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

Kirill A. Shutemov
2014-10-08T20:11:15Z

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.

> +
> + 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'?

> + 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.