linux-kernel - Re: [PATCH] zswap: only save zswap header if zpool is shrinkable

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Date:   Thu, 11 Jan 2018 15:57:57 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Dan Streetman <ddstreet@...e.org>
Cc:     Yu Zhao <yuzhao@...gle.com>, Seth Jennings <sjenning@...hat.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Minchan Kim <minchan@...nel.org>,
        Nitin Gupta <ngupta@...are.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux-MM <linux-mm@...ck.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] zswap: only save zswap header if zpool is shrinkable

Hello,

Yu Zhao, Dan, sorry for the delay


On (01/10/18 15:06), Dan Streetman wrote:
[..]
> Well, I think shrink vs evict an implementation detail, isn't it?
> That is, from zswap's perspective, there should be:
> 
> zpool_evictable()
> if true, zswap needs to include the header on each compressed page,
> because the zpool may callback zpool->ops->evict() which calls
> zswap_writeback_entry() which expects the entry to start with a zswap
> header.
> if false, zswap doesn't need to include the header, because the zpool
> will never, ever call zpool->ops->evict
> 
> zpool_shrink()
> this will try to shrink the zpool, using whatever
> zpool-implementation-specific shrinking method.  If zpool_evictable()
> is true for this zpool, then zpool_shrink() *might* callback to
> zpool->ops->evict(), although it doesn't have to if it can shrink
> without evictions.  If zpool_evictable() is false, then zpool_shrink()
> will never callback to zpool->ops->evict().

ACK on this!

	-ss