Re: [PATCH 14/15] dax: associate mappings with inodes, and warn if dma collides with truncate

[Jan Kara]

On Tue 19-12-17 17:11:38, Dan Williams wrote:
> On Fri, Nov 10, 2017 at 1:08 AM, Christoph Hellwig <hch@xxxxxx> wrote:
> >> +             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]

But this just shows that mapping->a_ops are wrong for this mapping, doesn't
it? ->set_page_dirty handler for DAX mapping should just properly handle
DAX pages... (and only those)

>  [..]
>  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

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR