linux-kernel - Re: [PATCH -v8 3/4] Enable the MS_ASYNC functionality in sys

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Message-Id: <E1JHc0S-00027S-8D@pomaz-ex.szeredi.hu>
Date:	Wed, 23 Jan 2008 10:34:52 +0100
From:	Miklos Szeredi <miklos@...redi.hu>
To:	a.p.zijlstra@...llo.nl
CC:	salikhmetov@...il.com, linux-mm@...ck.org, jakob@...hought.net,
	linux-kernel@...r.kernel.org, valdis.kletnieks@...edu,
	riel@...hat.com, ksm@...dk, staubach@...hat.com,
	jesper.juhl@...il.com, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, protasnb@...il.com, miklos@...redi.hu,
	r.e.wolff@...wizard.nl, hidave.darkstar@...il.com,
	hch@...radead.org
Subject: Re: [PATCH -v8 3/4] Enable the MS_ASYNC functionality in
	sys_msync()

> On Wed, 2008-01-23 at 09:47 +0100, Peter Zijlstra wrote:
> > On Wed, 2008-01-23 at 02:21 +0300, Anton Salikhmetov wrote:
> 
> > > +static void vma_wrprotect(struct vm_area_struct *vma)
> > > +{
> > > +	unsigned long addr = vma->vm_start;
> > > +	pgd_t *pgd = pgd_offset(vma->vm_mm, addr);
> > > +
> > > +	while (addr < vma->vm_end) {
> > > +		unsigned long next = pgd_addr_end(addr, vma->vm_end);
> > > +
> > > +		if (!pgd_none_or_clear_bad(pgd))
> > > +			vma_wrprotect_pgd_range(vma, pgd, addr, next);
> > > +
> > > +		++pgd;
> > > +		addr = next;
> > > +	}
> > > +}
> > 
> > I think you want to pass start, end here too, you might not need to
> > sweep the whole vma.
> 
> Also, it still doesn't make sense to me why we'd not need to walk the
> rmap, it is all the same file after all.

It's the same file, but not the same memory map.  It basically depends
on how you define msync:

 a) sync _file_ on region defined by this mmap/start/end-address
 b) sync _memory_region_ defined by start/end-address

b) is a perfectly fine definition, and it's consistent with what this
code does.  The fact that POSIX probably implies a) (in a rather
poorly defined way) doesn't make much difference, I think.

Miklos
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