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: <20170419115125.GA27790@bombadil.infradead.org>
Date:   Wed, 19 Apr 2017 04:51:25 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Minchan Kim <minchan@...nel.org>
Cc:     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 Wed, Apr 19, 2017 at 03:02:37PM +0900, Minchan Kim wrote:
> On Tue, Apr 18, 2017 at 09:33:07AM +0200, Michal Hocko wrote:
> > I do not follow. Why would you need kmap for something that is already
> > in the kernel space?
> 
> Because it can work with highmem pages.

That's copy_user_highpage().  If you want to define a new arch API
copy_highpage(), feel free to make a case for it ...

> > > 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?