[RFC PATCH 13/20] famfs: Add iomap_ops

John Groves
2024-02-23T17:42:56Z

This commit introduces the famfs iomap_ops. When either
dax_iomap_fault() or dax_iomap_rw() is called, we get a callback
via our iomap_begin() handler. The question being asked is
"please resolve (file, offset) to (daxdev, offset)". The function
famfs_meta_to_dax_offset() does this.

The per-file metadata is just an extent list to the
backing dax dev. The order of this resolution is O(N) for N
extents. Note with the current user space, files usually have
only one extent.

Signed-off-by: John Groves <john@groves.net>
---
fs/famfs/famfs_file.c | 245 ++++++++++++++++++++++++++++++++++++++++++
1 file changed, 245 insertions(+)
create mode 100644 fs/famfs/famfs_file.c

diff --git a/fs/famfs/famfs_file.c b/fs/famfs/famfs_file.c
new file mode 100644
index 000000000000..fc667d5f7be8
--- /dev/null
+++ b/fs/famfs/famfs_file.c
@@ -0,0 +1,245 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * famfs - dax file system for shared fabric-attached memory
+ *
+ * Copyright 2023-2024 Micron Technology, Inc.
+ *
+ * This file system, originally based on ramfs the dax support from xfs,
+ * is intended to allow multiple host systems to mount a common file system
+ * view of dax files that map to shared memory.
+ */
+
+#include <linux/module.h>
+#include <linux/fs.h>
+#include <linux/mm.h>
+#include <linux/sched.h>
+#include <linux/dax.h>
+#include <linux/uio.h>
+#include <linux/iomap.h>
+#include <uapi/linux/famfs_ioctl.h>
+#include "famfs_internal.h"
+
+/*********************************************************************
+ * iomap_operations
+ *
+ * This stuff uses the iomap (dax-related) helpers to resolve file offsets to
+ * offsets within a dax device.
+ */
+
+/**
+ * famfs_meta_to_dax_offset()
+ *
+ * This function is called by famfs_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 - struct iomap to be filled in to indicate where to find the right memory, relative
+ * to a dax device.
+ * @offset - the 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
+ */
+static int
+famfs_meta_to_dax_offset(
+ struct inode *inode,
+ struct iomap *iomap,
+ loff_t offset,
+ loff_t len,
+ unsigned int flags)
+{
+ struct famfs_file_meta *meta = (struct famfs_file_meta *)inode->i_private;
+ int i;
+ loff_t local_offset = offset;
+ struct famfs_fs_info *fsi = inode->i_sb->s_fs_info;
+
+ iomap->offset = offset; /* file offset */
+
+ for (i = 0; i < meta->tfs_extent_ct; i++) {
+ loff_t dax_ext_offset = meta->tfs_extents[i].offset;
+ loff_t dax_ext_len = meta->tfs_extents[i].len;
+
+ if ((dax_ext_offset == 0) && (meta->file_type != FAMFS_SUPERBLOCK))
+ pr_err("%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;
+
+ /*+
+ * 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
+ *
+ * iomap->addr is the offset within the dax device where that data
+ * starts
+ */
+ iomap->addr = dax_ext_offset + local_offset; /* dax dev offset */
+ iomap->offset = offset; /* file offset */
+ iomap->length = min_t(loff_t, len, ext_len_remainder);
+ iomap->dax_dev = fsi->dax_devp;
+ iomap->type = IOMAP_MAPPED;
+ iomap->flags = flags;
+
+ return 0;
+ }
+ local_offset -= dax_ext_len; /* Get ready for the next extent */
+ }
+
+ /* Set iomap to zero length in this case, and return 0
+ * This just means that the r/w is past EOF
+ */
+ iomap->addr = offset;
+ iomap->offset = offset; /* file offset */
+ iomap->length = 0; /* this had better result in no access to dax mem */
+ iomap->dax_dev = fsi->dax_devp;
+ iomap->type = IOMAP_MAPPED;
+ iomap->flags = flags;
+
+ return 0;
+}
+
+/**
+ * famfs_iomap_begin()
+ *
+ * This function is pretty simple because files are
+ * * never partially allocated
+ * * never have holes (never sparse)
+ * * never "allocate on write"
+ */
+static int
+famfs_iomap_begin(
+ struct inode *inode,
+ loff_t offset,
+ loff_t length,
+ unsigned int flags,
+ struct iomap *iomap,
+ struct iomap *srcmap)
+{
+ struct famfs_file_meta *meta = inode->i_private;
+ size_t size;
+ int rc;
+
+ size = i_size_read(inode);
+
+ WARN_ON(size != meta->file_size);
+
+ rc = famfs_meta_to_dax_offset(inode, iomap, offset, length, flags);
+
+ return rc;
+}
+
+/* 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_iomap_begin,
+};
+
+/*********************************************************************
+ * vm_operations
+ */
+static vm_fault_t
+__famfs_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;
+
+ if (write_fault) {
+ sb_start_pagefault(inode->i_sb);
+ file_update_time(vmf->vma->vm_file);
+ }
+
+ if (IS_DAX(inode)) {
+ pfn_t pfn;
+
+ 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);
+ } else {
+ /* All famfs faults will be dax... */
+ pr_err("%s: oops, non-dax fault\n", __func__);
+ ret = VM_FAULT_SIGBUS;
+ }
+
+ 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)
+{
+ /* DAX can shortcut the normal fault path on write faults! */
+ return __famfs_filemap_fault(vmf, 0,
+ IS_DAX(file_inode(vmf->vma->vm_file)) && famfs_is_write_fault(vmf));
+}
+
+static vm_fault_t
+famfs_filemap_huge_fault(
+ struct vm_fault *vmf,
+ unsigned int pe_size)
+{
+ if (!IS_DAX(file_inode(vmf->vma->vm_file))) {
+ pr_err("%s: file not marked IS_DAX!!\n", __func__);
+ return VM_FAULT_SIGBUS;
+ }
+
+ /* DAX can shortcut the normal fault path on write faults! */
+ return __famfs_filemap_fault(vmf, pe_size, famfs_is_write_fault(vmf));
+}
+
+static vm_fault_t
+famfs_filemap_page_mkwrite(
+ struct vm_fault *vmf)
+{
+ return __famfs_filemap_fault(vmf, 0, true);
+}
+
+static vm_fault_t
+famfs_filemap_pfn_mkwrite(
+ struct vm_fault *vmf)
+{
+ return __famfs_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)
+{
+ vm_fault_t ret;
+
+ ret = filemap_map_pages(vmf, start_pgoff, end_pgoff);
+ return ret;
+}
+
+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,
+};
+
--
2.43.0