linux-kernel - Re: [PATCH v1 5/7] dax: Add huge page fault support

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Message-ID: <20141009204716.GQ5098@wil.cx>
Date:	Thu, 9 Oct 2014 16:47:16 -0400
From:	Matthew Wilcox <willy@...ux.intel.com>
To:	"Kirill A. Shutemov" <kirill@...temov.name>
Cc:	Matthew Wilcox <matthew.r.wilcox@...el.com>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, Matthew Wilcox <willy@...ux.intel.com>
Subject: Re: [PATCH v1 5/7] dax: Add huge page fault support

On Wed, Oct 08, 2014 at 11:11:00PM +0300, Kirill A. Shutemov wrote:
> On Wed, Oct 08, 2014 at 09:25:27AM -0400, Matthew Wilcox wrote:
> > +	pgoff = ((address - vma->vm_start) >> PAGE_SHIFT) + vma->vm_pgoff;
> > +	size = (i_size_read(inode) + PAGE_SIZE - 1) >> PAGE_SHIFT;
> > +	if (pgoff >= size)
> > +		return VM_FAULT_SIGBUS;
> > +	/* If the PMD would cover blocks out of the file */
> > +	if ((pgoff | PG_PMD_COLOUR) >= size)
> > +		return VM_FAULT_FALLBACK;
> 
> IIUC, zero pading would work too.

The blocks after this file might be allocated to another file already.
I suppose we could ask the filesystem if it wants to allocate them to
this file.

Dave, Jan, is it acceptable to call get_block() for blocks that extend
beyond the current i_size?

> > +
> > +	memset(&bh, 0, sizeof(bh));
> > +	block = ((sector_t)pgoff & ~PG_PMD_COLOUR) << (PAGE_SHIFT - blkbits);
> > +
> > +	/* Start by seeing if we already have an allocated block */
> > +	bh.b_size = PMD_SIZE;
> > +	length = get_block(inode, block, &bh, 0);
> 
> This makes me confused. get_block() return zero on success, right?
> Why the var called 'lenght'?

Historical reasons.  I can go back and change the name of the variable.

> > +	sector = bh.b_blocknr << (blkbits - 9);
> > +	length = bdev_direct_access(bh.b_bdev, sector, &kaddr, &pfn, bh.b_size);
> > +	if (length < 0)
> > +		goto sigbus;
> > +	if (length < PMD_SIZE)
> > +		goto fallback;
> > +	if (pfn & PG_PMD_COLOUR)
> > +		goto fallback;	/* not aligned */
> 
> So, are you rely on pure luck to make get_block() allocate 2M aligned pfn?
> Not really productive. You would need assistance from fs and
> arch_get_unmapped_area() sides.

Certainly ext4 and XFS will align their allocations; if you ask it for a
2MB block, it will try to allocate a 2MB block aligned on a 2MB boundary.

I started looking into the get_unampped_area (and have the code sitting
around to align specially marked files on special boundaries), but when
I mentioned it to the author of the NVM Library, he said "Oh, I'll just
pick a 1GB aligned area to request it be mapped at", so I haven't taken
it any further.

The upshot is that (confirmed with debugging code), when the tests run,
they pretty much always get a correctly aligned block.
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