On 03/25/2015 10:00 PM, Dave Chinner wrote: > On Wed, Mar 25, 2015 at 12:19:50PM +0200, Boaz Harrosh wrote: >> On 03/25/2015 11:29 AM, Dave Chinner wrote: >>> On Wed, Mar 25, 2015 at 10:10:31AM +0200, Boaz Harrosh wrote: >>>> On 03/25/2015 04:22 AM, Dave Chinner wrote: >>>>> On Tue, Mar 24, 2015 at 08:14:59AM +0200, Boaz Harrosh wrote: >>>> <> >> <> >>>> The sync does happen, .fsync of the FS is called on each >>>> file just as if the user called it. If this is broken it just >>>> needs to be fixed there at the .fsync vector. POSIX mandate >>>> persistence at .fsync so at the vfs layer we rely on that. >>> >>> right now, the filesystems will see that there are no dirty pages >>> on the inode, and then just sync the inode metadata. They will do >>> nothing else as filesystems are not aware of CPU cachelines at all. >>> >> >> Sigh yes. There is this bug. And I am sitting on a wide fix for this. >> >> The strategy is. All Kernel writes are done with a new copy_user_nt. >> NT stands for none-temporal. This shows 20% improvements since cachelines >> need not be fetched when written too. > > That's unenforcable for mmap writes from userspace. And those are > the writes that will trigger the dirty write mapping problem. > So for them I was thinking of just doing the .fsync on every unmap (ie vm_operations_struct->close) So now we know that only inodes that have an active vm mapping are in need of sync. >>>> And because of that nothing turned the >>>> user mappings to read only. This is what I do here but >>>> instead of write-protecting I just unmap because it is >>>> easier for me to code it. >>> >>> That doesn't mean it is the correct solution. >> >> Please note that even if we properly .fsync cachlines the page-faults >> are orthogonal to this. There is no point in making mmapped dax pages >> read-only after every .fsync and pay a page-fault. We should leave them >> mapped has is. The only place that we need page protection is at freeze >> time. > > Actually, current behaviour of filesystems is that fsync cleans all > the pages in the range, and means all the mappings are marked > read-only and so we get new calls into .page_mkwrite when write > faults occur. We need that .page_mkwrite call to be able to a) > update the mtime of the inode, and b) mark the inode "data dirty" so > that fsync knows it needs to do something.... > > Hence I'd much prefer we start with identical behaviour to normal > files, then we can optimise from a sane start point when write page > faults show up as a performance problem. i.e. Correctness first, > performance second. > OK, (you see when you speak slow I understand fast ;-)). I agree then I'll see if I can schedule some time for this. My boss will be very angry with me about this. But I will need help please, and some hands holding. Unless someone else volunteers to work on this ? > Cheers, > Dave. > Thanks Boaz -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>