[PATCH V7 15/19] famfs_fuse: Plumb dax iomap and fuse read/write/mmap

John Groves
2026-01-18T22:33:33Z

From: John Groves <john@groves.net>

This commit fills in read/write/mmap handling for famfs files. The
dev_dax_iomap interface is used - just like xfs in fs-dax mode.

- Read/write are handled by famfs_fuse_[read|write]_iter() via
dax_iomap_rw() to fsdev_dax.
- Mmap is handled by famfs_fuse_mmap()
- Faults are handled by famfs_filemap_fault(), using dax_iomap_fault()
to fsdev_dax.
- File offset to dax offset resolution is handled via
famfs_fuse_iomap_begin(), which uses famfs "fmaps" to resolve the
the requested (file, offset) to an offset on a dax device (by way of
famfs_fileofs_to_daxofs() and famfs_interleave_fileofs_to_daxofs())

Signed-off-by: John Groves <john@groves.net>
---
fs/fuse/famfs.c | 448 +++++++++++++++++++++++++++++++++++++++++++++++
fs/fuse/file.c | 18 +-
fs/fuse/fuse_i.h | 19 ++
3 files changed, 483 insertions(+), 2 deletions(-)

diff --git a/fs/fuse/famfs.c b/fs/fuse/famfs.c
index 7aa2eb2e99bf..0218c2a61bc1 100644
--- a/fs/fuse/famfs.c
+++ b/fs/fuse/famfs.c
@@ -579,6 +579,454 @@ famfs_file_init_dax(
return rc;
}

+/*********************************************************************
+ * iomap_operations
+ *
+ * This stuff uses the iomap (dax-related) helpers to resolve file offsets to
+ * offsets within a dax device.
+ */
+
+static int famfs_file_bad(struct inode *inode);
+
+static int
+famfs_interleave_fileofs_to_daxofs(struct inode *inode, struct iomap *iomap,
+ loff_t file_offset, off_t len, unsigned int flags)
+{
+ struct fuse_inode *fi = get_fuse_inode(inode);
+ struct famfs_file_meta *meta = fi->famfs_meta;
+ struct fuse_conn *fc = get_fuse_conn(inode);
+ loff_t local_offset = file_offset;
+
+ /* This function is only for extent_type INTERLEAVED_EXTENT */
+ if (meta->fm_extent_type != INTERLEAVED_EXTENT) {
+ pr_err("%s: bad extent type\n", __func__);
+ goto err_out;
+ }
+
+ if (famfs_file_bad(inode))
+ goto err_out;
+
+ iomap->offset = file_offset;
+
+ for (int i = 0; i < meta->fm_niext; i++) {
+ struct famfs_meta_interleaved_ext *fei = &meta->ie[i];
+ u64 chunk_size = fei->fie_chunk_size;
+ u64 nstrips = fei->fie_nstrips;
+ u64 ext_size = min(fei->fie_nbytes, meta->file_size);
+
+ if (!IS_ALIGNED(chunk_size, PMD_SIZE)) {
+ pr_err("%s: chunk_size %lld not PMD-aligned\n",
+ __func__, meta->ie[i].fie_chunk_size);
+ return -EINVAL;
+ }
+ if (ext_size == 0) {
+ pr_err("%s: ext_size=%lld file_size=%ld\n",
+ __func__, fei->fie_nbytes, meta->file_size);
+ goto err_out;
+ }
+
+ /* Is the data is in this striped extent? */
+ if (local_offset < ext_size) {
+ u64 chunk_num = local_offset / chunk_size;
+ u64 chunk_offset = local_offset % chunk_size;
+ u64 chunk_remainder = chunk_size - chunk_offset;
+ u64 stripe_num = chunk_num / nstrips;
+ u64 strip_num = chunk_num % nstrips;
+ u64 strip_offset = chunk_offset + (stripe_num * chunk_size);
+ u64 strip_dax_ofs = fei->ie_strips[strip_num].ext_offset;
+ u64 strip_devidx = fei->ie_strips[strip_num].dev_index;
+
+ if (strip_devidx >= fc->dax_devlist->nslots) {
+ pr_err("%s: strip_devidx %llu >= nslots %d\n",
+ __func__, strip_devidx,
+ fc->dax_devlist->nslots);
+ goto err_out;
+ }
+
+ if (!fc->dax_devlist->devlist[strip_devidx].valid) {
+ pr_err("%s: daxdev=%lld invalid\n", __func__,
+ strip_devidx);
+ goto err_out;
+ }
+
+ iomap->addr = strip_dax_ofs + strip_offset;
+ iomap->offset = file_offset;
+ iomap->length = min_t(loff_t, len, chunk_remainder);
+
+ iomap->dax_dev = fc->dax_devlist->devlist[strip_devidx].devp;
+
+ iomap->type = IOMAP_MAPPED;
+ iomap->flags = flags;
+
+ return 0;
+ }
+ local_offset -= ext_size; /* offset is beyond this striped extent */
+ }
+
+ err_out:
+ pr_err("%s: err_out\n", __func__);
+
+ /* We fell out the end of the extent list.
+ * Set iomap to zero length in this case, and return 0
+ * This just means that the r/w is past EOF
+ */
+ iomap->addr = 0; /* there is no valid dax device offset */
+ iomap->offset = file_offset; /* file offset */
+ iomap->length = 0; /* this had better result in no access to dax mem */
+ iomap->dax_dev = NULL;
+ iomap->type = IOMAP_MAPPED;
+ iomap->flags = flags;
+
+ return -EIO;
+}
+
+/**
+ * famfs_fileofs_to_daxofs() - Resolve (file, offset, len) to (daxdev, offset, len)
+ *
+ * This function is called by famfs_fuse_iomap_begin() to resolve an offset in a
+ * file to an offset in a dax device. This is upcalled from dax from calls to
+ * both * dax_iomap_fault() and dax_iomap_rw(). Dax finishes the job resolving
+ * a fault to a specific physical page (the fault case) or doing a memcpy
+ * variant (the rw case)
+ *
+ * Pages can be PTE (4k), PMD (2MiB) or (theoretically) PuD (1GiB)
+ * (these sizes are for X86; may vary on other cpu architectures
+ *
+ * @inode: The file where the fault occurred
+ * @iomap: To be filled in to indicate where to find the right memory,
+ * relative to a dax device.
+ * @file_offset: Within the file where the fault occurred (will be page boundary)
+ * @len: The length of the faulted mapping (will be a page multiple)
+ * (will be trimmed in *iomap if it's disjoint in the extent list)
+ * @flags: flags passed to famfs_fuse_iomap_begin(), and sent back via
+ * struct iomap
+ *
+ * Return values: 0. (info is returned in a modified @iomap struct)
+ */
+static int
+famfs_fileofs_to_daxofs(struct inode *inode, struct iomap *iomap,
+ loff_t file_offset, off_t len, unsigned int flags)
+{
+ struct fuse_inode *fi = get_fuse_inode(inode);
+ struct famfs_file_meta *meta = fi->famfs_meta;
+ struct fuse_conn *fc = get_fuse_conn(inode);
+ loff_t local_offset = file_offset;
+
+ if (!fc->dax_devlist) {
+ pr_err("%s: null dax_devlist\n", __func__);
+ goto err_out;
+ }
+
+ if (famfs_file_bad(inode))
+ goto err_out;
+
+ if (meta->fm_extent_type == INTERLEAVED_EXTENT)
+ return famfs_interleave_fileofs_to_daxofs(inode, iomap,
+ file_offset,
+ len, flags);
+
+ iomap->offset = file_offset;
+
+ for (int i = 0; i < meta->fm_nextents; i++) {
+ /* TODO: check devindex too */
+ loff_t dax_ext_offset = meta->se[i].ext_offset;
+ loff_t dax_ext_len = meta->se[i].ext_len;
+ u64 daxdev_idx = meta->se[i].dev_index;
+
+
+ /* TODO: test that superblock and log offsets only happen
+ * with superblock and log files. Requires instrumentaiton
+ * from user space...
+ */
+
+ /* local_offset is the offset minus the size of extents skipped
+ * so far; If local_offset < dax_ext_len, the data of interest
+ * starts in this extent
+ */
+ if (local_offset < dax_ext_len) {
+ loff_t ext_len_remainder = dax_ext_len - local_offset;
+ struct famfs_daxdev *dd;
+
+ if (daxdev_idx >= fc->dax_devlist->nslots) {
+ pr_err("%s: daxdev_idx %llu >= nslots %d\n",
+ __func__, daxdev_idx,
+ fc->dax_devlist->nslots);
+ goto err_out;
+ }
+
+ dd = &fc->dax_devlist->devlist[daxdev_idx];
+
+ if (!dd->valid || dd->error) {
+ pr_err("%s: daxdev=%lld %s\n", __func__,
+ daxdev_idx,
+ dd->valid ? "error" : "invalid");
+ goto err_out;
+ }
+
+ /*
+ * OK, we found the file metadata extent where this
+ * data begins
+ * @local_offset - The offset within the current
+ * extent
+ * @ext_len_remainder - Remaining length of ext after
+ * skipping local_offset
+ * Outputs:
+ * iomap->addr: the offset within the dax device where
+ * the data starts
+ * iomap->offset: the file offset
+ * iomap->length: the valid length resolved here
+ */
+ iomap->addr = dax_ext_offset + local_offset;
+ iomap->offset = file_offset;
+ iomap->length = min_t(loff_t, len, ext_len_remainder);
+
+ iomap->dax_dev = fc->dax_devlist->devlist[daxdev_idx].devp;
+
+ iomap->type = IOMAP_MAPPED;
+ iomap->flags = flags;
+ return 0;
+ }
+ local_offset -= dax_ext_len; /* Get ready for the next extent */
+ }
+
+ err_out:
+ pr_err("%s: err_out\n", __func__);
+
+ /* We fell out the end of the extent list.
+ * Set iomap to zero length in this case, and return 0
+ * This just means that the r/w is past EOF
+ */
+ iomap->addr = 0; /* there is no valid dax device offset */
+ iomap->offset = file_offset; /* file offset */
+ iomap->length = 0; /* this had better result in no access to dax mem */
+ iomap->dax_dev = NULL;
+ iomap->type = IOMAP_MAPPED;
+ iomap->flags = flags;
+
+ return -EIO;
+}
+
+/**
+ * famfs_fuse_iomap_begin() - Handler for iomap_begin upcall from dax
+ *
+ * This function is pretty simple because files are
+ * * never partially allocated
+ * * never have holes (never sparse)
+ * * never "allocate on write"
+ *
+ * @inode: inode for the file being accessed
+ * @offset: offset within the file
+ * @length: Length being accessed at offset
+ * @flags: flags to be retured via struct iomap
+ * @iomap: iomap struct to be filled in, resolving (offset, length) to
+ * (daxdev, offset, len)
+ * @srcmap: source mapping if it is a COW operation (which it is not here)
+ */
+static int
+famfs_fuse_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
+ unsigned int flags, struct iomap *iomap, struct iomap *srcmap)
+{
+ struct fuse_inode *fi = get_fuse_inode(inode);
+ struct famfs_file_meta *meta = fi->famfs_meta;
+ size_t size;
+
+ size = i_size_read(inode);
+
+ WARN_ON(size != meta->file_size);
+
+ return famfs_fileofs_to_daxofs(inode, iomap, offset, length, flags);
+}
+
+/* Note: We never need a special set of write_iomap_ops because famfs never
+ * performs allocation on write.
+ */
+const struct iomap_ops famfs_iomap_ops = {
+ .iomap_begin = famfs_fuse_iomap_begin,
+};
+
+/*********************************************************************
+ * vm_operations
+ */
+static vm_fault_t
+__famfs_fuse_filemap_fault(struct vm_fault *vmf, unsigned int pe_size,
+ bool write_fault)
+{
+ struct inode *inode = file_inode(vmf->vma->vm_file);
+ vm_fault_t ret;
+ unsigned long pfn;
+
+ if (!IS_DAX(file_inode(vmf->vma->vm_file))) {
+ pr_err("%s: file not marked IS_DAX!!\n", __func__);
+ return VM_FAULT_SIGBUS;
+ }
+
+ if (write_fault) {
+ sb_start_pagefault(inode->i_sb);
+ file_update_time(vmf->vma->vm_file);
+ }
+
+ ret = dax_iomap_fault(vmf, pe_size, &pfn, NULL, &famfs_iomap_ops);
+ if (ret & VM_FAULT_NEEDDSYNC)
+ ret = dax_finish_sync_fault(vmf, pe_size, pfn);
+
+ if (write_fault)
+ sb_end_pagefault(inode->i_sb);
+
+ return ret;
+}
+
+static inline bool
+famfs_is_write_fault(struct vm_fault *vmf)
+{
+ return (vmf->flags & FAULT_FLAG_WRITE) &&
+ (vmf->vma->vm_flags & VM_SHARED);
+}
+
+static vm_fault_t
+famfs_filemap_fault(struct vm_fault *vmf)
+{
+ return __famfs_fuse_filemap_fault(vmf, 0, famfs_is_write_fault(vmf));
+}
+
+static vm_fault_t
+famfs_filemap_huge_fault(struct vm_fault *vmf, unsigned int pe_size)
+{
+ return __famfs_fuse_filemap_fault(vmf, pe_size,
+ famfs_is_write_fault(vmf));
+}
+
+static vm_fault_t
+famfs_filemap_mkwrite(struct vm_fault *vmf)
+{
+ return __famfs_fuse_filemap_fault(vmf, 0, true);
+}
+
+const struct vm_operations_struct famfs_file_vm_ops = {
+ .fault = famfs_filemap_fault,
+ .huge_fault = famfs_filemap_huge_fault,
+ .map_pages = filemap_map_pages,
+ .page_mkwrite = famfs_filemap_mkwrite,
+ .pfn_mkwrite = famfs_filemap_mkwrite,
+};
+
+/*********************************************************************
+ * file_operations
+ */
+
+/**
+ * famfs_file_bad() - Check for files that aren't in a valid state
+ *
+ * @inode: inode
+ *
+ * Returns: 0=success
+ * -errno=failure
+ */
+static int
+famfs_file_bad(struct inode *inode)
+{
+ struct fuse_inode *fi = get_fuse_inode(inode);
+ struct famfs_file_meta *meta = fi->famfs_meta;
+ size_t i_size = i_size_read(inode);
+
+ if (!meta) {
+ pr_err("%s: un-initialized famfs file\n", __func__);
+ return -EIO;
+ }
+ if (meta->error) {
+ pr_debug("%s: previously detected metadata errors\n", __func__);
+ return -EIO;
+ }
+ if (i_size != meta->file_size) {
+ pr_warn("%s: i_size overwritten from %ld to %ld\n",
+ __func__, meta->file_size, i_size);
+ meta->error = true;
+ return -ENXIO;
+ }
+ if (!IS_DAX(inode)) {
+ pr_debug("%s: inode %llx IS_DAX is false\n",
+ __func__, (u64)inode);
+ return -ENXIO;
+ }
+ return 0;
+}
+
+static ssize_t
+famfs_fuse_rw_prep(struct kiocb *iocb, struct iov_iter *ubuf)
+{
+ struct inode *inode = iocb->ki_filp->f_mapping->host;
+ size_t i_size = i_size_read(inode);
+ size_t count = iov_iter_count(ubuf);
+ size_t max_count;
+ ssize_t rc;
+
+ rc = famfs_file_bad(inode);
+ if (rc)
+ return (ssize_t)rc;
+
+ /* Avoid unsigned underflow if position is past EOF */
+ if (iocb->ki_pos >= i_size)
+ max_count = 0;
+ else
+ max_count = i_size - iocb->ki_pos;
+
+ if (count > max_count)
+ iov_iter_truncate(ubuf, max_count);
+
+ if (!iov_iter_count(ubuf))
+ return 0;
+
+ return rc;
+}
+
+ssize_t
+famfs_fuse_read_iter(struct kiocb *iocb, struct iov_iter *to)
+{
+ ssize_t rc;
+
+ rc = famfs_fuse_rw_prep(iocb, to);
+ if (rc)
+ return rc;
+
+ if (!iov_iter_count(to))
+ return 0;
+
+ rc = dax_iomap_rw(iocb, to, &famfs_iomap_ops);
+
+ file_accessed(iocb->ki_filp);
+ return rc;
+}
+
+ssize_t
+famfs_fuse_write_iter(struct kiocb *iocb, struct iov_iter *from)
+{
+ ssize_t rc;
+
+ rc = famfs_fuse_rw_prep(iocb, from);
+ if (rc)
+ return rc;
+
+ if (!iov_iter_count(from))
+ return 0;
+
+ return dax_iomap_rw(iocb, from, &famfs_iomap_ops);
+}
+
+int
+famfs_fuse_mmap(struct file *file, struct vm_area_struct *vma)
+{
+ struct inode *inode = file_inode(file);
+ ssize_t rc;
+
+ rc = famfs_file_bad(inode);
+ if (rc)
+ return rc;
+
+ file_accessed(file);
+ vma->vm_ops = &famfs_file_vm_ops;
+ vm_flags_set(vma, VM_HUGEPAGE);
+ return 0;
+}
+
#define FMAP_BUFSIZE PAGE_SIZE

int
diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index 1f64bf68b5ee..45a09a7f0012 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -1831,6 +1831,8 @@ static ssize_t fuse_file_read_iter(struct kiocb *iocb, struct iov_iter *to)

if (FUSE_IS_VIRTIO_DAX(fi))
return fuse_dax_read_iter(iocb, to);
+ if (fuse_file_famfs(fi))
+ return famfs_fuse_read_iter(iocb, to);

/* FOPEN_DIRECT_IO overrides FOPEN_PASSTHROUGH */
if (ff->open_flags & FOPEN_DIRECT_IO)
@@ -1853,6 +1855,8 @@ static ssize_t fuse_file_write_iter(struct kiocb *iocb, struct iov_iter *from)

if (FUSE_IS_VIRTIO_DAX(fi))
return fuse_dax_write_iter(iocb, from);
+ if (fuse_file_famfs(fi))
+ return famfs_fuse_write_iter(iocb, from);

/* FOPEN_DIRECT_IO overrides FOPEN_PASSTHROUGH */
if (ff->open_flags & FOPEN_DIRECT_IO)
@@ -1868,9 +1872,13 @@ static ssize_t fuse_splice_read(struct file *in, loff_t *ppos,
unsigned int flags)
{
struct fuse_file *ff = in->private_data;
+ struct inode *inode = file_inode(in);
+ struct fuse_inode *fi = get_fuse_inode(inode);

/* FOPEN_DIRECT_IO overrides FOPEN_PASSTHROUGH */
- if (fuse_file_passthrough(ff) && !(ff->open_flags & FOPEN_DIRECT_IO))
+ if (fuse_file_famfs(fi))
+ return -EIO; /* famfs does not use the page cache... */
+ else if (fuse_file_passthrough(ff) && !(ff->open_flags & FOPEN_DIRECT_IO))
return fuse_passthrough_splice_read(in, ppos, pipe, len, flags);
else
return filemap_splice_read(in, ppos, pipe, len, flags);
@@ -1880,9 +1888,13 @@ static ssize_t fuse_splice_write(struct pipe_inode_info *pipe, struct file *out,
loff_t *ppos, size_t len, unsigned int flags)
{
struct fuse_file *ff = out->private_data;
+ struct inode *inode = file_inode(out);
+ struct fuse_inode *fi = get_fuse_inode(inode);

/* FOPEN_DIRECT_IO overrides FOPEN_PASSTHROUGH */
- if (fuse_file_passthrough(ff) && !(ff->open_flags & FOPEN_DIRECT_IO))
+ if (fuse_file_famfs(fi))
+ return -EIO; /* famfs does not use the page cache... */
+ else if (fuse_file_passthrough(ff) && !(ff->open_flags & FOPEN_DIRECT_IO))
return fuse_passthrough_splice_write(pipe, out, ppos, len, flags);
else
return iter_file_splice_write(pipe, out, ppos, len, flags);
@@ -2390,6 +2402,8 @@ static int fuse_file_mmap(struct file *file, struct vm_area_struct *vma)
/* DAX mmap is superior to direct_io mmap */
if (FUSE_IS_VIRTIO_DAX(fi))
return fuse_dax_mmap(file, vma);
+ if (fuse_file_famfs(fi))
+ return famfs_fuse_mmap(file, vma);

/*
* If inode is in passthrough io mode, because it has some file open
diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
index 83e24cee994b..f5548466c2b2 100644
--- a/fs/fuse/fuse_i.h
+++ b/fs/fuse/fuse_i.h
@@ -1650,6 +1650,9 @@ extern void fuse_sysctl_unregister(void);
int famfs_file_init_dax(struct fuse_mount *fm,
struct inode *inode, void *fmap_buf,
size_t fmap_size);
+ssize_t famfs_fuse_write_iter(struct kiocb *iocb, struct iov_iter *from);
+ssize_t famfs_fuse_read_iter(struct kiocb *iocb, struct iov_iter *to);
+int famfs_fuse_mmap(struct file *file, struct vm_area_struct *vma);
void __famfs_meta_free(void *map);

void famfs_teardown(struct fuse_conn *fc);
@@ -1692,6 +1695,22 @@ int fuse_get_fmap(struct fuse_mount *fm, struct inode *inode);
static inline void famfs_teardown(struct fuse_conn *fc)
{
}
+static inline ssize_t famfs_fuse_write_iter(struct kiocb *iocb,
+ struct iov_iter *to)
+{
+ return -ENODEV;
+}
+static inline ssize_t famfs_fuse_read_iter(struct kiocb *iocb,
+ struct iov_iter *to)
+{
+ return -ENODEV;
+}
+static inline int famfs_fuse_mmap(struct file *file,
+ struct vm_area_struct *vma)
+{
+ return -ENODEV;
+}
+
static inline struct fuse_backing *famfs_meta_set(struct fuse_inode *fi,
void *meta)
{
--
2.52.0