On Sun, Mar 06, 2016 at 03:30:34AM +0300, Kirill A. Shutemov wrote: > On Sun, Mar 06, 2016 at 09:38:11AM +1100, Dave Chinner wrote: > > On Sat, Mar 05, 2016 at 02:24:12AM +0300, Kirill A. Shutemov wrote: > > > Would it be acceptable for fallocate(FALLOC_FL_PUNCH_HOLE) to return > > > -EBUSY (or other errno on your choice), if we cannot split the page > > > right away? > > > > Which means THP are not transparent any more. What does an > > application do when it gets an EBUSY, anyway? > > I guess it's reasonable to expect from an application to handle EOPNOTSUPP > as FALLOC_FL_PUNCH_HOLE is not supported by some filesystems. Yes, but this is usually done as a check at the program initialisation to determine whether to issue hole punches at all. It's not suppose to be a dynamic error. > Although, non-consistent result from the same fd can be confusing. Exactly. > > And it's not just hole punching that has this problem. Direct IO is > > going to have the same issue with invalidation of the mapped ranges > > over the IO being done. XFS already WARNs when page cache > > invalidation fails with EBUSY in direct IO, because that is > > indicative of an application with a potential data corruption vector > > and there's nothing we can do in the kernel code to prevent it. > > My current understanding is that for filesystems with persistent storage, > in order to make THP any useful, we would need to implement writeback > without splitting the huge page. Algorithmically it is no different to filesytem block size < page size writeback. > At the moment, I have no idea how hard it would be.. THP support would effectively require us to remove PAGE_CACHE_SIZE assumptions from all of the filesystem and buffer code. That's a large chunk of work e.g. fs/buffer.c and any filesystem that uses bufferheads for tracking filesystem block state through the page cache. Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
