linux-kernel - Re: [PATCH v3 04/11] ext2: remove support for DAX PMD faults

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Date:   Wed, 28 Sep 2016 07:47:20 +1000
From:   Dave Chinner <david@...morbit.com>
To:     Ross Zwisler <ross.zwisler@...ux.intel.com>
Cc:     linux-kernel@...r.kernel.org, Theodore Ts'o <tytso@....edu>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Hellwig <hch@....de>,
        Dan Williams <dan.j.williams@...el.com>,
        Jan Kara <jack@...e.com>,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-nvdimm@...ts.01.org,
        linux-xfs@...r.kernel.org
Subject: Re: [PATCH v3 04/11] ext2: remove support for DAX PMD faults

On Tue, Sep 27, 2016 at 02:47:55PM -0600, Ross Zwisler wrote:
> DAX PMD support was added via the following commit:
> 
> commit e7b1ea2ad658 ("ext2: huge page fault support")
> 
> I believe this path to be untested as ext2 doesn't reliably provide block
> allocations that are aligned to 2MiB.  In my testing I've been unable to
> get ext2 to actually fault in a PMD.  It always fails with a "pfn
> unaligned" message because the sector returned by ext2_get_block() isn't
> aligned.
> 
> I've tried various settings for the "stride" and "stripe_width" extended
> options to mkfs.ext2, without any luck.
> 
> Since we can't reliably get PMDs, remove support so that we don't have an
> untested code path that we may someday traverse when we happen to get an
> aligned block allocation.  This should also make 4k DAX faults in ext2 a
> bit faster since they will no longer have to call the PMD fault handler
> only to get a response of VM_FAULT_FALLBACK.
> 
> Signed-off-by: Ross Zwisler <ross.zwisler@...ux.intel.com>
....
> @@ -154,7 +133,6 @@ static int ext2_dax_pfn_mkwrite(struct vm_area_struct *vma,
>  
>  static const struct vm_operations_struct ext2_dax_vm_ops = {
>  	.fault		= ext2_dax_fault,
> -	.pmd_fault	= ext2_dax_pmd_fault,
>  	.page_mkwrite	= ext2_dax_fault,
>  	.pfn_mkwrite	= ext2_dax_pfn_mkwrite,
>  };

Would it be better to put a comment mentioning this here? So as the
years go by, this reminds people not to bother trying to implement
it?

/*
 * .pmd_fault is not supported for DAX because allocation in ext2
 * cannot be reliably aligned to huge page sizes and so pmd faults
 * will always fail and fail back to regular faults.
 */

-- 
Dave Chinner
david@...morbit.com