linux-kernel - Re: copy_page() on a kmalloc-ed page with DEBUG_SLAB enabled (was "zram: do not use copy

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Message-ID: <20170420014542.GA542@jagdpanzerIV.localdomain>
Date:   Thu, 20 Apr 2017 10:45:42 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Minchan Kim <minchan@...nel.org>, Michal Hocko <mhocko@...nel.org>,
        Christoph Lameter <cl@...ux.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kernel-team@....com,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: copy_page() on a kmalloc-ed page with DEBUG_SLAB enabled (was
 "zram: do not use copy_page with non-page alinged address")

On (04/19/17 04:51), Matthew Wilcox wrote:
[..]
> > > > Another approach is the API does normal thing for non-aligned prefix and
> > > > tail space and fast thing for aligned space.
> > > > Otherwise, it would be happy if the API has WARN_ON non-page SIZE aligned
> > > > address.
> 
> Why not just use memcpy()?  Is copy_page() significantly faster than
> memcpy() for a PAGE_SIZE amount of data?

that's a good point.

I was going to ask yesterday - do we even need copy_page()? arch that
provides well optimized copy_page() quite likely provides somewhat
equally optimized memcpy(). so may be copy_page() is not even needed?

	-ss