• Vivek Goyal's avatar
    dax: dax_layout_busy_page() should not unmap cow pages · d75996dd
    Vivek Goyal authored
    Vivek:
    
        "As of now dax_layout_busy_page() calls unmap_mapping_range() with last
         argument as 1, which says even unmap cow pages. I am wondering who needs
         to get rid of cow pages as well.
    
         I noticed one interesting side affect of this. I mount xfs with -o dax and
         mmaped a file with MAP_PRIVATE and wrote some data to a page which created
         cow page. Then I called fallocate() on that file to zero a page of file.
         fallocate() called dax_layout_busy_page() which unmapped cow pages as well
         and then I tried to read back the data I wrote and what I get is old
         data from persistent memory. I lost the data I had written. This
         read basically resulted in new fault and read back the data from
         persistent memory.
    
         This sounds wrong. Are there any users which need to unmap cow pages
         as well? If not, I am proposing changing it to not unmap cow pages.
    
         I noticed this while while writing virtio_fs code where when I tried
         to reclaim a memory range and that corrupted the executable and I
         was running from virtio-fs and program got segment violation."
    
    Dan:
    
        "In fact the unmap_mapping_range() in this path is only to synchronize
         against get_user_pages_fast() and force it to call back into the
         filesystem to re-establish the mapping. COW pages should be left
         untouched by dax_layout_busy_page()."
    
    Cc: <stable@vger.kernel.org>
    Fixes: 5fac7408 ("mm, fs, dax: handle layout changes to pinned dax mappings")
    Signed-off-by: default avatarVivek Goyal <vgoyal@redhat.com>
    Link: https://lore.kernel.org/r/20190802192956.GA3032@redhat.comSigned-off-by: default avatarDan Williams <dan.j.williams@intel.com>
    d75996dd
dax.c 45.8 KB