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    SubjectRe: [PATCH 14/15] dax: associate mappings with inodes, and warn if dma collides with truncate
    On Wed, Dec 20, 2017 at 6:38 AM, Jan Kara <jack@suse.cz> wrote:
    > On Tue 19-12-17 17:11:38, Dan Williams wrote:
    >> 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]
    >
    > 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@suse.com>
    > SUSE Labs, CR

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