RE: [PATCH v8 05/22] Add vm_replace_mixed()

["Zhang, Tianfei" <tianfei.zhang@intel.com>]

> -----Original Message-----
> From: owner-linux-mm@kvack.org [mailto:owner-linux-mm@kvack.org] On
> Behalf Of Kirill A. Shutemov
> Sent: Monday, July 28, 2014 9:26 PM
> To: Matthew Wilcox
> Cc: Wilcox, Matthew R; linux-fsdevel@vger.kernel.org; linux-mm@kvack.org;
> linux-kernel@vger.kernel.org
> Subject: Re: [PATCH v8 05/22] Add vm_replace_mixed()
> 
> On Fri, Jul 25, 2014 at 03:44:50PM -0400, Matthew Wilcox wrote:
> > On Wed, Jul 23, 2014 at 06:55:00PM +0300, Kirill A. Shutemov wrote:
> > > >         update_hiwater_rss(mm);
> > >
> > > No: you cannot end up with lower rss after replace, iiuc.
> >
> > Actually, you can ... when we replace a real page with a PFN, our rss
> > decreases.
> 
> Okay.
> 
> > > Do you mean you pointed to new file all the time? O_CREAT doesn't
> > > truncate file if it exists, iirc.
> >
> > It was pointing to a new file.  Still not sure why that one failed to
> > trigger the problem.  The slightly modified version attached triggered
> > the problem *just fine* :-)
> >
> > I've attached all the patches in my tree so far.  For the v9 patch
> > kit, I'll keep patch 3 as a separate patch, but roll patches 1, 2 and
> > 4 into other patches.
> >
> > I am seeing something odd though.  When I run double-map with
> > debugging printks inserted in strategic spots in the kernel, I see
> > four calls to do_dax_fault().  The first two, as expected, are the
> > loads from the two mapped addresses.  The third is via mkwrite, but
> > then the fourth time I get a regular page fault for write, and I don't
> understand why I get it.
> >
> > Any ideas?
> 
> unmap_mapping_range() clears pte you've just set by vm_replace_mixed() on
> third fault.
> 
> And locking looks wrong: it seems you need to hold i_mmap_mutex while
> replacing hole page with pfn. Your VM_BUG_ON() in zap_pte_single() triggers
> on my setup.
> 
> > +static void zap_pte_single(struct vm_area_struct *vma, pte_t *pte,
> > +				unsigned long addr)
> > +{
> > +	struct mm_struct *mm = vma->vm_mm;
> > +	int force_flush = 0;
> > +	int rss[NR_MM_COUNTERS];
> > +
> > +
> 	VM_BUG_ON(!mutex_is_locked(&vma->vm_file->f_mapping->i_mmap_m
> utex));
> 
> It's wrong place for VM_BUG_ON(): zap_pte_single() on anon mapping should
> work fine)

Hi Shutemov:
1. I am confuse that why insert_page() drop the PageAnon, insert_pfn() donot drop PageAnon?

2. 
remap_vmalloc_range() -> vm_insert_page()->insert_page() -> inc_mm_counter_fast(mm, MM_FILEPAGES);

so, in this scenario, this vmalloc page maybe not sure be a page cache, why increase the MM_FILEPAGES ?


> 
> > +
> > +	init_rss_vec(rss);
> 
> Vector to commit single update to mm counters? What about inline counters
> update for rss == NULL case?
> 
> > +	update_hiwater_rss(mm);
> > +	flush_cache_page(vma, addr, pte_pfn(*pte));
> > +	zap_pte(NULL, vma, pte, addr, NULL, rss, &force_flush);
> > +	flush_tlb_page(vma, addr);
> > +	add_mm_rss_vec(mm, rss);
> > +}
> > +
> 
> --
>  Kirill A. Shutemov
> 
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