diff options
Diffstat (limited to 'drivers')
| -rw-r--r-- | drivers/staging/zsmalloc/zsmalloc-main.c | 66 | ||||
| -rw-r--r-- | drivers/staging/zsmalloc/zsmalloc.h | 9 |
2 files changed, 64 insertions, 11 deletions
diff --git a/drivers/staging/zsmalloc/zsmalloc-main.c b/drivers/staging/zsmalloc/zsmalloc-main.c index 5bd53a8..b453d84 100644 --- a/drivers/staging/zsmalloc/zsmalloc-main.c +++ b/drivers/staging/zsmalloc/zsmalloc-main.c @@ -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) { diff --git a/drivers/staging/zsmalloc/zsmalloc.h b/drivers/staging/zsmalloc/zsmalloc.h index fbe6bec..c2eb174 100644 --- a/drivers/staging/zsmalloc/zsmalloc.h +++ b/drivers/staging/zsmalloc/zsmalloc.h @@ -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; |