On Wed, Nov 17, 2010 at 03:31:37PM -0800, Michel Lespinasse wrote: > On Wed, Nov 17, 2010 at 3:11 PM, Dave Chinner <david@xxxxxxxxxxxxx> wrote: > >> Really, my understanding is that not pre-allocating filesystem blocks > >> is just fine. This is, after all, what happens with ext3 and it's > >> never been reported as a bug (that I know of). > > > > It's not ext3 you have to worry about - it's the filesystems that > > need special state set up on their pages/buffers for ->writepage to > > work correctly that are the problem. You need to call > > ->write_begin/->write_end to get the state set up properly. > > > > If this state is not set up properly, silent data loss will occur > > during mmap writes either by ENOSPC or failing to set up writes into > > unwritten extents correctly (i.e. we'll be back to where we were in > > 2.6.15). > > > > I don't think ->page_mkwrite can be worked around - we need that to > > be called on the first write fault of any mmap()d page to ensure it > > is set up correctly for writeback. If we don't get write faults > > after the page is mlock()d, then we need the ->page_mkwrite() call > > during the mlock() call. > > Just to be clear - I'm proposing to skip the entire do_wp_page() call > by doing a read fault rather than a write fault. If the page wasn't > dirty already, it will stay clean and with a non-writable PTE until it > gets actually written to, at which point we'll get a write fault and > do_wp_page will be invoked as usual. I have no problem with that - I'm surprised that mlock didn't work that way in the first place. > I am not proposing to skip the page_mkwrite() while upgrading the PTE > permissions, which I think is what you were arguing against ? I wasn't arguing against anything, merely pointing out that the ->page_mkwrite call is aboslutely necessary. You've made clarified that it still occurs, so I'm happy... FWIW, what I was responding to was the assumption that "this is alright for ext3, so it must be OK" extrapolation about ->page_mkwrite behaviour. Especially as ext3 does not even implement the ->page_mkwrite operation (which means mmap writes into holes can't detect ENOSPC correctly)... Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom policy in Canada: sign http://dissolvethecrtc.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>