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SubjectRe: [PATCH 14/15] dax: associate mappings with inodes, and warn if dma collides with truncate
On Fri, Nov 10, 2017 at 1:08 AM, Christoph Hellwig <hch@lst.de> 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]
[..]
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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