Commit c3e3e88a authored by Nitin Cupta's avatar Nitin Cupta Committed by Greg Kroah-Hartman
Browse files

zsmalloc: add more comment



This patch adds lots of comments and it will help others
to review and enhance.
Signed-off-by: default avatarSeth Jennings <sjenning@linux.vnet.ibm.com>
Signed-off-by: default avatarNitin Gupta <ngupta@vflare.org>
Signed-off-by: default avatarMinchan Kim <minchan@kernel.org>
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
parent 1b945aee
......@@ -10,16 +10,14 @@
* 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).
* This allocator is designed for use with 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
......@@ -27,6 +25,21 @@
* 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.
*
......@@ -98,7 +111,7 @@
/*
* Object location (<PFN>, <obj_idx>) is encoded as
* as single (void *) handle value.
* 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
......@@ -264,6 +277,13 @@ static void set_zspage_mapping(struct page *page, unsigned int class_idx,
page->mapping = (struct address_space *)m;
}
/*
* zsmalloc divides the pool into various size classes where each
* class maintains a list of zspages where each zspage is divided
* into equal sized chunks. Each allocation falls into one of these
* classes depending on its size. This function returns index of the
* size class which has chunk size big enough to hold the give size.
*/
static int get_size_class_index(int size)
{
int idx = 0;
......@@ -275,6 +295,13 @@ static int get_size_class_index(int size)
return idx;
}
/*
* For each size class, zspages are divided into different groups
* depending on how "full" they are. This was done so that we could
* easily find empty or nearly empty zspages when we try to shrink
* the pool (not yet implemented). This function returns fullness
* status of the given page.
*/
static enum fullness_group get_fullness_group(struct page *page)
{
int inuse, max_objects;
......@@ -296,6 +323,12 @@ static enum fullness_group get_fullness_group(struct page *page)
return fg;
}
/*
* Each size class maintains various freelists and zspages are assigned
* to one of these freelists based on the number of live objects they
* have. This functions inserts the given zspage into the freelist
* identified by <class, fullness_group>.
*/
static void insert_zspage(struct page *page, struct size_class *class,
enum fullness_group fullness)
{
......@@ -313,6 +346,10 @@ static void insert_zspage(struct page *page, struct size_class *class,
*head = page;
}
/*
* This function removes the given zspage from the freelist identified
* by <class, fullness_group>.
*/
static void remove_zspage(struct page *page, struct size_class *class,
enum fullness_group fullness)
{
......@@ -334,6 +371,15 @@ static void remove_zspage(struct page *page, struct size_class *class,
list_del_init(&page->lru);
}
/*
* Each size class maintains zspages in different fullness groups depending
* on the number of live objects they contain. When allocating or freeing
* objects, the fullness status of the page can change, say, from ALMOST_FULL
* to ALMOST_EMPTY when freeing an object. This function checks if such
* a status change has occurred for the given page and accordingly moves the
* page from the freelist of the old fullness group to that of the new
* fullness group.
*/
static enum fullness_group fix_fullness_group(struct zs_pool *pool,
struct page *page)
{
......
......@@ -18,12 +18,19 @@
/*
* zsmalloc mapping modes
*
* NOTE: These only make a difference when a mapped object spans pages
* NOTE: These only make a difference when a mapped object spans pages.
* They also have no effect when PGTABLE_MAPPING is selected.
*/
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) */
/*
* NOTE: ZS_MM_WO should only be used for initializing new
* (uninitialized) allocations. Partial writes to already
* initialized allocations should use ZS_MM_RW to preserve the
* existing data.
*/
};
struct zs_pool;
......
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