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Message-ID: <gkasmy2ntypw725tlnxbrclrax43h44jcc5lyak3ezjtawjezl@66ychtusd3pr>
Date: Sat, 5 Jul 2025 14:44:19 -0500
From: John Groves <John@...ves.net>
To: Amir Goldstein <amir73il@...il.com>
Cc: Dan Williams <dan.j.williams@...el.com>,
Bernd Schubert <bschubert@....com>, John Groves <jgroves@...ron.com>,
Jonathan Corbet <corbet@....net>, Vishal Verma <vishal.l.verma@...el.com>,
Dave Jiang <dave.jiang@...el.com>, Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>,
Alexander Viro <viro@...iv.linux.org.uk>, Christian Brauner <brauner@...nel.org>,
"Darrick J . Wong" <djwong@...nel.org>, Randy Dunlap <rdunlap@...radead.org>,
Jeff Layton <jlayton@...nel.org>, Kent Overstreet <kent.overstreet@...ux.dev>,
linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org, nvdimm@...ts.linux.dev,
linux-cxl@...r.kernel.org, linux-fsdevel@...r.kernel.org,
Jonathan Cameron <Jonathan.Cameron@...wei.com>, Stefan Hajnoczi <shajnocz@...hat.com>,
Joanne Koong <joannelkoong@...il.com>, Josef Bacik <josef@...icpanda.com>,
Aravind Ramesh <arramesh@...ron.com>, Ajay Joshi <ajayjoshi@...ron.com>,
Miklos Szeredi <miklos@...redi.hu>
Subject: Re: [RFC V2 15/18] famfs_fuse: Plumb dax iomap and fuse
read/write/mmap
On 25/07/04 11:13AM, Amir Goldstein wrote:
> On Thu, Jul 3, 2025 at 8:51 PM John Groves <John@...ves.net> wrote:
> >
> > 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.
> >
> > Signed-off-by: John Groves <john@...ves.net>
> > ---
> > fs/fuse/famfs.c | 436 +++++++++++++++++++++++++++++++++++++++++++++++
> > fs/fuse/file.c | 14 ++
> > fs/fuse/fuse_i.h | 3 +
> > 3 files changed, 453 insertions(+)
> >
> > diff --git a/fs/fuse/famfs.c b/fs/fuse/famfs.c
> > index f5e01032b825..1973eb10b60b 100644
> > --- a/fs/fuse/famfs.c
> > +++ b/fs/fuse/famfs.c
> > @@ -585,3 +585,439 @@ 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 ssize_t 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;
> > + int i;
> > +
> > + /* 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 (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 = fei->fie_nbytes;
> > +
> > + ext_size = min_t(u64, ext_size, meta->file_size);
> > +
> > + 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 stripe_num = chunk_num / nstrips;
> > + u64 strip_num = chunk_num % nstrips;
> > + u64 chunk_remainder = chunk_size - chunk_offset;
> > + 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 (!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 0;
> > +}
> > +
> > +/**
> > + * 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:
> > + *
> > + * 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;
> > + int i;
> > +
> > + 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 (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;
> > +
> > + if ((dax_ext_offset == 0) &&
> > + (meta->file_type != FAMFS_SUPERBLOCK))
> > + pr_warn("%s: zero offset on non-superblock file!!\n",
> > + __func__);
> > +
> > + /* 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;
> > +
> > + 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 0;
> > +}
> > +
> > +/**
> > + * 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:
> > + * @iomap: iomap struct to be filled in, resolving (offset, length) to
> > + * (daxdev, offset, len)
> > + * @srcmap:
> > + */
> > +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;
> > + pfn_t 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_page_mkwrite(struct vm_fault *vmf)
> > +{
> > + return __famfs_fuse_filemap_fault(vmf, 0, true);
> > +}
> > +
> > +static vm_fault_t
> > +famfs_filemap_pfn_mkwrite(struct vm_fault *vmf)
> > +{
> > + return __famfs_fuse_filemap_fault(vmf, 0, true);
> > +}
> > +
> > +static vm_fault_t
> > +famfs_filemap_map_pages(struct vm_fault *vmf, pgoff_t start_pgoff,
> > + pgoff_t end_pgoff)
> > +{
> > + return filemap_map_pages(vmf, start_pgoff, end_pgoff);
> > +}
> > +
> > +const struct vm_operations_struct famfs_file_vm_ops = {
> > + .fault = famfs_filemap_fault,
> > + .huge_fault = famfs_filemap_huge_fault,
> > + .map_pages = famfs_filemap_map_pages,
> > + .page_mkwrite = famfs_filemap_page_mkwrite,
> > + .pfn_mkwrite = famfs_filemap_pfn_mkwrite,
> > +};
> > +
> > +/*********************************************************************
> > + * file_operations
> > + */
> > +
> > +/**
> > + * famfs_file_bad() - Check for files that aren't in a valid state
> > + *
> > + * @inode - inode
> > + *
> > + * Returns: 0=success
> > + * -errno=failure
> > + */
> > +static ssize_t
> > +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 rc;
> > +
> > + max_count = max_t(size_t, 0, 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 (int)rc;
> > +
> > + file_accessed(file);
> > + vma->vm_ops = &famfs_file_vm_ops;
> > + vm_flags_set(vma, VM_HUGEPAGE);
> > + return 0;
> > +}
> > diff --git a/fs/fuse/file.c b/fs/fuse/file.c
> > index 5d205eadb48f..24a14b176510 100644
> > --- a/fs/fuse/file.c
> > +++ b/fs/fuse/file.c
> > @@ -1874,6 +1874,8 @@ 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)
> > @@ -1896,6 +1898,8 @@ 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)
> > @@ -1911,10 +1915,14 @@ 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))
> > return fuse_passthrough_splice_read(in, ppos, pipe, len, flags);
> > + else if (fuse_file_famfs(fi))
> > + return -EIO; /* direct I/O doesn't make sense in dax_iomap */
> > else
> > return filemap_splice_read(in, ppos, pipe, len, flags);
> > }
> > @@ -1923,10 +1931,14 @@ 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))
> > return fuse_passthrough_splice_write(pipe, out, ppos, len, flags);
> > + else if (fuse_file_famfs(fi))
> > + return -EIO; /* direct I/O doesn't make sense in dax_iomap */
> > else
> > return iter_file_splice_write(pipe, out, ppos, len, flags);
> > }
>
> This looks odd.
>
> Usually, the methods first check for FUSE_IS_VIRTIO_DAX() and
> fuse_file_famfs() to get this condition out of the way so I never needed
> to think about whether or not the code verifies that fuse_file_passthrough()
> and fuse_file_famfs() cannot co-exist.
>
> Is there a reason why you did not follow the same pattern here?
I think I just got a little sloppy. I'll do the famfs test first. Unless we
can rule out this path for famfs. But unlike virtiofs, famfs doesn't have
separate file_operations, so I suppose it must be checked.
>
> Also, your comment makes no sense.
> splice is not the case of direct I/O - quite the contrary.
Sorry, brain fart on the comment. Splice doesn't make sense for famfs because
famfs doesn't use the page cache. Will fix the comment too.
>
> Thanks,
> Amir.
Thank you Amir!
John
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