linux-kernel - Re: [PATCH 2/2] block: use DAX for partition table reads

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Date:	Fri, 29 Jan 2016 09:54:28 -0800
From:	Dan Williams <dan.j.williams@...el.com>
To:	Jens Axboe <axboe@...com>
Cc:	linux-block@...r.kernel.org,
	"linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
	Dave Chinner <david@...morbit.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Christoph Hellwig <hch@....de>, Jeff Moyer <jmoyer@...hat.com>,
	Jan Kara <jack@...e.com>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Matthew Wilcox <willy@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ross Zwisler <ross.zwisler@...ux.intel.com>
Subject: Re: [PATCH 2/2] block: use DAX for partition table reads

On Fri, Jan 29, 2016 at 9:46 AM, Jens Axboe <axboe@...com> wrote:
> On 01/29/2016 08:18 AM, Dan Williams wrote:
>>
>> +unsigned char *read_dev_sector(struct block_device *bdev, sector_t n,
>> Sector *p)
>> +{
>>         struct page *page;
>>
>> -       page = read_mapping_page(mapping, (pgoff_t)(n >>
>> (PAGE_CACHE_SHIFT-9)),
>> -                                NULL);
>> +       /* don't populate page cache for dax capable devices */
>> +       if (IS_DAX(bdev->bd_inode))
>> +               page = read_dax_sector(bdev, n);
>> +       else
>> +               page = read_pagecache_sector(bdev, n);
>> +
>
>
> Fall back to non-dax, if dax fails?

I think we need to fail hard otherwise we're back to the original
problem of confusing the dax code that expects to find an empty page
cache.

>
>> +struct page *read_dax_sector(struct block_device *bdev, sector_t n)
>> +{
>> +       struct page *page = __page_cache_alloc(GFP_KERNEL | __GFP_COLD);
>
>
> Why isn't that just alloc_pages()?

Just for symmetry with the same allocation that the pagecache path
makes, but alloc_pages() works too...