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. > 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? -- 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
