On Wed 11-06-08 12:07:49, Andrew Morton wrote: > On Wed, 11 Jun 2008 20:38:45 +0530 > "Aneesh Kumar K.V" <aneesh.kumar@xxxxxxxxxxxxxxxxxx> wrote: > > > On Thu, Jun 05, 2008 at 12:30:45PM -0700, Andrew Morton wrote: > > > On Thu, 5 Jun 2008 22:35:12 +0530 > > > "Aneesh Kumar K.V" <aneesh.kumar@xxxxxxxxxxxxxxxxxx> wrote: > > > > > > > We would like to get notified when we are doing a write on mmap > > > > section. The changes are needed to handle ENOSPC when writing to an > > > > mmap section of files with holes. > > > > > > > > > > Whoa. You didn't copy anything like enough mailing lists for a change > > > of this magnitude. I added some. > > > > > > This is a large change in behaviour! > > > > > > a) applications will now get a synchronous SIGBUS when modifying a > > > page over an ENOSPC filesystem. Whereas previously they could have > > > proceeded to completion and then detected the error via an fsync(). > > > > Or not detect the error at all if we don't call fsync() right ? Isn't a > > synchronous SIGBUS the right behaviour ? > > > > Not according to POSIX. Or at least posix-several-years-ago, when this > last was discussed. The spec doesn't have much useful to say about any > of this. > > It's a significant change in the userspace interface. > > > > > > > > > It's going to take more than one skimpy little paragraph to > > > justify this, and to demonstrate that it is preferable, and to > > > convince us that nothing will break from this user-visible behaviour > > > change. > > > > > > b) we're now doing fs operations (and some I/O) in the pagefault > > > code. This has several implications: > > > > > > - performance changes > > > > > > - potential for deadlocks when a process takes the fault from > > > within a copy_to_user() in, say, mm/filemap.c > > > > > > - performing additional memory allocations within that > > > copy_to_user(). Possibility that these will reenter the > > > filesystem. > > > > > > And that's just ext2. > > > > > > For ext3 things are even more complex, because we have the > > > journal_start/journal_end pair which is effectively another "lock" for > > > ranking/deadlock purposes. And now we're taking i_alloc_sem and > > > lock_page and we're doing ->writepage() and its potential > > > journal_start(), all potentially within the context of a > > > copy_to_user(). > > > > One of the reason why we would need this in ext3/ext4 is that we cannot > > do block allocation in the writepage with the recent locking changes. > > Perhaps those recent locking changes were wrong. Well, the locking changes are those reverting locking ordering of transaction start and page lock - we have them in ext4 and Aneesh seems to be looking into porting them to ext3 (at least ordered mode rewrite needs them). I wouldn't say they are wrong in principle. It's easier to use page_mkwrite() to allocate blocks so that later in writepage() we don't have to do block allocation which needs to start a transaction (because that means unlocking the page which gets quickly nasty to handle properly...). BTW: XFS, OCFS2 or GFS2 define page_mkwrite() in this manner so they do return SIGBUS when you run out of space when writing to mmapped hole. So it's not like this change is introducing completely new behavior... I can understand that we might not want to change the behavior for ext2 or ext3 but ext4 is IMO definitely free to choose. > > The locking changes involve changing the locking order of journal_start > > and page_lock. With writepage we are already called with page_lock and > > we can't start new transaction needed for block allocation. > > ext3_write_begin() has journal_start() nesting inside the lock_page(). > > > But if we agree that we should not do block allocation in page_mkwrite > > we need to add writepages and allocate blocks in writepages. > > I'm not sure what writepages has to do with pagefaults? Honza -- Jan Kara <jack@xxxxxxx> SUSE Labs, CR -- 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
