Re: [External] Re: [PATCH] fuse: writeback_cache consistency enhancement (writeback_cache_v2)
From: Jiachen Zhang
Date: Tue Jun 28 2022 - 01:21:32 EST
On Sat, Jun 25, 2022 at 2:29 AM Vivek Goyal <vgoyal@xxxxxxxxxx> wrote:
>
> 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.
> >
> > 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(-)
>
> A quick comment without reading this patch, please do add some
> documentation which explains what's the existing behavior and what
> the new behavior.
>
> Probably Documentation/filesystem/fuse.rst is right place. We have
> so many fine knobs w.r.t attr caching and data caching and what kind
> of cache consistency to expect but there is no documentation. So
> very hard to figure out what to expect in different scenarios for
> a user.
>
> I think this patch series probably a good time to start some cache
> coherency related documentation in fuse and that will help users
> as well as will provide context for future changes.
>
> Thanks
> Vivek
Hi Vivek.
I found some good materials about fuse I/O and cache modes in
Documentation/filesystem/fuse-io.rst [1]. Maybe we could add more
information about cache coherence to that file?
[1] https://www.kernel.org/doc/html/v5.17/filesystems/fuse-io.html
Thanks,
Jiachen
>
> >
> > 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
> >
>