linux-kernel - Re: [PATCH 1/2] zbud: allow PAGE

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Date:	Tue, 22 Sep 2015 12:18:58 +0200
From:	Vitaly Wool <vitalywool@...il.com>
To:	Dan Streetman <ddstreet@...e.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Minchan Kim <minchan@...nel.org>,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>,
	Seth Jennings <sjennings@...iantweb.net>
Subject: Re: [PATCH 1/2] zbud: allow PAGE_SIZE allocations

Hi Dan,

On Mon, Sep 21, 2015 at 6:17 PM, Dan Streetman <ddstreet@...e.org> wrote:
> Please make sure to cc Seth also, he's the owner of zbud.

Sure :)

<snip>
>> @@ -514,8 +552,17 @@ int zbud_reclaim_page(struct zbud_pool *pool, unsigned int retries)
>>                 return -EINVAL;
>>         }
>>         for (i = 0; i < retries; i++) {
>> -               zhdr = list_tail_entry(&pool->lru, struct zbud_header, lru);
>> -               list_del(&zhdr->lru);
>> +               page = list_tail_entry(&pool->lru, struct page, lru);
>> +               zhdr = page_address(page);
>> +               list_del(&page->lru);
>> +               /* Uncompressed zbud page? just run eviction and free it */
>> +               if (page->flags & PG_uncompressed) {
>> +                       page->flags &= ~PG_uncompressed;
>> +                       spin_unlock(&pool->lock);
>> +                       pool->ops->evict(pool, encode_handle(zhdr, FULL));
>> +                       __free_page(page);
>> +                       return 0;
>
> again, don't be redundant.  change the function to handle full-sized
> pages, don't repeat the function in an if() block for a special case.

Well, this case is a little tricky. How to process a zbud page in
zbud_reclaim_page() is defined basing on the assumption there is a
zhdr at the beginning of the page. What can be done here IMV is either
of the following:
* add a constant magic number to zhdr and check for it, if the check
fails, it is a type FULL page
* add a CRC field to zhdr, if CRC check over assumed zhdr fails,  it
is a type FULL page
* use a field from struct page to indicate its type

The last option still looks better to me.

~vitaly
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