed-off-by: Seth Jennings <sjenning@...ux.vnet.ibm.com>
>>> ---
>>>  include/linux/zsmalloc.h |   49 ++
>>>  mm/Kconfig               |   24 +
>>>  mm/Makefile              |    1 +
>>>  mm/zsmalloc.c            | 1124 ++++++++++++++++++++++++++++++++++++++++++++++
>>>  4 files changed, 1198 insertions(+)
>>>  create mode 100644 include/linux/zsmalloc.h
>>>  create mode 100644 mm/zsmalloc.c
>>>
>>> diff --git a/include/linux/zsmalloc.h b/include/linux/zsmalloc.h
>>> new file mode 100644
>>> index 0000000..eb6efb6
>>> --- /dev/null
>>> +++ b/include/linux/zsmalloc.h
>>> @@ -0,0 +1,49 @@
>>> +/*
>>> + * zsmalloc memory allocator
>>> + *
>>> + * Copyright (C) 2011  Nitin Gupta
>>> + *
>>> + * This code is released using a dual license strategy: BSD/GPL
>>> + * You can choose the license that better fits your requirements.
>>> + *
>>> + * Released under the terms of 3-clause BSD License
>>> + * Released under the terms of GNU General Public License Version 2.0
>>> + */
>>> +
>>> +#ifndef _ZS_MALLOC_H_
>>> +#define _ZS_MALLOC_H_
>>> +
>>> +#include <linux/types.h>
>>> +#include <linux/mm_types.h>
>>> +
>>> +/*
>>> + * zsmalloc mapping modes
>>> + *
>>> + * NOTE: These only make a difference when a mapped object spans pages
>>> +*/
>>> +enum zs_mapmode {
>>> +	ZS_MM_RW, /* normal read-write mapping */
>>> +	ZS_MM_RO, /* read-only (no copy-out at unmap time) */
>>> +	ZS_MM_WO /* write-only (no copy-in at map time) */
>>> +};
>>
>>
>> These makes no difference for PGTABLE_MAPPING.
>> Please add some comment for this.
> 
> Yes. Will do.
> 
>>
>>> +struct zs_ops {
>>> +	struct page * (*alloc)(gfp_t);
>>> +	void (*free)(struct page *);
>>> +};
>>> +
>>> +struct zs_pool;
>>> +
>>> +struct zs_pool *zs_create_pool(gfp_t flags, struct zs_ops *ops);
>>> +void zs_destroy_pool(struct zs_pool *pool);
>>> +
>>> +unsigned long zs_malloc(struct zs_pool *pool, size_t size, gfp_t flags);
>>> +void zs_free(struct zs_pool *pool, unsigned long obj);
>>> +
>>> +void *zs_map_object(struct zs_pool *pool, unsigned long handle,
>>> +			enum zs_mapmode mm);
>>> +void zs_unmap_object(struct zs_pool *pool, unsigned long handle);
>>> +
>>> +u64 zs_get_total_size_bytes(struct zs_pool *pool);
>>> +
>>> +#endif
>>> diff --git a/mm/Kconfig b/mm/Kconfig
>>> index 278e3ab..25b8f38 100644
>>> --- a/mm/Kconfig
>>> +++ b/mm/Kconfig
>>> @@ -446,3 +446,27 @@ config FRONTSWAP
>>>  	  and swap data is stored as normal on the matching swap device.
>>>  
>>>  	  If unsure, say Y to enable frontswap.
>>> +
>>> +config ZSMALLOC
>>> +	tristate "Memory allocator for compressed pages"
>>> +	default n
>>> +	help
>>> +	  zsmalloc is a slab-based memory allocator designed to store
>>> +	  compressed RAM pages.  zsmalloc uses virtual memory mapping
>>> +	  in order to reduce fragmentation.  However, this results in a
>>> +	  non-standard allocator interface where a handle, not a pointer, is
>>> +	  returned by an alloc().  This handle must be mapped in order to
>>> +	  access the allocated space.
>>> +
>>> +config PGTABLE_MAPPING
>>> +	bool "Use page table mapping to access object in zsmalloc"
>>> +	depends on ZSMALLOC
>>> +	help
>>> +	  By default, zsmalloc uses a copy-based object mapping method to
>>> +	  access allocations that span two pages. However, if a particular
>>> +	  architecture (ex, ARM) performs VM mapping faster than copying,
>>> +	  then you should select this. This causes zsmalloc to use page table
>>> +	  mapping rather than copying for object mapping.
>>> +
>>> +	  You can check speed with zsmalloc benchmark[1].
>>> +	  [1] https://github.com/spartacus06/zsmalloc
>>> diff --git a/mm/Makefile b/mm/Makefile
>>> index 3a46287..0f6ef0a 100644
>>> --- a/mm/Makefile
>>> +++ b/mm/Makefile
>>> @@ -58,3 +58,4 @@ obj-$(CONFIG_DEBUG_KMEMLEAK) += kmemleak.o
>>>  obj-$(CONFIG_DEBUG_KMEMLEAK_TEST) += kmemleak-test.o
>>>  obj-$(CONFIG_CLEANCACHE) += cleancache.o
>>>  obj-$(CONFIG_MEMORY_ISOLATION) += page_isolation.o
>>> +obj-$(CONFIG_ZSMALLOC)	+= zsmalloc.o
>>> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
>>> new file mode 100644
>>> index 0000000..34378ef
>>> --- /dev/null
>>> +++ b/mm/zsmalloc.c
>>> @@ -0,0 +1,1124 @@
>>> +/*
>>> + * zsmalloc memory allocator
>>> + *
>>> + * Copyright (C) 2011  Nitin Gupta
>>> + *
>>> + * This code is released using a dual license strategy: BSD/GPL
>>> + * You can choose the license that better fits your requirements.
>>> + *
>>> + * Released under the terms of 3-clause BSD License
>>> + * Released under the terms of GNU General Public License Version 2.0
>>> + */
>>> +
>>> +
>>> +/*
>>> + * This allocator is designed for use with zcache and zram. Thus, the
>>> + * allocator is supposed to work well under low memory conditions. In
>>> + * particular, it never attempts higher order page allocation which is
>>> + * very likely to fail under memory pressure. On the other hand, if we
>>> + * just use single (0-order) pages, it would suffer from very high
>>> + * fragmentation -- any object of size PAGE_SIZE/2 or larger would occupy
>>> + * an entire page. This was one of the major issues with its predecessor
>>> + * (xvmalloc).
>>> + *
>>> + * To overcome these issues, zsmalloc allocates a bunch of 0-order pages
>>> + * and links them together using various 'struct page' fields. These linked
>>> + * pages act as a single higher-order page i.e. an object can span 0-order
>>> + * page boundaries. The code refers to these linked pages as a single entity
>>> + * called zspage.
>>> + *
>>> + * For simplicity, zsmalloc can only allocate objects of size up to PAGE_SIZE
>>> + * since this satisfies the requirements of all its current users (in the
>>> + * worst case, page is incompressible and is thus stored "as-is" i.e. in
>>> + * uncompressed form). For allocation requests larger than this size, failure
>>> + * is returned (see zs_malloc).
>>> + *
>>> + * Additionally, zs_malloc() does not return a dereferenceable pointer.
>>> + * Instead, it returns an opaque handle (unsigned long) which encodes actual
>>> + * location of the allocated object. The reason for this indirection is that
>>> + * zsmalloc does not keep zspages permanently mapped since that would cause
>>> + * issues on 32-bit systems where the VA region for kernel space mappings
>>> + * is very small. So, before using the allocating memory, the object has to
>>> + * be mapped using zs_map_object() to get a usable pointer and subsequently
>>> + * unmapped using zs_unmap_object().
>>> + *
>>> + * Following is how we use various fields and flags of underlying
>>> + * struct page(s) to form a zspage.
>>> + *
>>> + * Usage of struct page fields:
>>> + *	page->first_page: points to the first component (0-order) page
>>> + *	page->index (union with page->freelist): offset of the first object
>>> + *		starting in this page. For the first page, this is
>>> + *		always 0, so we use this field (aka freelist) to point
>>> + *		to the first free object in zspage.
>>> + *	page->lru: links together all component pages (except the first page)
>>> + *		of a zspage
>>> + *
>>> + *	For _first_ page only:
>>> + *
>>> + *	page->private (union with page->first_page): refers to the
>>> + *		component page after the first page
>>> + *	page->freelist: points to the first free object in zspage.
>>> + *		Free objects are linked together using in-place
>>> + *		metadata.
>>> + *	page->objects: maximum number of objects we can store in this
>>> + *		zspage (class->zspage_order * PAGE_SIZE / class->size)
>>
>> How about just embedding maximum number of objects to size_class?
>> For the SLUB, each slab can have difference number of objects.
>> But, for the zsmalloc, it is not possible, so there is no reason
>> to maintain it within metadata of zspage. Just to embed it to size_class
>> is sufficient.
> 
> Yes, a little code massaging and this can go away.
> 
> However, there might be some value in having variable sized zspages in
> the same size_class.  It could improve allocation success rate at the
> expense of efficiency by not failing in alloc_zspage() if we can't
> allocate the optimal number of pages.  As long as we can allocate the
> first page, then we can proceed.
> 

Yes, I remember trying to allow partial failures and thus allow variable
sized zspages within the same size class but I just skipped that since
the value wasn't clear: if the system cannot give us 4  (non-contiguous)
pages then it's probably going to fail allocation  requests for even
single pages, in not so far future. Also, for objects>PAGESIZE/2,
failing over to zspage of just PAGESIZE is not going to  help either.

>>
>>
>>> + *	page->lru: links together first pages of various zspages.
>>> + *		Basically forming list of zspages in a fullness group.
>>> + *	page->mapping: class index and fullness group of the zspage
>>> + *
>>> + * Usage of struct page flags:
>>> + *	PG_private: identifies the first component page
>>> + *	PG_private2: identifies the last component page
>>> + *
>>> + */
>>> +
>>> +#ifdef CONFIG_ZSMALLOC_DEBUG
>>> +#define DEBUG
>>> +#endif
>>
>> Is this obsolete?
> 
> Yes, I'll remove it.
> 
>>
>>> +#include <linux/module.h>
>>> +#include <linux/kernel.h>
>>> +#include <linux/bitops.h>
>>> +#include <linux/errno.h>
>>> +#include <linux/highmem.h>
>>> +#include <linux/init.h>
>>> +#include <linux/string.h>
>>> +#include <linux/slab.h>
>>> +#include <asm/tlbflush.h>
>>> +#include <asm/pgtable.h>
>>> +#include <linux/cpumask.h>
>>> +#include <linux/cpu.h>
>>> +#include <linux/vmalloc.h>
>>> +#include <linux/hardirq.h>
>>> +#include <linux/spinlock.h>
>>> +#include <linux/types.h>
>>> +
>>> +#include <linux/zsmalloc.h>
>>> +
>>> +/*
>>> + * This must be power of 2 and greater than of equal to sizeof(link_free).
>>> + * These two conditions ensure that any 'struct link_free' itself doesn't
>>> + * span more than 1 page which avoids complex case of mapping 2 pages simply
>>> + * to restore link_free pointer values.
>>> + */
>>> +#define ZS_ALIGN		8
>>> +
>>> +/*
>>> + * A single 'zspage' is composed of up to 2^N discontiguous 0-order (single)
>>> + * pages. ZS_MAX_ZSPAGE_ORDER defines upper limit on N.
>>> + */
>>> +#define ZS_MAX_ZSPAGE_ORDER 2
>>> +#define ZS_MAX_PAGES_PER_ZSPAGE (_AC(1, UL) << ZS_MAX_ZSPAGE_ORDER)
>>> +
>>> +/*
>>> + * Object location (<PFN>, <obj_idx>) is encoded as
>>> + * as single (unsigned long) handle value.
>>> + *
>>> + * Note that object index <obj_idx> is relative to system
>>> + * page <PFN> it is stored in, so for each sub-page belonging
>>> + * to a zspage, obj_idx starts with 0.
>>> + *
>>> + * This is made more complicated by various memory models and PAE.
>>> + */
>>> +
>>> +#ifndef MAX_PHYSMEM_BITS
>>> +#ifdef CONFIG_HIGHMEM64G
>>> +#define MAX_PHYSMEM_BITS 36
>>> +#else /* !CONFIG_HIGHMEM64G */
>>> +/*
>>> + * If this definition of MAX_PHYSMEM_BITS is used, OBJ_INDEX_BITS will just
>>> + * be PAGE_SHIFT
>>> + */
>>> +#define MAX_PHYSMEM_BITS BITS_PER_LONG
>>> +#endif
>>> +#endif
>>> +#define _PFN_BITS		(MAX_PHYSMEM_BITS - PAGE_SHIFT)
>>> +#define OBJ_INDEX_BITS	(BITS_PER_LONG - _PFN_BITS)
>>> +#define OBJ_INDEX_MASK	((_AC(1, UL) << OBJ_INDEX_BITS) - 1)
>>> +
>>> +#define MAX(a, b) ((a) >= (b) ? (a) : (b))
>>> +/* ZS_MIN_ALLOC_SIZE must be multiple of ZS_ALIGN */
>>> +#define ZS_MIN_ALLOC_SIZE \
>>> +	MAX(32, (ZS_MAX_PAGES_PER_ZSPAGE << PAGE_SHIFT >> OBJ_INDEX_BITS))
>>> +#define ZS_MAX_ALLOC_SIZE	PAGE_SIZE
>>> +
>>> +/*
>>> + * On systems with 4K page size, this gives 254 size classes! There is a
>>> + * trader-off here:
>>> + *  - Large number of size classes is potentially wasteful as free page are
>>> + *    spread across these classes
>>> + *  - Small number of size classes causes large internal fragmentation
>>> + *  - Probably its better to use specific size classes (empirically
>>> + *    determined). NOTE: all those class sizes must be set as multiple of
>>> + *    ZS_ALIGN to make sure link_free itself never has to span 2 pages.
>>> + *
>>> + *  ZS_MIN_ALLOC_SIZE and ZS_SIZE_CLASS_DELTA must be multiple of ZS_ALIGN
>>> + *  (reason above)
>>> + */
>>> +#define ZS_SIZE_CLASS_DELTA	(PAGE_SIZE >> 8)
>>> +#define ZS_SIZE_CLASSES		((ZS_MAX_ALLOC_SIZE - ZS_MIN_ALLOC_SIZE) / \
>>> +					ZS_SIZE_CLASS_DELTA + 1)
>>> +
>>> +/*
>>> + * We do not maintain any list for completely empty or full pages
>>> + */
>>> +enum fullness_group {
>>> +	ZS_ALMOST_FULL,
>>> +	ZS_ALMOST_EMPTY,
>>> +	_ZS_NR_FULLNESS_GROUPS,
>>> +
>>> +	ZS_EMPTY,
>>> +	ZS_FULL
>>> +};
>>> +
>>> +/*
>>> + * We assign a page to ZS_ALMOST_EMPTY fullness group when:
>>> + *	n <= N / f, where
>>> + * n = number of allocated objects
>>> + * N = total number of objects zspage can store
>>> + * f = 1/fullness_threshold_frac
>>> + *
>>> + * Similarly, we assign zspage to:
>>> + *	ZS_ALMOST_FULL	when n > N / f
>>> + *	ZS_EMPTY	when n == 0
>>> + *	ZS_FULL		when n == N
>>> + *
>>> + * (see: fix_fullness_group())
>>> + */
>>> +static const int fullness_threshold_frac = 4;
>>> +
>>> +struct size_class {
>>> +	/*
>>> +	 * Size of objects stored in this class. Must be multiple
>>> +	 * of ZS_ALIGN.
>>> +	 */
>>> +	int size;
>>> +	unsigned int index;
>>> +
>>> +	/* Number of PAGE_SIZE sized pages to combine to form a 'zspage' */
>>> +	int pages_per_zspage;
>>> +
>>> +	spinlock_t lock;
>>> +
>>> +	/* stats */
>>> +	u64 pages_allocated;
>>> +
>>> +	struct page *fullness_list[_ZS_NR_FULLNESS_GROUPS];
>>> +};
>>
>> Instead of simple pointer, how about using list_head?
>> With this, fullness_list management is easily consolidated to
>> set_zspage_mapping() and we can remove remove_zspage(), insert_zspage().
> 

Yes seems like a nice cleanup. However, I think we should defer it till
it's promoted out of staging.

Thanks a lot of reviews and comments.
Nitin

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