linux-ext4 - Re: [PATCH] mm: Fix warning in insert

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Message-ID: <20181003163557.GA18434@thunk.org>
Date:   Wed, 3 Oct 2018 12:35:58 -0400
From:   "Theodore Y. Ts'o" <tytso@....edu>
To:     Jan Kara <jack@...e.cz>
Cc:     linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Dan Williams <dan.j.williams@...el.com>, linux-mm@...ck.org,
        Dave Jiang <dave.jiang@...el.com>
Subject: Re: [PATCH] mm: Fix warning in insert_pfn()

On Fri, Aug 24, 2018 at 05:45:42PM +0200, Jan Kara wrote:
> In DAX mode a write pagefault can race with write(2) in the following
> way:
> 
> CPU0                            CPU1
>                                 write fault for mapped zero page (hole)
> dax_iomap_rw()
>   iomap_apply()
>     xfs_file_iomap_begin()
>       - allocates blocks
>     dax_iomap_actor()
>       invalidate_inode_pages2_range()
>         - invalidates radix tree entries in given range
>                                 dax_iomap_pte_fault()
>                                   grab_mapping_entry()
>                                     - no entry found, creates empty
>                                   ...
>                                   xfs_file_iomap_begin()
>                                     - finds already allocated block
>                                   ...
>                                   vmf_insert_mixed_mkwrite()
>                                     - WARNs and does nothing because there
>                                       is still zero page mapped in PTE
>         unmap_mapping_pages()
> 
> This race results in WARN_ON from insert_pfn() and is occasionally
> triggered by fstest generic/344. Note that the race is otherwise
> harmless as before write(2) on CPU0 is finished, we will invalidate page
> tables properly and thus user of mmap will see modified data from
> write(2) from that point on. So just restrict the warning only to the
> case when the PFN in PTE is not zero page.
> 
> Signed-off-by: Jan Kara <jack@...e.cz>

I don't see this in linux-next.  What's the status of this patch?

Thanks,

					- Ted