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

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Message-ID: <CALZtONCgqF7MiGUkLaGm8sTfPu-yd2gMTgq2Y1DJNOpFdBEt8A@mail.gmail.com>
Date:	Tue, 22 Sep 2015 13:09:09 -0400
From:	Dan Streetman <ddstreet@...e.org>
To:	Vitaly Wool <vitalywool@...il.com>
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

On Tue, Sep 22, 2015 at 6:18 AM, Vitaly Wool <vitalywool@...il.com> wrote:
> 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:

aha, this is why you used the page flag.

> * 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

neither of those; you can't guarantee the magic number won't naturally
occur in a page.

> * use a field from struct page to indicate its type

sure, you could use a pre-existing field from struct page, like the
page->private field.

>
> The last option still looks better to me.
>
> ~vitaly
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