linux-kernel - Re: [PATCH 1/1] lz4: Implement lz4 with dynamic offset length.

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Date:   Tue, 3 Apr 2018 21:26:42 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Maninder Singh <maninder1.s@...sung.com>
Cc:     "sergey.senozhatsky.work@...il.com" 
        <sergey.senozhatsky.work@...il.com>,
        "herbert@...dor.apana.org.au" <herbert@...dor.apana.org.au>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "minchan@...nel.org" <minchan@...nel.org>,
        "ngupta@...are.org" <ngupta@...are.org>,
        "keescook@...omium.org" <keescook@...omium.org>,
        "anton@...msg.org" <anton@...msg.org>,
        "ccross@...roid.com" <ccross@...roid.com>,
        "tony.luck@...el.com" <tony.luck@...el.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "colin.king@...onical.com" <colin.king@...onical.com>,
        "linux-crypto@...r.kernel.org" <linux-crypto@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        PANKAJ MISHRA <pankaj.m@...sung.com>,
        AMIT SAHRAWAT <a.sahrawat@...sung.com>,
        Vaneet Narang <v.narang@...sung.com>
Subject: Re: [PATCH 1/1] lz4: Implement lz4 with dynamic offset length.

On (04/02/18 11:21), Maninder Singh wrote:
[..]
> >>  static const char * const backends[] = {
> >>          "lzo",
> >>  #if IS_ENABLED(CONFIG_CRYPTO_LZ4)
> >>          "lz4",
> >> +#if (PAGE_SIZE < (32 * KB))
> >> +        "lz4_dyn",
> >> +#endif
> > 
> >This is not the list of supported algorithms. It's the list of
> >recommended algorithms. You can configure zram to use any of
> >available and known to Crypto API algorithms. Including lz4_dyn
> >on PAGE_SIZE > 32K systems.
> > 
> Yes, we want to integrate new compression(lz4_dyn) for ZRAM 
> only if PAGE_SIZE is less than 32KB to get maximum benefit. 
> so we added lz4_dyn to available list of ZRAM compression alhorithms.

Which is not what I was talking about.

You shrink a 2 bytes offset down to a 1 byte offset, thus you enforce that
'page should be less than 32KB', which I'm sure will be confusing. And you
rely on lz4_dyn users to do the right thing - namely, to use that 'nice'
`#if (PAGE_SIZE < (32 * KB))'. Apart from that, lz4_dyn supports only data
in up to page_size chunks. Suppose my system has page_size of less than 32K,
so I legitimately can enable lz4_dyn, but suppose that I will use it
somewhere where I don't work with page_size-d chunks. Will I able to just
do tfm->compress(src, sz) on random buffers? The whole thing looks to be
quite fragile.

	-ss