Re: [PATCH] fuse: writeback_cache consistency enhancement (writeback_cache_v2)
From: Vivek Goyal
Date: Wed Jun 29 2022 - 13:58:13 EST
On Fri, Jun 24, 2022 at 01:58:25PM +0800, Jiachen Zhang 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.
We have a init flag to negotiate FUSE_AUTO_INVAL_DATA to enable/disable
fc->auto_inval_data. So should it be filesystem's (user space) responsibility
to not use FUSE_AUTO_INVAL_DATA when using FUSE_WRITEBACK_CACHE. I mean,
otherwise it is confusing. On one hand filesystem is using FUSE_AUTO_INVAL_DATA
but it silently is ingored by kernel because FUSE_WRITEBACK_CACHE is
being used as well.
will it make more sense to just document it and let filesystem authors
enable options carefully.
BTW, this probably should be a separate patch in the patch series anyway
with proper explanation.
Thanks
Vivek
>
> 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;
> +
> /*
> * fs kills suid/sgid/cap on write/chown/trunc. suid is killed on
> * write/trunc only if caller did not have CAP_FSETID. sgid is killed
> diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
> index 8c0665c5dff8..2d5fa82b08b6 100644
> --- a/fs/fuse/inode.c
> +++ b/fs/fuse/inode.c
> @@ -237,14 +237,41 @@ void fuse_change_attributes(struct inode *inode, struct fuse_attr *attr,
> u32 cache_mask;
> loff_t oldsize;
> struct timespec64 old_mtime;
> + bool try_wb_update = false;
> +
> + if (fc->writeback_cache_v2 && S_ISREG(inode->i_mode)) {
> + inode_lock(inode);
> + try_wb_update = true;
> + }
>
> spin_lock(&fi->lock);
> /*
> * In case of writeback_cache enabled, writes update mtime, ctime and
> * may update i_size. In these cases trust the cached value in the
> * inode.
> + *
> + * In writeback_cache_v2 mode, if all the following conditions are met,
> + * then we allow the attributes to be refreshed:
> + *
> + * - inode is not in the process of being written (I_SYNC)
> + * - inode has no dirty pages (I_DIRTY_PAGES)
> + * - inode data-related attributes are clean (I_DIRTY_DATASYNC)
> + * - inode does not have any page writeback in progress
> + *
> + * Note: checking PAGECACHE_TAG_WRITEBACK is not sufficient in fuse,
> + * since inode can appear to have no PageWriteback pages, yet still have
> + * outstanding write request.
> */
> cache_mask = fuse_get_cache_mask(inode);
> + if (try_wb_update && !(inode->i_state & (I_DIRTY_PAGES | I_SYNC |
> + I_DIRTY_DATASYNC)) && RB_EMPTY_ROOT(&fi->writepages))
> + cache_mask &= ~(STATX_MTIME | STATX_CTIME | STATX_SIZE);
> + /*
> + * Ensure the ordering of cleanness checking and following attr_version
> + * comparison.
> + */
> + smp_mb();
> +
> if (cache_mask & STATX_SIZE)
> attr->size = i_size_read(inode);
>
> @@ -283,7 +310,13 @@ void fuse_change_attributes(struct inode *inode, struct fuse_attr *attr,
> truncate_pagecache(inode, attr->size);
> if (!fc->explicit_inval_data)
> inval = true;
> - } else if (fc->auto_inval_data) {
> + } else if (!fc->writeback_cache_v2 && fc->auto_inval_data) {
> + /*
> + * When fc->writeback_cache_v2 is set, the old_mtime
> + * can be generated by kernel and must not equal to
> + * new_mtime generated by server. So skip in such
> + * case.
> + */
> struct timespec64 new_mtime = {
> .tv_sec = attr->mtime,
> .tv_nsec = attr->mtimensec,
> @@ -303,6 +336,9 @@ void fuse_change_attributes(struct inode *inode, struct fuse_attr *attr,
>
> if (IS_ENABLED(CONFIG_FUSE_DAX))
> fuse_dax_dontcache(inode, attr->flags);
> +
> + if (try_wb_update)
> + inode_unlock(inode);
> }
>
> static void fuse_init_inode(struct inode *inode, struct fuse_attr *attr)
> @@ -1153,6 +1189,10 @@ static void process_init_reply(struct fuse_mount *fm, struct fuse_args *args,
> fc->async_dio = 1;
> if (flags & FUSE_WRITEBACK_CACHE)
> fc->writeback_cache = 1;
> + if (flags & FUSE_WRITEBACK_CACHE_V2) {
> + fc->writeback_cache = 1;
> + fc->writeback_cache_v2 = 1;
> + }
> if (flags & FUSE_PARALLEL_DIROPS)
> fc->parallel_dirops = 1;
> if (flags & FUSE_HANDLE_KILLPRIV)
> @@ -1234,7 +1274,7 @@ void fuse_send_init(struct fuse_mount *fm)
> FUSE_ABORT_ERROR | FUSE_MAX_PAGES | FUSE_CACHE_SYMLINKS |
> FUSE_NO_OPENDIR_SUPPORT | FUSE_EXPLICIT_INVAL_DATA |
> FUSE_HANDLE_KILLPRIV_V2 | FUSE_SETXATTR_EXT | FUSE_INIT_EXT |
> - FUSE_SECURITY_CTX;
> + FUSE_SECURITY_CTX | FUSE_WRITEBACK_CACHE_V2;
> #ifdef CONFIG_FUSE_DAX
> if (fm->fc->dax)
> flags |= FUSE_MAP_ALIGNMENT;
> diff --git a/include/uapi/linux/fuse.h b/include/uapi/linux/fuse.h
> index d6ccee961891..b474763bcf59 100644
> --- a/include/uapi/linux/fuse.h
> +++ b/include/uapi/linux/fuse.h
> @@ -194,6 +194,7 @@
> * - add FUSE_SECURITY_CTX init flag
> * - add security context to create, mkdir, symlink, and mknod requests
> * - add FUSE_HAS_INODE_DAX, FUSE_ATTR_DAX
> + * - add FUSE_WRITEBACK_CACHE_V2 init flag
> */
>
> #ifndef _LINUX_FUSE_H
> @@ -353,6 +354,9 @@ struct fuse_file_lock {
> * FUSE_SECURITY_CTX: add security context to create, mkdir, symlink, and
> * mknod
> * FUSE_HAS_INODE_DAX: use per inode DAX
> + * FUSE_WRITEBACK_CACHE_V2:
> + * allow time/size to be refreshed if no pending write
> + * c/mtime not updated from kernel to server
> */
> #define FUSE_ASYNC_READ (1 << 0)
> #define FUSE_POSIX_LOCKS (1 << 1)
> @@ -389,6 +393,7 @@ struct fuse_file_lock {
> /* bits 32..63 get shifted down 32 bits into the flags2 field */
> #define FUSE_SECURITY_CTX (1ULL << 32)
> #define FUSE_HAS_INODE_DAX (1ULL << 33)
> +#define FUSE_WRITEBACK_CACHE_V2 (1ULL << 34)
>
> /**
> * CUSE INIT request/reply flags
> --
> 2.20.1
>