Documentation of various struct page fields
used by zsmalloc.
Signed-off-by: Nitin Gupta <[email protected]>
Changes for v2:
- Regroup descriptions as suggested by Seth
---
drivers/staging/zsmalloc/zsmalloc-main.c | 48 ++++++++++++++++++++++++++++++
1 file changed, 48 insertions(+)
diff --git a/drivers/staging/zsmalloc/zsmalloc-main.c b/drivers/staging/zsmalloc/zsmalloc-main.c
index 4496737..1db76ec 100644
--- a/drivers/staging/zsmalloc/zsmalloc-main.c
+++ b/drivers/staging/zsmalloc/zsmalloc-main.c
@@ -10,6 +10,54 @@
* 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.
+ *
+ * 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)
+ * 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
--
1.7.10.2
On Sat, Jun 09, 2012 at 05:41:14PM -0700, Nitin Gupta wrote:
> Documentation of various struct page fields
> used by zsmalloc.
>
> Signed-off-by: Nitin Gupta <[email protected]>
>
> Changes for v2:
> - Regroup descriptions as suggested by Seth
^^ - Konrad
Otherwise: Acked-by: Konrad Rzeszutek Wilk <[email protected]>
> ---
> drivers/staging/zsmalloc/zsmalloc-main.c | 48 ++++++++++++++++++++++++++++++
> 1 file changed, 48 insertions(+)
>
> diff --git a/drivers/staging/zsmalloc/zsmalloc-main.c b/drivers/staging/zsmalloc/zsmalloc-main.c
> index 4496737..1db76ec 100644
> --- a/drivers/staging/zsmalloc/zsmalloc-main.c
> +++ b/drivers/staging/zsmalloc/zsmalloc-main.c
> @@ -10,6 +10,54 @@
> * 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.
> + *
> + * 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)
> + * 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
> --
> 1.7.10.2
>
> --
> To unsubscribe, send a message with 'unsubscribe linux-mm' in
> the body to [email protected]. For more info on Linux MM,
> see: http://www.linux-mm.org/ .
> Don't email: <a href=mailto:"[email protected]"> [email protected] </a>
>
On 06/09/2012 05:44 PM, Konrad Rzeszutek Wilk wrote:
> On Sat, Jun 09, 2012 at 05:41:14PM -0700, Nitin Gupta wrote:
>> Documentation of various struct page fields
>> used by zsmalloc.
>>
>> Signed-off-by: Nitin Gupta <[email protected]>
>>
>> Changes for v2:
>> - Regroup descriptions as suggested by Seth
> ^^ - Konrad
>
> Otherwise: Acked-by: Konrad Rzeszutek Wilk <[email protected]>
>
Sorry about that, Konrad!
Greg: Please let me know if I should resend the patch with corrected
name in the changelog.
Thanks,
Nitin
>> ---
>> drivers/staging/zsmalloc/zsmalloc-main.c | 48 ++++++++++++++++++++++++++++++
>> 1 file changed, 48 insertions(+)
>>
>> diff --git a/drivers/staging/zsmalloc/zsmalloc-main.c b/drivers/staging/zsmalloc/zsmalloc-main.c
>> index 4496737..1db76ec 100644
>> --- a/drivers/staging/zsmalloc/zsmalloc-main.c
>> +++ b/drivers/staging/zsmalloc/zsmalloc-main.c
>> @@ -10,6 +10,54 @@
>> * 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.
>> + *
>> + * 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)
>> + * 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
>> --
>> 1.7.10.2
>>
>> --
>> To unsubscribe, send a message with 'unsubscribe linux-mm' in
>> the body to [email protected]. For more info on Linux MM,
>> see: http://www.linux-mm.org/ .
>> Don't email: <a href=mailto:"[email protected]"> [email protected] </a>
>>
On 06/10/2012 09:41 AM, Nitin Gupta wrote:
> Documentation of various struct page fields
> used by zsmalloc.
>
> Signed-off-by: Nitin Gupta <[email protected]>
>
Reviewed-by: Minchan Kim <[email protected]>
--
Kind regards,
Minchan Kim
On Sat, Jun 09, 2012 at 05:49:42PM -0700, Nitin Gupta wrote:
> On 06/09/2012 05:44 PM, Konrad Rzeszutek Wilk wrote:
>
> > On Sat, Jun 09, 2012 at 05:41:14PM -0700, Nitin Gupta wrote:
> >> Documentation of various struct page fields
> >> used by zsmalloc.
> >>
> >> Signed-off-by: Nitin Gupta <[email protected]>
> >>
> >> Changes for v2:
> >> - Regroup descriptions as suggested by Seth
> > ^^ - Konrad
> >
> > Otherwise: Acked-by: Konrad Rzeszutek Wilk <[email protected]>
> >
>
>
> Sorry about that, Konrad!
> Greg: Please let me know if I should resend the patch with corrected
> name in the changelog.
No need, I've fixed it up.
greg k-h