linux-kernel - Re: [PATCH] mm: extend zero pages to same element pages for zram

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Date:   Mon, 6 Feb 2017 06:14:48 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     zhouxianrong <zhouxianrong@...wei.com>
Cc:     Minchan Kim <minchan@...nel.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
        sergey.senozhatsky@...il.com, iamjoonsoo.kim@....com,
        ngupta@...are.org, Mi.Sophia.Wang@...wei.com, zhouxiyu@...wei.com,
        weidu.du@...wei.com, zhangshiming5@...wei.com,
        won.ho.park@...wei.com
Subject: Re: [PATCH] mm: extend zero pages to same element pages for zram

On Mon, Feb 06, 2017 at 09:28:18AM +0800, zhouxianrong wrote:
> > > +static inline void zram_fill_page_partial(char *ptr, unsigned int size,
> > > +		unsigned long value)
> > > +{
> > > +	int i;
> > > +	unsigned long *page;
> > > +
> > > +	if (likely(value == 0)) {
> > > +		memset(ptr, 0, size);
> > > +		return;
> > > +	}
> > > +
> > > +	i = ((unsigned long)ptr) % sizeof(*page);
> > > +	if (i) {
> > > +		while (i < sizeof(*page)) {
> > > +			*ptr++ = (value >> (i * 8)) & 0xff;
> > > +			--size;
> > > +			++i;
> > > +		}
> > > +	}
> > > +
> > 
> > I don't think we need this part because block layer works with sector
> > size or multiple times of it so it must be aligned unsigned long.
> 
> Minchan and Matthew Wilcox:
> 
> 1. right, but users could open /dev/block/zram0 file and do any read operations.

But any such read operation would go through the page cache, so will
be page aligned.  Unless they do an O_DIRECT operation, in which case
it must be aligned to block size.  Please, try it.  I/Os which are not
aligned should be failed long before they reach your driver.