On Tue, Dec 19, 2017 at 05:11:38PM -0800, 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.
That means every single filesystem has an incorrect assumption for
DAX pages. xfs_vm_set_page_dirty() is derived directly from
__set_page_dirty_buffers(), which is the default function that
set_page_dirty() calls to do it's work. Indeed, ext4 also calls
__set_page_dirty_buffers(), so whatever problem XFS has here with
DAX and racing truncates is going to manifest in ext4 as well.
> 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;
> }
This is likely to be a problem in lots more places if we have to
treat "has page been truncated away" race checks on dax mappings
differently to page cache mappings. This smells of a whack-a-mole
style bandaid to me....
Cheers,
Dave.
--
Dave Chinner
david@xxxxxxxxxxxxx
