Subject: Re: [PATCH -v6 2/2] Updating ctime and mtime for memory-mapped
	files
From: Peter Zijlstra <peterz@infradead.org>
To: Miklos Szeredi <miklos@szeredi.hu>
Cc: salikhmetov@gmail.com, linux-mm@kvack.org, jakob@unthought.net,
       linux-kernel@vger.kernel.org, valdis.kletnieks@vt.edu, riel@redhat.com,
       ksm@42.dk, staubach@redhat.com, jesper.juhl@gmail.com,
       torvalds@linux-foundation.org, akpm@linux-foundation.org,
       protasnb@gmail.com, r.e.wolff@bitwizard.nl, hidave.darkstar@gmail.com,
       hch@infradead.org
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	 <E1JFnsg-0008UU-LU@pomaz-ex.szeredi.hu>  <1200651337.5920.9.camel@twins>
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Date: Fri, 18 Jan 2008 11:25:58 +0100
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On Fri, 2008-01-18 at 11:15 +0100, Peter Zijlstra wrote:
> On Fri, 2008-01-18 at 10:51 +0100, Miklos Szeredi wrote:
> 
> > > diff --git a/mm/msync.c b/mm/msync.c
> > > index a4de868..a49af28 100644
> > > --- a/mm/msync.c
> > > +++ b/mm/msync.c
> > > @@ -13,11 +13,33 @@
> > >  #include <linux/syscalls.h>
> > >  
> > >  /*
> > > + * Scan the PTEs for pages belonging to the VMA and mark them read-only.
> > > + * It will force a pagefault on the next write access.
> > > + */
> > > +static void vma_wrprotect(struct vm_area_struct *vma)
> > > +{
> > > +	unsigned long addr;
> > > +
> > > +	for (addr = vma->vm_start; addr < vma->vm_end; addr += PAGE_SIZE) {
> > > +		spinlock_t *ptl;
> > > +		pgd_t *pgd = pgd_offset(vma->vm_mm, addr);
> > > +		pud_t *pud = pud_offset(pgd, addr);
> > > +		pmd_t *pmd = pmd_offset(pud, addr);
> > > +		pte_t *pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
> > > +
> > > +		if (pte_dirty(*pte) && pte_write(*pte))
> > > +			*pte = pte_wrprotect(*pte);
> > > +		pte_unmap_unlock(pte, ptl);
> > > +	}
> > > +}
> > 
> > What about ram based filesystems?  They don't start out with read-only
> > pte's, so I think they don't want them read-protected now either.
> > Unless this is essential for correct mtime/ctime accounting on these
> > filesystems (I don't think it really is).  But then the mapping should
> > start out read-only as well, otherwise the time update will only work
> > after an msync(MS_ASYNC).
> 
> page_mkclean() has all the needed logic for this, it also walks the rmap
> and cleans out all other users, which I think is needed too for
> consistencies sake:
> 
> Process A			Process B
> 
> mmap(foo.txt)			mmap(foo.txt)
> 
> dirty page
> 				dirty page
> 
> msync(MS_ASYNC)
> 
> 				dirty page
> 
> msync(MS_ASYNC) <--- now what?!
> 
> 
> So what I would suggest is using the page table walkers from mm, and
> walks the page range, obtain the page using vm_normal_page() and call
> page_mkclean(). (Oh, and ensure you don't nest the pte lock :-)
> 
> All in all, that sounds rather expensive..

Bah, and will break on s390... so we'd need a page_mkclean() variant
that doesn't actually clear dirty.

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