linux-kernel - Re: [PATCH v2 5/5] dax: handle media errors in dax_do

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Message-ID: <CAPcyv4ijf=LDLytVRn_UptVVi5G=7r9bkTkkJHYgBF78tffh9w@mail.gmail.com>
Date:	Mon, 25 Apr 2016 10:21:44 -0700
From:	Dan Williams <dan.j.williams@...el.com>
To:	"Verma, Vishal L" <vishal.l.verma@...el.com>
Cc:	"hch@...radead.org" <hch@...radead.org>,
	"axboe@...com" <axboe@...com>, "jack@...e.cz" <jack@...e.cz>,
	"linux-nvdimm@...1.01.org" <linux-nvdimm@...1.01.org>,
	"david@...morbit.com" <david@...morbit.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"xfs@....sgi.com" <xfs@....sgi.com>,
	"linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"Wilcox, Matthew R" <matthew.r.wilcox@...el.com>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	"viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>
Subject: Re: [PATCH v2 5/5] dax: handle media errors in dax_do_io

On Mon, Apr 25, 2016 at 10:14 AM, Verma, Vishal L
<vishal.l.verma@...el.com> wrote:
> On Mon, 2016-04-25 at 01:31 -0700, hch@...radead.org wrote:
>> On Sat, Apr 23, 2016 at 06:08:37PM +0000, Verma, Vishal L wrote:
>> >
>> > direct_IO might fail with -EINVAL due to misalignment, or -ENOMEM
>> > due
>> > to some allocation failing, and I thought we should return the
>> > original
>> > -EIO in such cases so that the application doesn't lose the
>> > information
>> > that the bad block is actually causing the error.
>> EINVAL is a concern here.  Not due to the right error reported, but
>> because it means your current scheme is fundamentally broken - we
>> need to support I/O at any alignment for DAX I/O, and not fail due to
>> alignbment concernes for a highly specific degraded case.
>>
>> I think this whole series need to go back to the drawing board as I
>> don't think it can actually rely on using direct I/O as the EIO
>> fallback.
>>
> Agreed that DAX I/O can happen with any size/alignment, but how else do
> we send an IO through the driver without alignment restrictions? Also,
> the granularity at which we store badblocks is 512B sectors, so it
> seems natural that to clear such a sector, you'd expect to send a write
> to the whole sector.
>
> The expected usage flow is:
>
> - Application hits EIO doing dax_IO or load/store io
>
> - It checks badblocks and discovers it's files have lost data
>
> - It write()s those sectors (possibly converted to file offsets using
> fiemap)
>     * This triggers the fallback path, but if the application is doing
> this level of recovery, it will know the sector is bad, and write the
> entire sector
>
> - Or it replaces the entire file from backup also using write() (not
> mmap+stores)
>     * This just frees the fs block, and the next time the block is
> reallocated by the fs, it will likely be zeroed first, and that will be
> done through the driver and will clear errors
>
>
> I think if we want to keep allowing arbitrary alignments for the
> dax_do_io path, we'd need:
> 1. To represent badblocks at a finer granularity (likely cache lines)
> 2. To allow the driver to do IO to a *block device* at sub-sector
> granularity

3. Arrange for O_DIRECT to bypass dax_do_io(), and leave the
optimization only for the dax "buffered I/O" case.

4. Skip dax_do_io() entirely in the presence of errors

I think 3 is the most closely aligned with the typical block device
model.  In the typical case a buffered write may fail due to a
badblock read when filling the page cache, but an O_DIRECT write would
bypass the page cache and potentially clear the error / cause the
block to be reallocated internally to the drive.