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    Subject[PATCH] fuse: writeback_cache consistency enhancement (writeback_cache_v2)
    On Thu, Jun 30, 2022 at 1:57 AM Vivek Goyal <vgoyal@redhat.com> 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.
    >
    > 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.
    >

    Hi Vivek,

    The fc->auto_inval_data branch code is also skipped in writeback_cache
    v1 mode, so I think we can also skip it in writeback_cache_v2. I
    listed this point here because it is not skipped in Miklos'
    writeback_cache_v2 initial patch.

    Thanks,
    Jiachen

    > 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@miu.piliscsaba.redhat.com
    > > [2] https://lore.kernel.org/linux-fsdevel/20220608104202.19461-1-zhangjiachen.jaycee@bytedance.com/
    > >
    > > Suggested-by: Miklos Szeredi <mszeredi@redhat.com>
    > > Signed-off-by: Jiachen Zhang <zhangjiachen.jaycee@bytedance.com>
    > > ---
    > > 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
    > >
    >

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