linux-kernel - Re: [PATCH] tmpfs: fix regressions from wider use of ZERO

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Message-ID: <20220409050638.GB17755@lst.de>
Date:   Sat, 9 Apr 2022 07:06:38 +0200
From:   Christoph Hellwig <hch@....de>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Chuck Lever III <chuck.lever@...cle.com>,
        Mark Hemment <markhemm@...glemail.com>,
        Patrice CHOTARD <patrice.chotard@...s.st.com>,
        Mikulas Patocka <mpatocka@...hat.com>,
        Lukas Czerner <lczerner@...hat.com>,
        Christoph Hellwig <hch@....de>,
        "Darrick J. Wong" <djwong@...nel.org>, linux-mm@...ck.org,
        linux-nfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        linux-stm32@...md-mailman.stormreply.com, viro@...iv.linux.org.uk
Subject: Re: [PATCH] tmpfs: fix regressions from wider use of ZERO_PAGE

On Fri, Apr 08, 2022 at 01:38:41PM -0700, Hugh Dickins wrote:
> +		} else if (iter_is_iovec(to)) {
> +			/*
> +			 * Copy to user tends to be so well optimized, but
> +			 * clear_user() not so much, that it is noticeably
> +			 * faster to copy the zero page instead of clearing.
> +			 */
> +			ret = copy_page_to_iter(ZERO_PAGE(0), offset, nr, to);

Is the offset and length guaranteed to be less than PAGE_SIZE here?

Either way I'd rather do this optimization in iov_iter_zero rather
than hiding it in tmpfs.