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

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Date:   Thu, 21 Dec 2017 09:14:47 +1100
From:   Dave Chinner <david@...morbit.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Christoph Hellwig <hch@....de>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        Jan Kara <jack@...e.cz>,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>, Jeff Moyer <jmoyer@...hat.com>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 14/15] dax: associate mappings with inodes, and warn if
 dma collides with truncate

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@....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.

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@...morbit.com