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

[Dan Williams]

On Wed, Dec 20, 2017 at 6:38 AM, Jan Kara <jack@xxxxxxx> wrote:
> 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)

Ah, yes. Now that I change ->mapping to be non-NULL for DAX pages I
enable all the address_space_operations to start firing. However,
instead of adding DAX specific address_space_operations it appears
->mapping should never be set for DAX pages, because DAX pages are
disconnected from the page-writeback machinery. In other words never
setting ->mapping bypasses all the possible broken assumptions and
code paths that take page-cache specific actions before calling an
address_space_operation.

>
>>  [..]
>>  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