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Re: xfs: invalid requests to request_fn from xfs_repair

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: xfs: invalid requests to request_fn from xfs_repair
From: Jamie Pocas <pocas.jamie@xxxxxxxxx>
Date: Tue, 1 Apr 2014 18:28:19 -0400
Cc: xfs@xxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
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I have to say, thanks very much for the lightning fast response. Comments are inline. I think the punchline is going to end up being that I probably need to learn some more about the effects of O_DIRECT on the I/O path and how it ends up in the request_queue. It's tempting now to regress back to a bio-handling function, do my own merging, and see if I can avoid the problem, but I am stubborn so I will probably stick it out for as long as I can.

On Tue, Apr 1, 2014 at 4:42 PM, Dave Chinner <david@xxxxxxxxxxxxx> wrote:
On Tue, Apr 01, 2014 at 04:16:39PM -0400, Jamie Pocas wrote:
> blk_queue_physical_block_size(q, 512) // should be no surprise
> blk_queue_logical_block_size(q, 512) // should be no surprise

512 byte sectors.

> blk_queue_max_segments(q, 128); /* 128 memory segments (page +
> offset/length pairs) per request! */
> blk_queue_max_hw_sectors(q, CA_MAX_REQUEST_SECTORS); /* Up to 1024 sectors
> (512k) per request hard limit in the kernel */
> blk_queue_max_segment_size(q, CA_MAX_REQUEST_BYTES); /* 512k (1024 sectors)
> is the hard limit in the kernel */

And up to 512KB per IO.

> While iterating through segments in rq_for_each_segment(), for some
> requests I am seeing some odd behavior.
> segment 0: iter.bio->bi_sector = 0, blk_rq_cur_sectors(rq) = 903   // Ok,
> this looks normal
> segment 1: iter.bio->bi_sector = 1023, blk_rq_cur_sectors(rq) = 7 //
> Whoah... this doesn't look right to me

Seems fine to me. There's absolutely no reason two separate IOs
can't be sub-page sector aligned or discontiguous given the above
configuration. If that's what the getblocks callback returned to the
DIO layer, then that's what you're going to see in the bios...

I guess my assumption that sectors would be adjacent was invalid, but this is what I have read in ldd3 and other books and seen in some driver examples. So I apologize for my folly and it's another lesson in not believing everything you see on the net. The back half of my driver is really optimized for transferring a single contiguous lba & transfer length pair (ala SCSI WRITE 16 for example). However in desperation, I had changed the driver to support requests like this (with disjoint sector ranges) but it still seemed odd to me that occasionally some requests would even have multiple segments that would *write* the same sectors/lengths. For example I also see requests like the following.

Given: rq_data_dir(rq) == 1 (i.e. a write)
segment   0: iter.bio->bi_sector = 0, blk_rq_cur_sectors(rq) = 7    // Ok
segment   1: iter.bio->bi_sector = 0, blk_rq_cur_sectors(rq) = 7    // Again?
segment   2: iter.bio->bi_sector = 0, blk_rq_cur_sectors(rq) = 7    // Yet again?
segment   3: iter.bio->bi_sector = 0, blk_rq_cur_sectors(rq) = 7    // Really?
segment 126: iter.bio->bi_sector = 0, blk_rq_cur_sectors(rq) = 7    // What's going on?
// Reminder: segment limit is 128
segment 127: iter.bio->bi_sector = 1023, blk_rq_cur_sectors(rq) = 1 // Haha, bet you though I was going to say sector = 0!

This makes sense for reads, since maybe multiple user buffers wanted to read the same sectors at [around] the same time so it batched them in one request (temporal + spatial locality), but it doesn't make a lot of sense to me for writes. That seems inefficient at best. Wouldn't the last write win?

Even still, I occasionally see requests with more segments than the max (perhaps I am counting segments wrong too). This was commented on by Jens Axboe years ago here, but still seems to happen on my Ubuntu 12.04 w/ 3.2 kernel occasionally: https://lkml.org/lkml/2009/2/4/465

> You can see with segment 1, that the start sector is *NOT*
> adjacent to the the previous segment's sectors (there's a gap from
> sector 903 through 1022) and that the "sparse" request, for lack
> of a better term, extends beyond the max I/O boundary of 512k.
> Furthermore, this doesn't seem to jibe with what userspace is
> doing, which is a simple 512k read all in one chunk with a single
> userspace address.

The read syscall is for a byte offset (from the fd, set by lseek)
and a length, not a range of contiguous sectors on the device. That
off/len tuple gets mapped by the underlying filesystem or device
into an sector/len via a getblocks callback in the dio code and the
bios are then built according to the mappings that are returned. So
in many cases the IO that hits the block device looks nothing at all
like the IO that came from userspace.

In general if I were reading a file I agree 100%, but this was the read call issued by xfs_repair to read the superblock directly from a block device. So I, perhaps wrongly, anticipated straightforward requests or maybe even a single 512k request coming down to the request_fn. I should have mentioned that this was observed while using xfs_repair earlier in the email though, so no offense intended. There's more of the xfs_repair strace in my response to your next comment below. It fails very early as you can imagine when xfs_repair can't read the superblock.

> But when you look at the strace of what xfs_repair is doing, it's just an
> innocuous read of 512k from sector 0.
> write(2, "Phase 1 - find and verify superb"..., 40Phase 1 - find and verify
> superblock...
> ) = 40
> = 0x7f00e2f42000
> = 0x7f00e2ec1000
> lseek(4, 0, SEEK_SET)                   = 0
> read(4, 0x7f00e2ec1200, 524288)         = -1 EIO (Input/output error)
> write(2, "superblock read failed, offset 0"..., 61superblock read failed,
> offset 0, size 524288, ag 0, rval -1
> ) = 61

This is mostly meaningless without the command line you used for
xfs_repair and the trace from the open() syscall parameters that
returned fd 4 because we have no idea what the IO context actually

Yes, sorry about that I was trying to not bloat the thread with all the localizations loading noise. I'm going to try to strip some of those out and post more of the strace below. I *do* see the O_DIRECT flag as expected. Here's the block device being opened. All I did was "xfs_repair -n /dev/tsca0", hence the O_RDONLY flag.

open("/dev/tsca0", O_RDONLY|O_DIRECT)   = 4
fstat(4, {st_mode=S_IFBLK|0660, st_rdev=makedev(251, 0), ...}) = 0
fstat(4, {st_mode=S_IFBLK|0660, st_rdev=makedev(251, 0), ...}) = 0
ioctl(4, BLKGETSIZE64, 0x7fff8f759270)  = 0
ioctl(4, BLKSSZGET, 0x6732e0)           = 0
chdir("/home/jpocas/source/xfs/drivers") = 0
brk(0x23f9000)                          = 0x23f9000
close(3)                                = 0
getrlimit(RLIMIT_FSIZE, {rlim_cur=RLIM_INFINITY, rlim_max=RLIM_INFINITY}) = 0
open("/usr/share/locale/locale.alias", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=2570, ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f00e2fc2000
read(3, "# Locale name alias data base.\n#"..., 4096) = 2570
read(3, "", 4096)                       = 0
close(3)                                = 0
munmap(0x7f00e2fc2000, 4096)            = 0
write(2, "Phase 1 - find and verify superb"..., 40Phase 1 - find and verify superblock...
) = 40
mmap(NULL, 528384, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f00e2f42000
mmap(NULL, 528384, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f00e2ec1000
lseek(4, 0, SEEK_SET)                   = 0
read(4, 0x7f00e2ec1200, 524288)         = -1 EIO (Input/output error)
write(2, "superblock read failed, offset 0"..., 61superblock read failed, offset 0, size 524288, ag 0, rval -1
) = 61
write(2, "\nfatal error -- ", 16
fatal error -- )       = 16
write(2, "Input/output error\n", 19Input/output error
)    = 19
exit_group(1)                           = ?

> The reason you see the EIO is because I am failing a request in the driver
> since it violates the restrictions I set earlier, is non-adjacent, and so I
> am unable to satisfy it.
> *Point 1:* Shouldn't requests contain all segments that are adjacent on
> disk

Not necessarily, see above.

Got it. I need to look at the DIO code path in the kernel.
> *Point 2:* If I ignore the incorrect iter.bio->bi_sector, and just
> read/write the request out as if it were adjacent, I xfs_repair reports
> corruption,

Of course, because you read data from different sectors than was
asked for by the higher layers.

Sorry, this was just out of desperation and another data point, ridiculous as it may sound. I have been looking at this for a couple weeks now so I am taking wild shots in the dark.
> and sure enough there are inodes which are zeroed out instead
> of having the inode magic 0x494e ( "IN") as expected. So mkfs.xfs, while
> not sending what appear to be illegal requests, is still resulting in
> corruption.
> *Point 3:* Interestingly this goes away when I set
> blk_queue_max_segments(q, 1), but this obviously cuts down on clustering,
> and this of course kills performance. Is this indicative of anything in
> particular that I could be doing wrong?

Probably does, but I can't tell you what it may be...

> Please cut me some slack when I say something like xfs_repair is "sending"
> invalid requests. I know that there is the C library, system call
> interface, block layer, etc.. in between, but I just mean to say simply
> that using this tool results in this unexpected behavior. I don't mean to
> point blame at xfs or xfsprogs. If this turns out to be a block layer
> issue, and this posting needs to be sent elsewhere, I apologize and would
> appreciate being pointed in the right direction.
> It almost feels like the block layer is splitting the bios up wrongly, is
> corrupting the bvecs, or is introducing a race. What's strange again, is
> that I have only seen this behavior with xfs tools, but not ext3, or ext4
> and e2fsprogs which has been working for years. It really shouldn't matter

Because the XFS tools use direct IO, and the ext tools don't.
Therefore the IO that the different tools pass through are completely
different code paths in the kernel that have different constraints.
e.g. buffered IO will always be page aligned, direct IO can be
sector aligned....

That makes sense and I do see that O_DIRECT is being used by both mkfs.xfs and xfs_repair. I guess I am off to investigate the O_DIRECT code path.
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