On Tue, 24 Mar 2009 18:44:21 +1100 Nick Piggin <nickpiggin@xxxxxxxxxxxx> wrote: > On Friday 20 March 2009 03:46:39 Jan Kara wrote: > > On Fri 20-03-09 02:48:21, Nick Piggin wrote: > > > > Holding mapping->private_lock over the __set_page_dirty should > > > fix it, although I guess you'd want to release it before calling > > > __mark_inode_dirty so as not to put inode_lock under there. I > > > have a patch for this if it sounds reasonable. > > > > Yes, that seems to be a bug - the function actually looked suspitious to > > me yesterday but I somehow convinced myself that it's fine. Probably > > because fsx-linux is single-threaded. > > > After a whole lot of chasing my own tail in the VM and buffer layers, > I think it is a problem in ext2 (and I haven't been able to reproduce > with ext3 yet, which might lend weight to that, although as we have > seen, it is very timing dependent). > > That would be slightly unfortunate because we still have Jan's ext3 > problem, and also another reported problem of corruption on ext3 (on > brd driver). > > Anyway, when I have reproduced the problem with the test case, the > "lost" writes are all reported to be holes. Unfortunately, that doesn't > point straight to the filesystem, because ext2 allocates blocks in this > case at writeout time, so if dirty bits are getting lost, then it would > be normal to see holes. > > I then put in a whole lot of extra infrastructure to track metadata about > each struct page (when it was last written out, when it last had the number > of writable ptes reach 0, when the dirty bits were last cleared etc). And > none of the normal asertions were triggering: eg. when any page is removed > from pagecache (except truncates), it has always had all its buffers > written out *after* all ptes were made readonly or unmapped. Lots of other > tests and crap like that. > > So I tried what I should have done to start with and did an e2fsck after > seeing corruption. Yes, it comes up with errors. Do you recall what the errors were? > Now that is unusual > because that should be largely insulated from the vm: if a dirty bit gets > lost, then the filesystem image should be quite happy and error-free with > a hole or unwritten data there. > > I don't know ext? locking very well, except that it looks pretty overly > complex and crufty. > > Usually, blocks are instantiated by write(2), under i_mutex, serialising > the allocator somewhat. mmap-write blocks are instantiated at writeout > time, unserialised. I moved truncate_mutex to cover the entire get_blocks > function, and can no longer trigger the problem. Might be a timing issue > though -- Ying, can you try this and see if you can still reproduce? > > I close my eyes and pick something out of a hat. a686cd89. Search for XXX. > Nice. Whether or not this cased the problem, can someone please tell me > why it got merged in that state? > > I'm leaving ext3 running for now. It looks like a nasty task to bisect > ext2 down to that commit :( and I would be more interested in trying to > reproduce Jan's ext3 problem, however, because I'm not too interested in > diving into ext2 locking to work out exactly what is racing and how to > fix it properly. I suspect it would be most productive to wire up some > ioctls right into the block allocator/lookup and code up a userspace > tester for it that could probably stress it a lot harder than kernel > writeout can. -- 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
