On Dec 1, 2009, at 11:03 AM, Jan Kara wrote: > On Tue 01-12-09 15:35:59, Jan Kara wrote: >> On Tue 01-12-09 12:42:45, Mike Galbraith wrote: >>> I bisected it this morning. Bisected cleanly to... >>> >>> 9eaaa2d5759837402ec5eee13b2a97921808c3eb is the first bad commit >> OK, I've debugged it. This commit is really at fault. The problem is >> following: >> When using writev, the page we copy from is not paged in (while when we >> use ordinary write, it is paged in). This difference might be worth >> investigation on its own (as it is likely to heavily impact performance of >> writev) but is irrelevant for us now - we should handle this without data >> corruption anyway. Because the source page is not available, we pass 0 as >> the number of copied bytes to write_end and thus ext3_write_end decides to >> truncate the file to original size. This is perfectly fine. The problem is >> that we do this by ext3_truncate() which just frees corresponding block but >> does not unmap buffers. So we leave mapped buffers beyond i_size (they >> actually never were inside i_size) but the blocks they are mapped to are >> already free. The write is then retried (after mapping the page), >> block_write_begin() sees the buffer is mapped (although it is beyond >> i_size) and thus it does not call ext3_get_block() anymore. So as a result, >> data is written to a block that is no longer allocated to the file. Bummer >> - welcome filesystem corruption. >> Ext4 also has this problem but delayed allocation mitigates the effect to >> an error in accounting of blocks reserved for delayed allocation and thus >> under normal circumstances nothing bad happens. >> The question is how to solve this in the cleanest way. We can call >> vmtruncate() instead of ext3_truncate() as we used to do but Nick wants to >> get rid of that (that's why I originally changed the code to what it is >> now). So probably we could just manually call truncate_pagecache() instead. >> Nick, I think your truncate calling sequence patch set needs similar fix >> for all filesystems as well. > The patch below fixes the issue for me... Thank you! I can confirm that the patch fixes the issue in my real application as well. James-- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
