Hi, I've been rewriting fscache and cachefiles to massively simplify it and make use of the kiocb interface to do direct-I/O to/from the netfs's pages which didn't exist when I first did this. Instead it has been attempting to monitor the page bit waitqueues to see when the backing filesystem's pages become up to date. https://lore.kernel.org/lkml/24942.1573667720@xxxxxxxxxxxxxxxxxxxxxx/ https://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs.git/log/?h=fscache-iter To make it more efficient, following other network filesystems implementations outside of the Linux kernel, I'm attempting to move to requiring reads and writes to the cache in much bigger granules (fixed at 256KiB initially), which means that I can represent the presence of a granule of that much data with a single bit. So far, I've done this for ->readpages(), ->readpage() and ->write_begin() by taking the requested page or pages and expanding/contracting the set of pages as necessary so that the first (or only) actually requested page is in there and both ends of the sequence are appropriate aligned. This, however, is at odds with the VM and *its* idea of how to do things - particularly for ->readpages(). The logic of my fscache_read_helper()[*] is applied after the VM's readahead logic, and the two don't necessarily see eye to eye at present. [*] This is in the patch named "fscache: Add read helper" in the above-mentioned git tree and "afs: Use new fscache I/O API" which has examples of using it. There are some things that need to be taken into consideration: (1) I might want to make the granule size variable both by file and over the length of a file. So for a file that's, say, <=512MiB in size, I might want 1 bit per 256KiB granule, but over 512MiB I might want to switch to 1 bit per 1MiB granule. Or for files that large, just use 1MiB granules all the way through. (2) The granule size might also need vary by cache. (3) Some files I want to treat as monolithic. The file is either all there or none of it is. Examples might be non-regular files such as symlinks or directories. (4) These parameters might be tunable by the admin. So how best to make the VM deal with this? Is it better to integrate such logic into the VM or leave it on top? David
