On Fri, Nov 10, 2017 at 1:08 AM, Christoph Hellwig [off-list ref] wrote:

quoted

+             struct {
+                     /*
+                      * ZONE_DEVICE pages are never on an lru or handled by
+                      * a slab allocator, this points to the hosting device
+                      * page map.
+                      */
+                     struct dev_pagemap *pgmap;
+                     /*
+                      * inode association for MEMORY_DEVICE_FS_DAX page-idle
+                      * callbacks. Note that we don't use ->mapping since
+                      * that has hard coded page-cache assumptions in
+                      * several paths.
+                      */

What assumptions?  I'd much rather fix those up than having two fields
that have the same functionality.

[ Reviving this old thread where you asked why I introduce page->inode
instead of reusing page->mapping ]

For example, xfs_vm_set_page_dirty() assumes that page->mapping being
non-NULL indicates a typical page cache page, this is a false
assumption for DAX. My guess at a fix for this is to add
pagecache_page() checks to locations like this, but I worry about how
to find them all. Where pagecache_page() is:

bool pagecache_page(struct page *page)
{
        if (!page->mapping)
                return false;
        if (!IS_DAX(page->mapping->host))
                return false;
        return true;
}

Otherwise we go off the rails:

 WARNING: CPU: 27 PID: 1783 at fs/xfs/xfs_aops.c:1468
xfs_vm_set_page_dirty+0xf3/0x1b0 [xfs]
 [..]
 CPU: 27 PID: 1783 Comm: dma-collision Tainted: G           O
4.15.0-rc2+ #984
 [..]
 Call Trace:
  set_page_dirty_lock+0x40/0x60
  bio_set_pages_dirty+0x37/0x50
  iomap_dio_actor+0x2b7/0x3b0
  ? iomap_dio_zero+0x110/0x110
  iomap_apply+0xa4/0x110
  iomap_dio_rw+0x29e/0x3b0
  ? iomap_dio_zero+0x110/0x110
  ? xfs_file_dio_aio_read+0x7c/0x1a0 [xfs]
  xfs_file_dio_aio_read+0x7c/0x1a0 [xfs]
  xfs_file_read_iter+0xa0/0xc0 [xfs]
  __vfs_read+0xf9/0x170
  vfs_read+0xa6/0x150
  SyS_pread64+0x93/0xb0
  entry_SYSCALL_64_fastpath+0x1f/0x96

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