> Yes, I do - I'd been tripping over it once every couple weeks,
> and I finally figured out how to hold my mouth right so that it
> fails (almost) every time.
OK, I tested and verified Karl's fix and wrote some commentary around it.
(Would a aio-dio git repo on kernel.org for these kind of fixes be well
received?)
----
dio: fix cache invalidation after sync writes
Commit commit 65b8291c4000e5f38fc94fb2ca0cb7e8683c8a1b ("dio: invalidate clean
pages before dio write") introduced a bug which stopped dio from ever
invalidating the page cache after writes. It still invalidated it before
writes so most users were fine.
Karl Schendel reported hitting this bug ( http://lkml.org/lkml/2007/10/26/481 )
when he had a buffered reader immediately reading file data after an O_DIRECT
wirter had written the data. The kernel issued read-ahead beyond the position
of the reader which overlapped with the O_DIRECT writer. The failure to
invalidate after writes caused the reader to see stale data from the
read-ahead.
The following patch is originally from Karl. The following commentary is his:
The below 3rd try takes on your suggestion of just invalidating
no matter what the retval from the direct_IO call. I ran it
thru the test-case several times and it has worked every time.
The post-invalidate is probably still too early for async-directio,
but I don't have a testcase for that; just sync. And, this
won't be any worse in the async case.
I added a test to the aio-dio-regress repository which mimics Karl's IO
pattern. It verifed the bad behaviour and that the patch fixed it. I agree
with Karl, this still doesn't help the case where a buffered reader follows an
AIO O_DIRECT writer. That will require a bit more work.
This gives up on the idea of returning EIO to indicate to userspace that stale
data remains if the invalidation failed.
Signed-off-by: Zach Brown <[email protected]>
--- linux-2.6.23.1-base/mm/filemap.c 2007-10-12 12:43:44.000000000 -0400
+++ linux-2.6.23.1/mm/filemap.c 2007-10-26 19:21:20.000000000 -0400
@@ -2194,21 +2194,17 @@ generic_file_direct_IO(int rw, struct ki
}
retval = mapping->a_ops->direct_IO(rw, iocb, iov, offset, nr_segs);
- if (retval)
- goto out;
/*
* Finally, try again to invalidate clean pages which might have been
- * faulted in by get_user_pages() if the source of the write was an
- * mmap()ed region of the file we're writing. That's a pretty crazy
- * thing to do, so we don't support it 100%. If this invalidation
- * fails and we have -EIOCBQUEUED we ignore the failure.
+ * cached by non-direct readahead, or faulted in by get_user_pages()
+ * if the source of the write was an mmap'ed region of the file
+ * we're writing. Either one is a pretty crazy thing to do,
+ * so we don't support it 100%. If this invalidation
+ * fails, tough, the write still worked...
*/
if (rw == WRITE && mapping->nrpages) {
- int err = invalidate_inode_pages2_range(mapping,
- offset >> PAGE_CACHE_SHIFT, end);
- if (err && retval >= 0)
- retval = err;
+ invalidate_inode_pages2_range(mapping, offset >> PAGE_CACHE_SHIFT, end);
}
out:
return retval;
-
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