[Bugs] [Bug 1651433] Two data files in a small window during file migration and resultant corruption

bugzilla at redhat.com bugzilla at redhat.com
Tue Nov 20 05:02:29 UTC 2018


--- Comment #1 from Susant Kumar Palai <spalai at redhat.com> ---
Description of problem:

At the end of migration, following are the sequence of steps related to
conversion of src and dst file modes:

1. remove sticky and sgid bits from dst (this essentially converts dst to
2. set phase2 flags on src (this converts src to linkto file)
3. remove linkto xattr on dst (this essentially is a NOP as step 1 converts dst
to datafile).
4. unlink the src.

In the above scheme of things, in the time-window after 1 but before 2, both
src and dst are data files. This can cause some subtle data corruptions in read
path (though the window is very narrow).
1. Assume there is an fd opened by an application app1 - fd1 - while the file
was in migration (before step 1 in migration algo).
2. Assume another fd opened by another application app2 - fd2 - after 1, but
before 2. This fd would be opened on dst.
3. some writes are done on fd2. These writes end up on dst. The application
app2 does an fsync and on successful completion, notifies app1.
4. app1 does a read to the region written by app2. But if this read is done
before sticky bit on src is removed (step 2 in migration algo), the read is not
attempted on dst as dht_readv thinks file is still under phase 1 of migration.
This results in app1 not seeing data written by app2 (a stale read).

The above scenario is a very minute race corresponding to two co-ordinated
applications. But the steps expectations are POSIX complaint. Also, note that
if rebalance process crashes in this window, the windown becomes bigger (till
next invocation of rebalance). I think the root of the problem is that
dht_readv expects file to be in phase2 of migration before attempting read on
dst, but dst is converted to data file before we set phase2 flags on src. The
problem can be solved if we set phase2 flags on src before converting dst to
data file. However, this has a side-effect of having no data file in that
window as linkto xattrs are already set on src. This problem can be overcome if
we follow following steps:

1. remove linkto xattr from src (to ensure just setting phase2 flags on src
doesn't covert it into a linkto file).
2. set phase2 flags on src (src will remain a dst file even after this).
3. remove sticky and sgid bits from dst (this essentially converts dst to
4. set the linkto xattr on src to dst.
5. remove linkto xattr on dst (this essentially is a NOP as step 1 converts dst
to datafile).
6. unlink the src.

However, this has a problem that if rebalance process crashes after step 2 but
before step 3. All the reads will be from dst though dst is not a data-file
yet. But again, dst will be in sync with src even if it is not a data-file (as
all writes still go to dst). In fact this situation can be identified if reads
redirected to dst return with iatts stating that dst is still not a data file.
Couple this with an identification of rebalance process crash (like maintaining
some state - analogous to meta-lock - which gets cleaned up after a disconnect
from rebalance process), we can prevent reads to dst in this specific scenario.

Kind of a complicated scenario (less probable too?), but something interesting

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