linux-kernel - Re: clear_page, copy_page address align question?

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Message-ID: <1491880344.4166.224.camel@kernel.crashing.org>
Date:   Tue, 11 Apr 2017 13:12:24 +1000
From:   Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: clear_page, copy_page address align question?

On Tue, 2017-04-11 at 12:08 +0900, Minchan Kim wrote:
> Hello,
> 
> When I tested zram in ppc64, I got random corruption.
> With investigation, it seems clear_page corrupted the memory.
> I passed 64K kmalloced(kmalloc(PAGE_SIZE)) address to clear_page
> and turned on slub debug so address is not aligned with PAGE_SIZE.
> Is it a valid usecase that non-PAGE_SIZE aligned address is
> used for clear_page in ppc64?
> 
> As well, copy_page have same rule, too?
> 
> Anyway, when I changed clear_page to memset, it seems the problem
> is gone.

Yes, both clear_page and copy_page assume a PAGE_SHIFT alignment and
are highly optimize according to this.

I wouldn't be surprised of other architectures implementations are the
same.

I don't think it's ever legit to call these functions for something
that isn't a naturally aligned page.

Cheers,
Ben.