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Re: [dm-devel] [BUG] pvmove corrupting XFS filesystems (was Re: [BUG] In

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [dm-devel] [BUG] pvmove corrupting XFS filesystems (was Re: [BUG] Internal error xfs_dir2_data_reada_verify)
From: Matteo Frigo <athena@xxxxxxxx>
Date: Thu, 07 Mar 2013 07:13:27 -0500
Cc: dm-devel@xxxxxxxxxx, xfs@xxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <87y5eah4xz.fsf@xxxxxxxx> (Matteo Frigo's message of "Tue, 26 Feb 2013 21:21:44 -0500")
References: <87d2vnc34r.fsf@xxxxxxxx> <20130226044039.GM5551@dastard> <20130227010414.GD1514@xxxxxxxxxxxxxxxxxx> <20130227014900.GY5551@dastard> <87y5eah4xz.fsf@xxxxxxxx>
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.1 (gnu/linux)
Matteo Frigo <athena@xxxxxxxx> writes:

> Well, I don't really know anything about this topic, so I may be
> completely off the mark, but dm-raid1.c:mirror_map() does indeed return
> EWOULDBLOCK, and EWOULDBLOCK is #define'd to be EAGAIN, so it seems to
> me that dm-raid1 does indeed return EAGAIN for "rw == READA" (which I
> assume is read-ahead) if the "region is not in-sync":
>       /*
>        * If region is not in-sync queue the bio.
>        */
>       if (!r || (r == -EWOULDBLOCK)) {
>               if (rw == READA)
>                       return -EWOULDBLOCK;
>               queue_bio(ms, bio, rw);
>               return DM_MAPIO_SUBMITTED;
>       }

Dave (and others),

do you have any suggestion on what should be done to fix this bug?

I have tried returning -EIO instead of -EWOULDBLOCK, but xfs does not
like that.  dm-zero.c:zero_map() appears to return -EIO too, so this is
another potential issue.

I have verified that removing the READA special case, treating READA
like READ, fixes the problem:

                if(0) if (rw == READA)
                        return -EWOULDBLOCK;

Of course this "fix" throws away the baby with the bath water.

I am willing to write and submit a patch, but I would appreciate
directions as to what the correct protocol between xfs and dm is
supposed to be.


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