On Fri, Jun 24, 2022 at 9:03 AM Jiachen Zhang <zhangjiachen.jaycee@xxxxxxxxxxxxx> wrote: > > Some users may want both the high performance of the writeback_cahe mode and > a little bit more consistency among FUSE mounts. In the current writeback > mode implementation, users of one FUSE mount can never see the file > expansion done by other FUSE mounts. > > Based on the suggested writeback V2 patch in the upstream mailing-list [1], > this commit allows the cmtime and size to be updated from server in > writeback mode. Compared with the writeback V2 patch in [1], this commit has > several differences: > > 1. Ensure c/mtime are not updated from kernel to server. IOW, the cmtime > generated by kernel are just temporary values that are never flushed to > server, and they can also be updated by the official server cmtime when > the writeback cache is clean. > > 2. Skip mtime-based revalidation when fc->auto_inval_data is set with > fc->writeback_cache_v2. Because the kernel-generated temporary cmtime > are likely not equal to the offical server cmtime. > > 3. If any page is ever flushed to the server during FUSE_GETATTR > handling on fuse server, even if the cache is clean when > fuse_change_attributes() checks, we should not update the i_size. This > is because the FUSE_GETATTR may get a staled size before the FUSE_WRITE > request changes server inode size. This commit ensures this by > increasing attr_version after writeback for writeback_cache_v2. In that > case, we should also ensure the ordering of the attr_version updating > and the fi->writepages RB-tree updating. So that if a fuse page > writeback ever happens during fuse_change_attributes(), either the > fi->writepages is not empty, or the attr_version is increased. So we > never mistakenly update a stale file size from server to kernel. > > With this patch, writeback mode can consider the server c/mtime as the > official one. When inode attr is timeout or invalidated, kernel has chance > to see size and c/mtime modified by others. > > Together with another patch [2], a FUSE daemon is able to implement > close-to-open (CTO) consistency like what is done in NFS clients. > > [1] https://lore.kernel.org/linux-fsdevel/Ymfu8fGbfYi4FxQ4@xxxxxxxxxxxxxxxxxxxxxxxxx > [2] https://lore.kernel.org/linux-fsdevel/20220608104202.19461-1-zhangjiachen.jaycee@xxxxxxxxxxxxx/ > > Suggested-by: Miklos Szeredi <mszeredi@xxxxxxxxxx> > Signed-off-by: Jiachen Zhang <zhangjiachen.jaycee@xxxxxxxxxxxxx> > --- > fs/fuse/file.c | 17 +++++++++++++++ > fs/fuse/fuse_i.h | 3 +++ > fs/fuse/inode.c | 44 +++++++++++++++++++++++++++++++++++++-- > include/uapi/linux/fuse.h | 5 +++++ > 4 files changed, 67 insertions(+), 2 deletions(-) > > diff --git a/fs/fuse/file.c b/fs/fuse/file.c > index 9b64e2ff1c96..35bdc7af8468 100644 > --- a/fs/fuse/file.c > +++ b/fs/fuse/file.c > @@ -1829,6 +1829,15 @@ static void fuse_writepage_end(struct fuse_mount *fm, struct fuse_args *args, > */ > fuse_send_writepage(fm, next, inarg->offset + inarg->size); > } > + > + if (fc->writeback_cache_v2) > + fi->attr_version = atomic64_inc_return(&fc->attr_version); > + /* > + * Ensure attr_version increases before the page is move out of the > + * writepages rb-tree. > + */ > + smp_mb(); > + > fi->writectr--; > fuse_writepage_finish(fm, wpa); > spin_unlock(&fi->lock); > @@ -1858,10 +1867,18 @@ static struct fuse_file *fuse_write_file_get(struct fuse_inode *fi) > > int fuse_write_inode(struct inode *inode, struct writeback_control *wbc) > { > + struct fuse_conn *fc = get_fuse_conn(inode); > struct fuse_inode *fi = get_fuse_inode(inode); > struct fuse_file *ff; > int err; > > + /* > + * Kernel c/mtime should not be updated to the server in the > + * writeback_cache_v2 mode as server c/mtime are official. > + */ > + if (fc->writeback_cache_v2) > + return 0; > + > /* > * Inode is always written before the last reference is dropped and > * hence this should not be reached from reclaim. > diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h > index 488b460e046f..47de36146fb8 100644 > --- a/fs/fuse/fuse_i.h > +++ b/fs/fuse/fuse_i.h > @@ -654,6 +654,9 @@ struct fuse_conn { > /* show legacy mount options */ > unsigned int legacy_opts_show:1; > > + /* Improved writeback cache policy */ > + unsigned writeback_cache_v2:1; > + Seeing that writeback_cache_v2 depends on writeback_cache I wonder whether that will not be better represented as: /** write-back cache policy (default is write-through) */ - unsigned writeback_cache:1; + unsigned writeback_cache:2; Looking at the recently added handle_killpriv_v2, I also wonder if that would not have been better. Vivek, is handle_killpriv_v2 really independent of handle_killpriv? Seeing test like these worry me as they are inviting bugs: if (!fc->handle_killpriv && !fc->handle_killpriv_v2) { Thanks, Amir.
