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SubjectRe: [PATCH v2] zbud: allow up to PAGE_SIZE allocations
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Hello Seth,

On Thu, Sep 24, 2015 at 12:41 AM, Seth Jennings
<sjennings@variantweb.net> wrote:
> On Wed, Sep 23, 2015 at 10:59:00PM +0200, Vitaly Wool wrote:
>> Okay, how about this? It's gotten smaller BTW :)
>>
>> zbud: allow up to PAGE_SIZE allocations
>>
>> Currently zbud is only capable of allocating not more than
>> PAGE_SIZE - ZHDR_SIZE_ALIGNED - CHUNK_SIZE. This is okay as
>> long as only zswap is using it, but other users of zbud may
>> (and likely will) want to allocate up to PAGE_SIZE. This patch
>> addresses that by skipping the creation of zbud internal
>> structure in the beginning of an allocated page. As a zbud page
>> is no longer guaranteed to contain zbud header, the following
>> changes have to be applied throughout the code:
>> * page->lru to be used for zbud page lists
>> * page->private to hold 'under_reclaim' flag
>>
>> page->private will also be used to indicate if this page contains
>> a zbud header in the beginning or not ('headless' flag).
>>
>> Signed-off-by: Vitaly Wool <vitalywool@gmail.com>
>> ---
>> mm/zbud.c | 167 ++++++++++++++++++++++++++++++++++++++++++--------------------
>> 1 file changed, 113 insertions(+), 54 deletions(-)
>>
>> diff --git a/mm/zbud.c b/mm/zbud.c
>> index fa48bcdf..3946fba 100644
>> --- a/mm/zbud.c
>> +++ b/mm/zbud.c
>> @@ -105,18 +105,20 @@ struct zbud_pool {
>>
>> /*
>> * struct zbud_header - zbud page metadata occupying the first chunk of each
>> - * zbud page.
>> + * zbud page, except for HEADLESS pages
>> * @buddy: links the zbud page into the unbuddied/buddied lists in the pool
>> - * @lru: links the zbud page into the lru list in the pool
>> * @first_chunks: the size of the first buddy in chunks, 0 if free
>> * @last_chunks: the size of the last buddy in chunks, 0 if free
>> */
>> struct zbud_header {
>> struct list_head buddy;
>> - struct list_head lru;
>> unsigned int first_chunks;
>> unsigned int last_chunks;
>> - bool under_reclaim;
>> +};
>> +
>> +enum zbud_page_flags {
>> + UNDER_RECLAIM = 0,
>
> Don't need the "= 0"
>
>> + PAGE_HEADLESS,
>
> Also I think we should prefix the enum values here. With ZPF_ ?
>
>> };
>>
>> /*****************
>> @@ -221,6 +223,7 @@ MODULE_ALIAS("zpool-zbud");
>> *****************/
>> /* Just to make the code easier to read */
>> enum buddy {
>> + HEADLESS,
>> FIRST,
>> LAST
>> };
>> @@ -238,11 +241,14 @@ static int size_to_chunks(size_t size)
>> static struct zbud_header *init_zbud_page(struct page *page)
>> {
>> struct zbud_header *zhdr = page_address(page);
>> +
>> + INIT_LIST_HEAD(&page->lru);
>> + clear_bit(UNDER_RECLAIM, &page->private);
>> + clear_bit(HEADLESS, &page->private);
>
> I know we are using private in a bitwise flags mode, but maybe we
> should just init with page->private = 0
>
>> +
>> zhdr->first_chunks = 0;
>> zhdr->last_chunks = 0;
>> INIT_LIST_HEAD(&zhdr->buddy);
>> - INIT_LIST_HEAD(&zhdr->lru);
>> - zhdr->under_reclaim = 0;
>> return zhdr;
>> }
>>
>> @@ -267,11 +273,22 @@ static unsigned long encode_handle(struct zbud_header *zhdr, enum buddy bud)
>> * over the zbud header in the first chunk.
>> */
>> handle = (unsigned long)zhdr;
>> - if (bud == FIRST)
>> + switch (bud) {
>> + case FIRST:
>> /* skip over zbud header */
>> handle += ZHDR_SIZE_ALIGNED;
>> - else /* bud == LAST */
>> + break;
>> + case LAST:
>> handle += PAGE_SIZE - (zhdr->last_chunks << CHUNK_SHIFT);
>> + break;
>> + case HEADLESS:
>> + break;
>> + default:
>> + /* this should never happen */
>> + pr_err("zbud: invalid buddy value %d\n", bud);
>> + handle = 0;
>> + break;
>> + }
>
> Don't need this default case since we have a case for each valid value
> of the enum.
>
> Also, I think we want to add some code to free_zbud_page() to clear
> page->private and init page->lru so we don't leave dangling pointers.

Right, maybe it makes sense for free_zbud_page() to take struct page
pointer as an argument, too, to minimize back-and-forth conversions?

> Looks good though :)

Thanks, I'll come up with the new version shortly. :)

~vitaly


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