linux-kernel - Re: memfill

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Date:   Wed, 8 Feb 2017 10:04:47 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     James Bottomley <James.Bottomley@...senPartnership.com>
Cc:     Minchan Kim <minchan@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        sergey.senozhatsky@...il.com, iamjoonsoo.kim@....com,
        ngupta@...are.org, zhouxianrong@...wei.com, zhouxiyu@...wei.com,
        weidu.du@...wei.com, zhangshiming5@...wei.com,
        Mi.Sophia.Wang@...wei.com, won.ho.park@...wei.com, liw@....fi
Subject: Re: memfill

On Tue, Feb 07, 2017 at 11:07:34AM -0800, James Bottomley wrote:
> > /** 
> >  * memfill - Fill a region of memory with the given value
> >  * @s: Pointer to the start of the region.
> >  * @v: The word to fill the region with.
> >  * @n: The size of the region.
> >  * 
> >  * Differs from memset() in that it fills with an unsigned long
> > instead of 
> >  * a byte.  The pointer and the size must be aligned to unsigned
> > long.
> >  */
> > void memfill(unsigned long *s, unsigned long v, size_t n)
> 
> If we're going to do this, are you sure we wouldn't be wanting a string
> fill instead of a memfill (because filling either by byte or long looks
> a bit restrictive) assuming static strings that we can tell the compile
> time size of, it would be easy for generic code to optimise.

When you say 'string fill', do you mean this?

void memfill(void *dst, size_t dsz, void *src, size_t ssz)
{
	if (ssz == 1) {
		memset(dst, *(unsigned char *)src, dsz);
	} else if (ssz == sizeof(short)) {
		memset_s(dst, *(unsigned short *)src, dsz);
	} else if (ssz == sizeof(int)) {
		memset_i(dst, *(unsigned int *)src, dsz);
	} else if (ssz == sizeof(long)) {
		memset_l(dst, *(unsigned long *)src, dsz);
	} else {
		while (dsz >= ssz) {
			memcpy(dst, src, ssz);
			dsz -= ssz;
			dst += ssz;
		}
		if (dsz)
			memcpy(dst. src. dsz);
	}
}

(with memset_[sil] being obvious, I hope).  Possibly some variation on
this to optimise compile-time constant dsz.

I have no objection to that.  Indeed, it would match Lars Wirzenius'
memfill() definition (if not implementation) which makes me quite happy.