linux-kernel - making x86 clear_user not suck, was Re: [PATCH] tmpfs: fix regressions from wider use of ZERO

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Date:   Tue, 12 Apr 2022 06:57:57 +0200
From:   Christoph Hellwig <hch@....de>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     Christoph Hellwig <hch@....de>,
        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,
        x86@...nel.org
Subject: making x86 clear_user not suck, was Re: [PATCH] tmpfs: fix
 regressions from wider use of ZERO_PAGE

On Fri, Apr 08, 2022 at 11:08:29PM -0700, Hugh Dickins wrote:
> > 
> > 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.

I agree with that.

> 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).

Maybe.  OTOH I'd hate to have iov_iter_zero not used much because it
sucks too much.

So how can we entice someone with the right knowledge to implement a
decent clear_user for x86?