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

Matthew Wilcox
2014-10-09T20:47:33Z

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.