linux-kernel - Re: open(2) says O_DIRECT works on 512 byte boundries?

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Message-ID: <20090206183833.GU14011@random.random>
Date:	Fri, 6 Feb 2009 19:38:33 +0100
From:	Andrea Arcangeli <aarcange@...hat.com>
To:	Greg KH <greg@...ah.com>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	mtk.manpages@...il.com, linux-man@...r.kernel.org,
	linux-kernel@...r.kernel.org, Nick Piggin <npiggin@...e.de>,
	Izik Eidus <ieidus@...hat.com>, Hugh Dickins <hugh@...itas.com>
Subject: Re: open(2) says O_DIRECT works on 512 byte boundries?

On Fri, Feb 06, 2009 at 06:54:14PM +0100, Andrea Arcangeli wrote:
> +	if (is_cow_mapping(vm_flags)) {
> +		if (pte_write(pte)) {
> +			ptep_set_wrprotect(src_mm, addr, src_pte);
> +			pte = pte_wrprotect(pte);
> +		}

While working on the mainline version that will definitely require a
tlb flush here if forcecow/PageGUP is set, I just realized to provide
an atomic per-page copy in the fork_pre_cow an explicit tlb flush is
needed in the pre-gup-fast version too.

	if (is_cow_mapping(vm_flags)) {
		if (pte_write(pte)) {
			ptep_set_wrprotect(src_mm, addr, src_pte);
			pte = pte_wrprotect(pte);
			if (forcecow)
				flush_tlb_page(src_vma, addr);
		}
	}

However to flush the 'src_mm' I feel I can't pass the 'dst_vma' that
fork is passing to copy_page_range. OTOH the dst_vma is needed to be
passed to the fork_pre_cow which is why fork.c was changed in the
patch to pass dst_vma instead of src_vma.

So I think I want to avoid all further confusion if the 'vma' belongs
to the src_mm or the dst_mm by passing both src_vma, and dst_vma from
fork to copy_page_tables. In the pre-gup-fast version the tlb flush is
mostly a nitpick and it would never lead to any practical issue, but
for mostly theoretical reasons it may be good idea to have the
per-page atomic copy there too, comments?
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