Re: [PATCH 09/31] fuse: isolate the other regular file IO paths from iomap

["Darrick J. Wong" <djwong@kernel.org>]

On Thu, Nov 06, 2025 at 03:02:14PM -0800, Darrick J. Wong wrote:
> On Thu, Nov 06, 2025 at 07:44:46PM +0100, Amir Goldstein wrote:
> > On Wed, Oct 29, 2025 at 1:47 AM Darrick J. Wong <djwong@kernel.org> wrote:
> > >
> > > From: Darrick J. Wong <djwong@kernel.org>
> > >
> > > iomap completely takes over all regular file IO, so we don't need to
> > > access any of the other mechanisms at all.  Gate them off so that we can
> > > eventually overlay them with a union to save space in struct fuse_inode.
> > >
> > > Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
> > > ---
> > 
> > I apologize for providing criticism which is not very productive.
> > 
> > Looking at this I don't feel much confidence to say that all cases are covered
> > and worse, I don't have much confidence that future coder won't easily
> > write some code that accesses the wrong side of the union.
> > 
> > I don't really have great ideas on how to improve.
> > Maybe at least some of the gates can be inside the _nowrite accessors?
> > 
> > Please don't take this as any sort of objection to this patch.
> 
> I'll happily drop "fuse: overlay iomap inode info in struct fuse_inode"
> because C unions are ***awful bug nests anyway.  Maybe someday when
> we're porting fuse to Rust. ;)

I should add that gating the !iomap code paths mean that we don't have
to waste time messing around with otherwise unused data structures, so
I'll keep this patch even if I drop the unionizing one.

--D

> 
> > Thanks,
> > Amir.
> > 
> > >  fs/fuse/dir.c    |   14 +++++++++-----
> > >  fs/fuse/file.c   |   18 +++++++++++++-----
> > >  fs/fuse/inode.c  |    3 ++-
> > >  fs/fuse/iomode.c |    2 +-
> > >  4 files changed, 25 insertions(+), 12 deletions(-)
> > >
> > >
> > > diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c
> > > index 3c222b99d6e699..18eb1bb192bb58 100644
> > > --- a/fs/fuse/dir.c
> > > +++ b/fs/fuse/dir.c
> > > @@ -1991,6 +1991,7 @@ int fuse_do_setattr(struct mnt_idmap *idmap, struct dentry *dentry,
> > >         FUSE_ARGS(args);
> > >         struct fuse_setattr_in inarg;
> > >         struct fuse_attr_out outarg;
> > > +       const bool is_iomap = fuse_inode_has_iomap(inode);
> > >         bool is_truncate = false;
> > >         bool is_wb = fc->writeback_cache && S_ISREG(inode->i_mode);
> > >         loff_t oldsize;
> > > @@ -2048,12 +2049,15 @@ int fuse_do_setattr(struct mnt_idmap *idmap, struct dentry *dentry,
> > >                 if (err)
> > >                         return err;
> > >
> > > -               fuse_set_nowrite(inode);
> > > -               fuse_release_nowrite(inode);
> > > +               if (!is_iomap) {
> > > +                       fuse_set_nowrite(inode);
> > > +                       fuse_release_nowrite(inode);
> > > +               }
> > >         }
> > >
> > >         if (is_truncate) {
> > > -               fuse_set_nowrite(inode);
> > > +               if (!is_iomap)
> > > +                       fuse_set_nowrite(inode);
> > >                 set_bit(FUSE_I_SIZE_UNSTABLE, &fi->state);
> > >                 if (trust_local_cmtime && attr->ia_size != inode->i_size)
> > >                         attr->ia_valid |= ATTR_MTIME | ATTR_CTIME;
> > > @@ -2125,7 +2129,7 @@ int fuse_do_setattr(struct mnt_idmap *idmap, struct dentry *dentry,
> > >         if (!is_wb || is_truncate)
> > >                 i_size_write(inode, outarg.attr.size);
> > >
> > > -       if (is_truncate) {
> > > +       if (is_truncate && !is_iomap) {
> > >                 /* NOTE: this may release/reacquire fi->lock */
> > >                 __fuse_release_nowrite(inode);
> > >         }
> > > @@ -2149,7 +2153,7 @@ int fuse_do_setattr(struct mnt_idmap *idmap, struct dentry *dentry,
> > >         return 0;
> > >
> > >  error:
> > > -       if (is_truncate)
> > > +       if (is_truncate && !is_iomap)
> > >                 fuse_release_nowrite(inode);
> > >
> > >         clear_bit(FUSE_I_SIZE_UNSTABLE, &fi->state);
> > > diff --git a/fs/fuse/file.c b/fs/fuse/file.c
> > > index 42c85c19f3b13b..bd9c208a46c78d 100644
> > > --- a/fs/fuse/file.c
> > > +++ b/fs/fuse/file.c
> > > @@ -238,6 +238,7 @@ static int fuse_open(struct inode *inode, struct file *file)
> > >         struct fuse_conn *fc = fm->fc;
> > >         struct fuse_file *ff;
> > >         int err;
> > > +       const bool is_iomap = fuse_inode_has_iomap(inode);
> > >         bool is_truncate = (file->f_flags & O_TRUNC) && fc->atomic_o_trunc;
> > >         bool is_wb_truncate = is_truncate && fc->writeback_cache;
> > >         bool dax_truncate = is_truncate && FUSE_IS_DAX(inode);
> > > @@ -259,7 +260,7 @@ static int fuse_open(struct inode *inode, struct file *file)
> > >                         goto out_inode_unlock;
> > >         }
> > >
> > > -       if (is_wb_truncate || dax_truncate)
> > > +       if ((is_wb_truncate || dax_truncate) && !is_iomap)
> > >                 fuse_set_nowrite(inode);
> > >
> > >         err = fuse_do_open(fm, get_node_id(inode), file, false);
> > > @@ -272,7 +273,7 @@ static int fuse_open(struct inode *inode, struct file *file)
> > >                         fuse_truncate_update_attr(inode, file);
> > >         }
> > >
> > > -       if (is_wb_truncate || dax_truncate)
> > > +       if ((is_wb_truncate || dax_truncate) && !is_iomap)
> > >                 fuse_release_nowrite(inode);
> > >         if (!err) {
> > >                 if (is_truncate)
> > > @@ -520,12 +521,14 @@ static int fuse_fsync(struct file *file, loff_t start, loff_t end,
> > >  {
> > >         struct inode *inode = file->f_mapping->host;
> > >         struct fuse_conn *fc = get_fuse_conn(inode);
> > > +       const bool need_sync_writes = !fuse_inode_has_iomap(inode);
> > >         int err;
> > >
> > >         if (fuse_is_bad(inode))
> > >                 return -EIO;
> > >
> > > -       inode_lock(inode);
> > > +       if (need_sync_writes)
> > > +               inode_lock(inode);
> > >
> > >         /*
> > >          * Start writeback against all dirty pages of the inode, then
> > > @@ -536,7 +539,8 @@ static int fuse_fsync(struct file *file, loff_t start, loff_t end,
> > >         if (err)
> > >                 goto out;
> > >
> > > -       fuse_sync_writes(inode);
> > > +       if (need_sync_writes)
> > > +               fuse_sync_writes(inode);
> > >
> > >         /*
> > >          * Due to implementation of fuse writeback
> > > @@ -560,7 +564,8 @@ static int fuse_fsync(struct file *file, loff_t start, loff_t end,
> > >                 err = 0;
> > >         }
> > >  out:
> > > -       inode_unlock(inode);
> > > +       if (need_sync_writes)
> > > +               inode_unlock(inode);
> > >
> > >         return err;
> > >  }
> > > @@ -1942,6 +1947,9 @@ static struct fuse_file *__fuse_write_file_get(struct fuse_inode *fi)
> > >  {
> > >         struct fuse_file *ff;
> > >
> > > +       if (fuse_inode_has_iomap(&fi->inode))
> > > +               return NULL;
> > > +
> > >         spin_lock(&fi->lock);
> > >         ff = list_first_entry_or_null(&fi->write_files, struct fuse_file,
> > >                                       write_entry);
> > > diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
> > > index 9b9e7b2dd0d928..7602595006a19d 100644
> > > --- a/fs/fuse/inode.c
> > > +++ b/fs/fuse/inode.c
> > > @@ -191,7 +191,8 @@ static void fuse_evict_inode(struct inode *inode)
> > >                 if (inode->i_nlink > 0)
> > >                         atomic64_inc(&fc->evict_ctr);
> > >         }
> > > -       if (S_ISREG(inode->i_mode) && !fuse_is_bad(inode)) {
> > > +       if (S_ISREG(inode->i_mode) && !fuse_is_bad(inode) &&
> > > +           !fuse_inode_has_iomap(inode)) {
> > >                 WARN_ON(fi->iocachectr != 0);
> > >                 WARN_ON(!list_empty(&fi->write_files));
> > >                 WARN_ON(!list_empty(&fi->queued_writes));
> > > diff --git a/fs/fuse/iomode.c b/fs/fuse/iomode.c
> > > index 3728933188f307..0a534e5a6db5f6 100644
> > > --- a/fs/fuse/iomode.c
> > > +++ b/fs/fuse/iomode.c
> > > @@ -203,7 +203,7 @@ int fuse_file_io_open(struct file *file, struct inode *inode)
> > >          * io modes are not relevant with DAX and with server that does not
> > >          * implement open.
> > >          */
> > > -       if (FUSE_IS_DAX(inode) || !ff->args)
> > > +       if (fuse_inode_has_iomap(inode) || FUSE_IS_DAX(inode) || !ff->args)
> > >                 return 0;
> > >
> > >         /*
> > >
> > >
>