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Date: Thu, 21 Apr 2016 03:06:25 -0400
From: Matthew Wilcox <willy@linux.intel.com>
To: Toshi Kani <toshi.kani@hpe.com>
Cc: Jan Kara <jack@suse.cz>, Andrew Morton <akpm@linux-foundation.org>,
        dan.j.williams@intel.com, viro@zeniv.linux.org.uk,
        ross.zwisler@linux.intel.com, kirill.shutemov@linux.intel.com,
        david@fromorbit.com, tytso@mit.edu, adilger.kernel@dilger.ca,
        linux-nvdimm@ml01.01.org, linux-fsdevel@vger.kernel.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3 0/2] Align mmap address for DAX pmd mappings
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On Wed, Apr 20, 2016 at 11:10:25PM -0400, Toshi Kani wrote:
> How about moving the function (as is) to mm/huge_memory.c, rename it to
> get_hugepage_unmapped_area(), which is defined to NULL in huge_mm.h
> when TRANSPARENT_HUGEPAGE is unset?

Great idea.  Perhaps it should look something like this?

unsigned long thp_get_unmapped_area(struct file *filp, unsigned long addr,
                unsigned long len, unsigned long pgoff, unsigned long flags)
{
        loff_t off, off_end, off_pmd;
        unsigned long len_pmd, addr_pmd;

        if (addr)
                goto out;
        if (IS_DAX(filp->f_mapping->host) && !IS_ENABLED(CONFIG_FS_DAX_PMD))
                goto out;
        /* Kirill, please fill in the right condition here for THP pagecache */

        off = (loff_t)pgoff << PAGE_SHIFT;
        off_end = off + len;
        off_pmd = round_up(off, PMD_SIZE);      /* pmd-aligned start offset */

        if ((off_end <= off_pmd) || ((off_end - off_pmd) < PMD_SIZE))
                goto out;

        len_pmd = len + PMD_SIZE;
        if ((off + len_pmd) < off)
                goto out;

        addr_pmd = current->mm->get_unmapped_area(filp, NULL, len_pmd,
                                                pgoff, flags);
        if (!IS_ERR_VALUE(addr_pmd)) {
                addr_pmd += (off - addr_pmd) & (PMD_SIZE - 1);
                return addr_pmd;
        }
 out:
        return current->mm->get_unmapped_area(filp, addr, len, pgoff, flags);
}

 - I deleted the check for filp == NULL.  It can't be NULL ... this is a
   file_operation ;-)
 - Why is len_pmd len + PMD_SIZE instead of round_up(len, PMD_SIZE)?
 - I'm still in two minds about passing 'addr' to the first call to
   get_unmapped_area() instead of NULL.