On Tue, Oct 7, 2014 at 3:16 PM, Jan Kara <jack@xxxxxxx> wrote: > On Tue 07-10-14 12:30:59, Dave Chinner wrote: >> On Mon, Oct 06, 2014 at 04:30:19PM +0200, Jan Kara wrote: >> > On Mon 06-10-14 11:33:23, Thanos Makatos wrote: >> > > > > Trond also had a comment that if we extended the ioctl to work for all >> > > > > inodes (not just blkdev) and allowed some additional flags of what >> > > > > needs to be invalidated, the new ioctl would be also useful to NFS >> > > > > userspace - see Trond's email at >> > > > > >> > > > > http://www.spinics.net/lists/linux-fsdevel/msg78917.html >> > > > > >> > > > > and the following thread. I would prefer to cover that usecase when we >> > > > > are introducing new invalidation ioctl. Have you considered that Thanos? >> > > > >> > > > Sure, though I don't really know how to do it. I'll start by looking at the code >> > > > flow when someone does " echo 3 > /proc/sys/vm/drop_caches", unless you >> > > > already have a rough idea how to do that. >> > > >> > > I realise I haven't clearly understood what the semantics of this new ioctl >> > > should be. >> > > >> > > My initial goal was to implement an ioctl that would _completely_ invalidate >> > > the buffer cache of a block device when there is no file-system involved. >> > > Unless I'm mistaken the patch I posted achieves this goal. >> > Yes. >> > >> > > We now want to extend this patch to take care of cached metadata, which seems >> > > to be of particular importance for NFS, and I suspect that this piece of >> > > functionality will still be applicable to any kind of file-system, correct? >> > So most notably they want the ioctl to work not only for block devices >> > but also for any regular file. That's easily doable - you just call >> > filemap_write_and_wait() and invalidate_inode_pages2() in the ioctl handler >> > for regular files. >> > >> > Also they wanted to be able to specify a range of a mapping to invalidate - >> > that's easily doable as well. Finally they wanted a 'flags' argument so you >> > can additionally ask fs to invalidate also some metadata. How invalidation >> > is done will be a fs specific thing and for now I guess we don't need to go >> > into details. NFS guys can sort that out when they decide to implement it. >> > So in the beginning we can just have u64 flags argument and in >> > it a single 'INVAL_DATA' flag meaning that invalidation of data in a given >> > range is requested. Later NFS guys can add further flags. >> >> Why do we need a new ioctl to do this? fadvise64() seems like it's >> the exact fit for "FADV_INVALIDATE_[META]DATA" flags... > Well, fadvise() is currently a hint to kernel. In this case we would > really like the call to do the invalidation and return error if it fails > for some reason. So I'm not sure fadvise() is a perfect fit. But I wouldn't > be strongly opposed to it either. > fadvise is about giving programs the ability to "announce an intention to access file data in a specific pattern in the future, thus allowing the kernel to perform appropriate optimizations" according to the manpage. Cache invalidation and revalidation, OTOH, is about ensuring meta/data consistency between the disk and inode/page cache. I'm not seeing a perfect match. :-) -- Trond Myklebust Linux NFS client maintainer, PrimaryData trond.myklebust@xxxxxxxxxxxxxxx -- 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
