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

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Date:   Fri, 8 Apr 2022 23:08:29 -0700 (PDT)
From:   Hugh Dickins <hughd@...gle.com>
To:     Christoph Hellwig <hch@....de>
cc:     Hugh Dickins <hughd@...gle.com>,
        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>,
        "Darrick J. Wong" <djwong@...nel.org>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        Borislav Petkov <bp@...en8.de>, 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 Sat, 9 Apr 2022, Christoph Hellwig wrote:
> 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?

Almost :) The offset is guaranteed to be less than PAGE_SIZE here, and
the length is guaranteed to be less than or equal to PAGE_SIZE - offset.

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

Let's see what others say.  I think we would all prefer clear_user() to be
enhanced, and hack around it neither here in tmpfs nor in iov_iter_zero().
But that careful work won't get done by magic, nor by me.

And iov_iter_zero() has to deal with a wider range of possibilities,
when pulling in cache lines of ZERO_PAGE(0) will be less advantageous,
than in tmpfs doing a large dd - the case I'm aiming not to regress here
(tmpfs has been copying ZERO_PAGE(0) like this for years).

Hugh