2012/6/5 Dave Chinner <david@xxxxxxxxxxxxx>: > On Thu, May 24, 2012 at 02:35:38PM +0200, Jan Kara wrote: >> On Sat 19-05-12 11:40:24, Dave Chinner wrote: >> > So let's step back a moment and have a look at how we've got here. >> > The problem is that we've optimised ourselves into a corner with the >> > way we handle page cache truncation - we don't need mmap >> > serialisation because of the combination of i_size and page locks >> > mean we can detect truncated pages safely at page fault time. With >> > hole punching, we don't have that i_size safety blanket, and so we >> > need some other serialisation mechanism to safely detect whether a >> > page is valid or not at any given point in time. >> > >> > Because it needs to serialise against IO operations, we need a >> > sleeping lock of some kind, and it can't be the existing IO lock. >> > And now we are looking at needing a new lock for hole punching, I'm >> > really wondering if the i_size/page lock truncation optimisation >> > should even continue to exist. i.e. replace it with a single >> > mechanism that works for both hole punching, truncation and other >> > functions that require exclusive access or exclusion against >> > modifications to the mapping tree. >> > >> > But this is only one of the problems in this area.The way I see it >> > is that we have many kludges in the area of page invalidation w.r.t. >> > different types of IO, the page cache and mmap, especially when we >> > take into account direct IO. What we are seeing here is we need >> > some level of _mapping tree exclusion_ between: >> > >> > 1. mmap vs hole punch (broken) >> > 2. mmap vs truncate (i_size/page lock) >> > 3. mmap vs direct IO (non-existent) >> > 4. mmap vs buffered IO (page lock) >> > 5. writeback vs truncate (i_size/page lock) >> > 6. writeback vs hole punch (page lock, possibly broken) >> > 7. direct IO vs buffered IO (racy - flush cache before/after DIO) >> Yes, this is a nice summary of the most interesting cases. For completeness, >> here are the remaining cases: >> 8. mmap vs writeback (page lock) >> 9. writeback vs direct IO (as direct IO vs buffered IO) >> 10. writeback vs buffered IO (page lock) >> 11. direct IO vs truncate (dio_wait) >> 12. direct IO vs hole punch (dio_wait) >> 13. buffered IO vs truncate (i_mutex for writes, i_size/page lock for reads) >> 14. buffered IO vs hole punch (fs dependent, broken for ext4) >> 15. truncate vs hole punch (fs dependent) >> 16. mmap vs mmap (page lock) >> 17. writeback vs writeback (page lock) >> 18. direct IO vs direct IO (i_mutex or fs dependent) >> 19. buffered IO vs buffered IO (i_mutex for writes, page lock for reads) >> 20. truncate vs truncate (i_mutex) >> 21. punch hole vs punch hole (fs dependent) > I think we have even the xip cases here. Marco -- 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
