Re: [RFC PATCH] Support map_pages() for DAX

[Toshi Kani <toshi.kani@hp.com>]

On Sat, 2014-03-15 at 01:32 +0200, Kirill A. Shutemov wrote:
> On Fri, Mar 14, 2014 at 05:03:19PM -0600, Toshi Kani wrote:
> > +void dax_map_pages(struct vm_area_struct *vma, struct vm_fault *vmf,
> > +		get_block_t get_block)
> > +{
> > +	struct file *file = vma->vm_file;
> > +	struct inode *inode = file_inode(file);
> > +	struct buffer_head bh;
> > +	struct address_space *mapping = file->f_mapping;
> > +	unsigned long vaddr = (unsigned long)vmf->virtual_address;
> > +	pgoff_t pgoff = vmf->pgoff;
> > +	sector_t block;
> > +	pgoff_t size;
> > +	unsigned long pfn;
> > +	pte_t *pte = vmf->pte;
> > +	int error;
> > +
> > +	while (pgoff < vmf->max_pgoff) {
> > +		size = (i_size_read(inode) + PAGE_SIZE - 1) >> PAGE_SHIFT;
> > +		if (pgoff >= size)
> > +			return;
> > +
> > +		memset(&bh, 0, sizeof(bh));
> > +		block = (sector_t)pgoff << (PAGE_SHIFT - inode->i_blkbits);
> > +		bh.b_size = PAGE_SIZE;
> > +		error = get_block(inode, block, &bh, 0);
> > +		if (error || bh.b_size < PAGE_SIZE)
> > +			goto next;
> > +
> > +		if (!buffer_mapped(&bh) || buffer_unwritten(&bh) ||
> > +		    buffer_new(&bh))
> > +			goto next;
> > +
> > +		/* Recheck i_size under i_mmap_mutex */
> > +		mutex_lock(&mapping->i_mmap_mutex);
> 
> NAK. Have you tested this with lockdep enabled?
>
> ->map_pages() called with page table lock taken and ->i_mmap_mutex
> should be taken before it. It seems we need to take ->i_mmap_mutex in
> do_read_fault() before calling ->map_pages().

Thanks for pointing this out! I will make sure to test with lockdep next
time.

> Side note: I'm sceptical about whole idea to use i_mmap_mutux to protect
> against truncate. It will not scale good enough comparing lock_page()
> with its granularity.

I see.  I will think about it as well.  

Thanks,
-Toshi



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