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References: <20220624055825.29183-1-zhangjiachen.jaycee@bytedance.com> <YryWjGe6MgBmSkhO@redhat.com>
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From:   Jiachen Zhang <zhangjiachen.jaycee@bytedance.com>
Date:   Mon, 18 Jul 2022 13:48:30 +0800
Message-ID: <CAFQAk7iNc0aOHh4aYg4zuoUsxeBzPYo21VdU+pEhsWs2uixwoQ@mail.gmail.com>
Subject: Re: [External] Re: [PATCH] fuse: writeback_cache consistency
 enhancement (writeback_cache_v2)
To:     Vivek Goyal <vgoyal@redhat.com>
Cc:     miklos@szeredi.hu, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org, xieyongji@bytedance.com,
        fam.zheng@bytedance.com, Miklos Szeredi <mszeredi@redhat.com>
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On Thu, Jun 30, 2022 at 2:14 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.
>
> Can this give the appearance of time going backwards. If file server
> is getting time stamps from another remote filesystem (say NFS), it
> is possible that client and server clocks are not in sync. So for
> a stat() might return temporary value of c/mtime from local kernel and
> once it gets udpated, it returns a time which might be behind previous
> time. Sounds like trouble to me.
>

Hi Vivek,

Yes, the time is likely to be changed silently on a client after the
data is flush to the server. But in terms of consistency, some FS
implementations may rely on the server mtime to revalidate cache. In
such a case, if the server time is updated from multiple client time
sources, it may lead to false cache revalidation results.

> This can happen more often on virtiofs (even without a remote filesystem
> being involved).
>

Maybe if there is no need of multiple VM-guest consistency, we can
just disable the writeback_cache_v2 mode.


> >
> >     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.
>
> If your goal is to implement NFS style CTO with writeback cache mode, then
> is it not enough to query and udpate attributes at file open time. Instead
> of worrying about also updating attrs while file is open (and cache is
> clean). Updating attrs when cache is clean, is non-deterministic anyway,
> so I don't see how it can be very useful for applications.
>

IIUC, the NFS-style CTO consistency only requires updating invalidated
data & attr caches on file open, and the cache updating will be
delayed if there is any writing opens, even if the cache is clean. So
writeback_cache_v2 updates even more radical than NFS-style CTO.

Thanks,
Jiachen

> Thanks
> Vivek
>
>
> >
> > [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
> >
>