[PATCH 03/15] fs: introduce iomap infrastructure

Christoph Hellwig
2016-05-09T08:47:42Z

Add infrastructure for multipage buffered writes. This is implemented
using an main iterator that applies an actor function to a range that
can be written.

This infrastucture is used to implement a buffered write helper, one
to zero file ranges and one to implement the ->page_mkwrite VM
operations. All of them borrow a fair amount of code from fs/buffers.
for now by using an internal version of __block_write_begin that
gets passed an iomap and builds the corresponding buffer head.

The file system is gets a set of paired ->iomap_begin and ->iomap_end
calls which allow it to map/reserve a range and get a notification
once the write code is finished with it.

Based on earlier code from Dave Chinner.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Bob Peterson <rpeterso@redhat.com>
---
fs/Kconfig | 3 +
fs/Makefile | 1 +
fs/buffer.c | 76 +++++++++-
fs/internal.h | 3 +
fs/iomap.c | 394 ++++++++++++++++++++++++++++++++++++++++++++++++++
include/linux/iomap.h | 56 ++++++-
6 files changed, 523 insertions(+), 10 deletions(-)
create mode 100644 fs/iomap.c

diff --git a/fs/Kconfig b/fs/Kconfig
index 6725f59..276fcfb 100644
--- a/fs/Kconfig
+++ b/fs/Kconfig
@@ -10,6 +10,9 @@ config DCACHE_WORD_ACCESS

if BLOCK

+config FS_IOMAP
+ bool
+
source "fs/ext2/Kconfig"
source "fs/ext4/Kconfig"
source "fs/jbd2/Kconfig"
diff --git a/fs/Makefile b/fs/Makefile
index 85b6e13..ed2b632 100644
--- a/fs/Makefile
+++ b/fs/Makefile
@@ -49,6 +49,7 @@ obj-$(CONFIG_COREDUMP) += coredump.o
obj-$(CONFIG_SYSCTL) += drop_caches.o

obj-$(CONFIG_FHANDLE) += fhandle.o
+obj-$(CONFIG_FS_IOMAP) += iomap.o

obj-y += quota/

diff --git a/fs/buffer.c b/fs/buffer.c
index 33be296..d4429c3 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -21,6 +21,7 @@
#include <linux/kernel.h>
#include <linux/syscalls.h>
#include <linux/fs.h>
+#include <linux/iomap.h>
#include <linux/mm.h>
#include <linux/percpu.h>
#include <linux/slab.h>
@@ -1891,8 +1892,62 @@ void page_zero_new_buffers(struct page *page, unsigned from, unsigned to)
}
EXPORT_SYMBOL(page_zero_new_buffers);

-int __block_write_begin(struct page *page, loff_t pos, unsigned len,
- get_block_t *get_block)
+static void
+iomap_to_bh(struct inode *inode, sector_t block, struct buffer_head *bh,
+ struct iomap *iomap)
+{
+ loff_t offset = block << inode->i_blkbits;
+
+ bh->b_bdev = iomap->bdev;
+
+ /*
+ * Block points to offset in file we need to map, iomap contains
+ * the offset at which the map starts. If the map ends before the
+ * current block, then do not map the buffer and let the caller
+ * handle it.
+ */
+ BUG_ON(offset >= iomap->offset + iomap->length);
+
+ switch (iomap->type) {
+ case IOMAP_HOLE:
+ /*
+ * If the buffer is not up to date or beyond the current EOF,
+ * we need to mark it as new to ensure sub-block zeroing is
+ * executed if necessary.
+ */
+ if (!buffer_uptodate(bh) ||
+ (offset >= i_size_read(inode)))
+ set_buffer_new(bh);
+ break;
+ case IOMAP_DELALLOC:
+ if (!buffer_uptodate(bh) ||
+ (offset >= i_size_read(inode)))
+ set_buffer_new(bh);
+ set_buffer_uptodate(bh);
+ set_buffer_mapped(bh);
+ set_buffer_delay(bh);
+ break;
+ case IOMAP_UNWRITTEN:
+ /*
+ * For unwritten regions, we always need to ensure that
+ * sub-block writes cause the regions in the block we are not
+ * writing to are zeroed. Set the buffer as new to ensure this.
+ */
+ set_buffer_new(bh);
+ set_buffer_unwritten(bh);
+ /* FALLTHRU */
+ case IOMAP_MAPPED:
+ if (offset >= i_size_read(inode))
+ set_buffer_new(bh);
+ bh->b_blocknr = (iomap->blkno >> (inode->i_blkbits - 9)) +
+ ((offset - iomap->offset) >> inode->i_blkbits);
+ set_buffer_mapped(bh);
+ break;
+ }
+}
+
+int __block_write_begin_int(struct page *page, loff_t pos, unsigned len,
+ get_block_t *get_block, struct iomap *iomap)
{
unsigned from = pos & (PAGE_CACHE_SIZE - 1);
unsigned to = from + len;
@@ -1928,9 +1983,14 @@ int __block_write_begin(struct page *page, loff_t pos, unsigned len,
clear_buffer_new(bh);
if (!buffer_mapped(bh)) {
WARN_ON(bh->b_size != blocksize);
- err = get_block(inode, block, bh, 1);
- if (err)
- break;
+ if (get_block) {
+ err = get_block(inode, block, bh, 1);
+ if (err)
+ break;
+ } else {
+ iomap_to_bh(inode, block, bh, iomap);
+ }
+
if (buffer_new(bh)) {
unmap_underlying_metadata(bh->b_bdev,
bh->b_blocknr);
@@ -1971,6 +2031,12 @@ int __block_write_begin(struct page *page, loff_t pos, unsigned len,
page_zero_new_buffers(page, from, to);
return err;
}
+
+int __block_write_begin(struct page *page, loff_t pos, unsigned len,
+ get_block_t *get_block)
+{
+ return __block_write_begin_int(page, pos, len, get_block, NULL);
+}
EXPORT_SYMBOL(__block_write_begin);

static int __block_commit_write(struct inode *inode, struct page *page,
diff --git a/fs/internal.h b/fs/internal.h
index b71deee..c0c6f49 100644
--- a/fs/internal.h
+++ b/fs/internal.h
@@ -11,6 +11,7 @@

struct super_block;
struct file_system_type;
+struct iomap;
struct linux_binprm;
struct path;
struct mount;
@@ -39,6 +40,8 @@ static inline int __sync_blockdev(struct block_device *bdev, int wait)
* buffer.c
*/
extern void guard_bio_eod(int rw, struct bio *bio);
+extern int __block_write_begin_int(struct page *page, loff_t pos, unsigned len,
+ get_block_t *get_block, struct iomap *iomap);

/*
* char_dev.c
diff --git a/fs/iomap.c b/fs/iomap.c
new file mode 100644
index 0000000..fac9285
--- /dev/null
+++ b/fs/iomap.c
@@ -0,0 +1,394 @@
+/*
+ * Copyright (C) 2010 Red Hat, Inc.
+ * Copyright (c) 2016 Christoph Hellwig.
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms and conditions of the GNU General Public License,
+ * version 2, as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
+ * more details.
+ */
+#include <linux/module.h>
+#include <linux/compiler.h>
+#include <linux/fs.h>
+#include <linux/iomap.h>
+#include <linux/uaccess.h>
+#include <linux/gfp.h>
+#include <linux/mm.h>
+#include <linux/swap.h>
+#include <linux/pagemap.h>
+#include <linux/file.h>
+#include <linux/uio.h>
+#include <linux/backing-dev.h>
+#include <linux/buffer_head.h>
+#include "internal.h"
+
+typedef loff_t (*iomap_actor_t)(struct inode *inode, loff_t pos, loff_t len,
+ void *data, struct iomap *iomap);
+
+/*
+ * Execute a iomap write on a segment of the mapping that spans a
+ * contiguous range of pages that have identical block mapping state.
+ *
+ * This avoids the need to map pages individually, do individual allocations
+ * for each page and most importantly avoid the need for filesystem specific
+ * locking per page. Instead, all the operations are amortised over the entire
+ * range of pages. It is assumed that the filesystems will lock whatever
+ * resources they require in the iomap_begin call, and release them in the
+ * iomap_end call.
+ */
+static loff_t
+iomap_apply(struct inode *inode, loff_t pos, loff_t length, unsigned flags,
+ struct iomap_ops *ops, void *data, iomap_actor_t actor)
+{
+ struct iomap iomap = { 0 };
+ loff_t written = 0, ret;
+
+ /*
+ * Need to map a range from start position for count bytes. This can
+ * span multiple pages - it is only guaranteed to return a range of a
+ * single type of pages (e.g. all into a hole, all mapped or all
+ * unwritten). Failure at this point has nothing to undo.
+ *
+ * If allocation is required for this range, reserve the space now so
+ * that the allocation is guaranteed to succeed later on. Once we copy
+ * the data into the page cache pages, then we cannot fail otherwise we
+ * expose transient stale data. If the reserve fails, we can safely
+ * back out at this point as there is nothing to undo.
+ */
+ ret = ops->iomap_begin(inode, pos, length, flags, &iomap);
+ if (ret)
+ return ret;
+ if (WARN_ON(iomap.offset > pos))
+ return -EIO;
+
+ /*
+ * Cut down the length to the one actually provided by the filesystem,
+ * as it might not be able to give us the whole size that we requested.
+ */
+ if (iomap.offset + iomap.length < pos + length)
+ length = iomap.offset + iomap.length - pos;
+
+ /*
+ * Now that we have guaranteed that the space allocation will succeed.
+ * we can do the copy-in page by page without having to worry about
+ * failures exposing transient data.
+ */
+ written = actor(inode, pos, length, data, &iomap);
+
+ /*
+ * Now the data has been copied, commit the range we've copied. This
+ * should not fail unless the filesystem has had a fatal error.
+ */
+ ret = ops->iomap_end(inode, pos, length, written > 0 ? written : 0,
+ flags, &iomap);
+
+ return written ? written : ret;
+}
+
+static void
+iomap_write_failed(struct inode *inode, loff_t pos, unsigned len)
+{
+ loff_t i_size = i_size_read(inode);
+
+ /*
+ * Only truncate newly allocated pages beyoned EOF, even if the
+ * write started inside the existing inode size.
+ */
+ if (pos + len > i_size)
+ truncate_pagecache_range(inode, max(pos, i_size), pos + len);
+}
+
+static int
+iomap_write_begin(struct inode *inode, loff_t pos, unsigned len, unsigned flags,
+ struct page **pagep, struct iomap *iomap)
+{
+ pgoff_t index = pos >> PAGE_SHIFT;
+ struct page *page;
+ int status = 0;
+
+ BUG_ON(pos + len > iomap->offset + iomap->length);
+
+ page = grab_cache_page_write_begin(inode->i_mapping, index, flags);
+ if (!page)
+ return -ENOMEM;
+
+ status = __block_write_begin_int(page, pos, len, NULL, iomap);
+ if (unlikely(status)) {
+ unlock_page(page);
+ page_cache_release(page);
+ page = NULL;
+
+ iomap_write_failed(inode, pos, len);
+ }
+
+ *pagep = page;
+ return status;
+}
+
+static int
+iomap_write_end(struct inode *inode, loff_t pos, unsigned len,
+ unsigned copied, struct page *page)
+{
+ int ret;
+
+ ret = generic_write_end(NULL, inode->i_mapping, pos, len,
+ copied, page, NULL);
+ if (ret < len)
+ iomap_write_failed(inode, pos, len);
+ return ret;
+}
+
+static loff_t
+iomap_write_actor(struct inode *inode, loff_t pos, loff_t length, void *data,
+ struct iomap *iomap)
+{
+ struct iov_iter *i = data;
+ long status = 0;
+ ssize_t written = 0;
+ unsigned int flags = AOP_FLAG_NOFS;
+
+ /*
+ * Copies from kernel address space cannot fail (NFSD is a big user).
+ */
+ if (!iter_is_iovec(i))
+ flags |= AOP_FLAG_UNINTERRUPTIBLE;
+
+ do {
+ struct page *page;
+ unsigned long offset; /* Offset into pagecache page */
+ unsigned long bytes; /* Bytes to write to page */
+ size_t copied; /* Bytes copied from user */
+
+ offset = (pos & (PAGE_SIZE - 1));
+ bytes = min_t(unsigned long, PAGE_SIZE - offset,
+ iov_iter_count(i));
+again:
+ if (bytes > length)
+ bytes = length;
+
+ /*
+ * Bring in the user page that we will copy from _first_.
+ * Otherwise there's a nasty deadlock on copying from the
+ * same page as we're writing to, without it being marked
+ * up-to-date.
+ *
+ * Not only is this an optimisation, but it is also required
+ * to check that the address is actually valid, when atomic
+ * usercopies are used, below.
+ */
+ if (unlikely(iov_iter_fault_in_readable(i, bytes))) {
+ status = -EFAULT;
+ break;
+ }
+
+ status = iomap_write_begin(inode, pos, bytes, flags, &page,
+ iomap);
+ if (unlikely(status))
+ break;
+
+ if (mapping_writably_mapped(inode->i_mapping))
+ flush_dcache_page(page);
+
+ pagefault_disable();
+ copied = iov_iter_copy_from_user_atomic(page, i, offset, bytes);
+ pagefault_enable();
+
+ flush_dcache_page(page);
+ mark_page_accessed(page);
+
+ status = iomap_write_end(inode, pos, bytes, copied, page);
+ if (unlikely(status < 0))
+ break;
+ copied = status;
+
+ cond_resched();
+
+ iov_iter_advance(i, copied);
+ if (unlikely(copied == 0)) {
+ /*
+ * If we were unable to copy any data at all, we must
+ * fall back to a single segment length write.
+ *
+ * If we didn't fallback here, we could livelock
+ * because not all segments in the iov can be copied at
+ * once without a pagefault.
+ */
+ bytes = min_t(unsigned long, PAGE_SIZE - offset,
+ iov_iter_single_seg_count(i));
+ goto again;
+ }
+ pos += copied;
+ written += copied;
+ length -= copied;
+
+ balance_dirty_pages_ratelimited(inode->i_mapping);
+ } while (iov_iter_count(i) && length);
+
+ return written ? written : status;
+}
+
+ssize_t
+iomap_file_buffered_write(struct kiocb *iocb, struct iov_iter *iter,
+ struct iomap_ops *ops)
+{
+ struct inode *inode = iocb->ki_filp->f_mapping->host;
+ loff_t pos = iocb->ki_pos, ret = 0, written = 0;
+
+ while (iov_iter_count(iter)) {
+ ret = iomap_apply(inode, pos, iov_iter_count(iter),
+ IOMAP_WRITE, ops, iter, iomap_write_actor);
+ if (ret <= 0)
+ break;
+ pos += ret;
+ written += ret;
+ }
+
+ return written ? written : ret;
+}
+EXPORT_SYMBOL_GPL(iomap_file_buffered_write);
+
+static int iomap_zero(struct inode *inode, loff_t pos, unsigned offset,
+ unsigned bytes, struct iomap *iomap)
+{
+ struct page *page;
+ int status;
+
+ status = iomap_write_begin(inode, pos, bytes,
+ AOP_FLAG_UNINTERRUPTIBLE | AOP_FLAG_NOFS, &page, iomap);
+ if (status)
+ return status;
+
+ zero_user(page, offset, bytes);
+ mark_page_accessed(page);
+
+ return iomap_write_end(inode, pos, bytes, bytes, page);
+}
+
+static loff_t
+iomap_zero_range_actor(struct inode *inode, loff_t pos, loff_t count,
+ void *data, struct iomap *iomap)
+{
+ bool *did_zero = data;
+ loff_t written = 0;
+ int status;
+
+ /* already zeroed? we're done. */
+ if (iomap->type == IOMAP_HOLE || iomap->type == IOMAP_UNWRITTEN)
+ return count;
+
+ do {
+ unsigned offset, bytes;
+
+ offset = pos & (PAGE_SIZE - 1); /* Within page */
+ bytes = min_t(unsigned, PAGE_SIZE - offset, count);
+
+ status = iomap_zero(inode, pos, offset, bytes, iomap);
+ if (status < 0)
+ return status;
+
+ pos += bytes;
+ count -= bytes;
+ written += bytes;
+ if (did_zero)
+ *did_zero = true;
+ } while (count > 0);
+
+ return written;
+}
+
+int
+iomap_zero_range(struct inode *inode, loff_t pos, loff_t len, bool *did_zero,
+ struct iomap_ops *ops)
+{
+ loff_t ret;
+
+ while (len > 0) {
+ ret = iomap_apply(inode, pos, len, IOMAP_ZERO,
+ ops, did_zero, iomap_zero_range_actor);
+ if (ret <= 0)
+ return ret;
+
+ pos += ret;
+ len -= ret;
+ }
+
+ return 0;
+}
+EXPORT_SYMBOL_GPL(iomap_zero_range);
+
+int
+iomap_truncate_page(struct inode *inode, loff_t pos, bool *did_zero,
+ struct iomap_ops *ops)
+{
+ unsigned blocksize = (1 << inode->i_blkbits);
+ unsigned off = pos & (blocksize - 1);
+
+ /* Block boundary? Nothing to do */
+ if (!off)
+ return 0;
+ return iomap_zero_range(inode, pos, blocksize - off, did_zero, ops);
+}
+EXPORT_SYMBOL_GPL(iomap_truncate_page);
+
+static loff_t
+iomap_page_mkwrite_actor(struct inode *inode, loff_t pos, loff_t length,
+ void *data, struct iomap *iomap)
+{
+ struct page *page = data;
+ int ret;
+
+ ret = __block_write_begin_int(page, pos & ~PAGE_MASK, length,
+ NULL, iomap);
+ if (ret)
+ return ret;
+
+ block_commit_write(page, 0, length);
+ return length;
+}
+
+int iomap_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf,
+ struct iomap_ops *ops)
+{
+ struct page *page = vmf->page;
+ struct inode *inode = file_inode(vma->vm_file);
+ unsigned long length;
+ loff_t offset, size;
+ ssize_t ret;
+
+ lock_page(page);
+ size = i_size_read(inode);
+ if ((page->mapping != inode->i_mapping) ||
+ (page_offset(page) > size)) {
+ /* We overload EFAULT to mean page got truncated */
+ ret = -EFAULT;
+ goto out_unlock;
+ }
+
+ /* page is wholly or partially inside EOF */
+ if (((page->index + 1) << PAGE_SHIFT) > size)
+ length = size & ~PAGE_MASK;
+ else
+ length = PAGE_SIZE;
+
+ offset = page_offset(page);
+ while (length > 0) {
+ ret = iomap_apply(inode, offset, length, IOMAP_WRITE,
+ ops, page, iomap_page_mkwrite_actor);
+ if (unlikely(ret <= 0))
+ goto out_unlock;
+ offset += ret;
+ length -= ret;
+ }
+
+ set_page_dirty(page);
+ wait_for_stable_page(page);
+ return 0;
+out_unlock:
+ unlock_page(page);
+ return ret;
+}
+EXPORT_SYMBOL_GPL(iomap_page_mkwrite);
diff --git a/include/linux/iomap.h b/include/linux/iomap.h
index 1b22197..854766f 100644
--- a/include/linux/iomap.h
+++ b/include/linux/iomap.h
@@ -3,19 +3,65 @@

#include <linux/types.h>

-/* types of block ranges for multipage write mappings. */
+struct inode;
+struct iov_iter;
+struct kiocb;
+struct vm_area_struct;
+struct vm_fault;
+
+/*
+ * Types of block ranges for iomap mappings:
+ */
#define IOMAP_HOLE 0x01 /* no blocks allocated, need allocation */
#define IOMAP_DELALLOC 0x02 /* delayed allocation blocks */
#define IOMAP_MAPPED 0x03 /* blocks allocated @blkno */
#define IOMAP_UNWRITTEN 0x04 /* blocks allocated @blkno in unwritten state */

+/*
+ * Magic value for blkno:
+ */
#define IOMAP_NULL_BLOCK -1LL /* blkno is not valid */

struct iomap {
- sector_t blkno; /* first sector of mapping */
- loff_t offset; /* file offset of mapping, bytes */
- u64 length; /* length of mapping, bytes */
- int type; /* type of mapping */
+ sector_t blkno; /* first sector of mapping, fs blocks */
+ loff_t offset; /* file offset of mapping, bytes */
+ u64 length; /* length of mapping, bytes */
+ int type; /* type of mapping */
+ struct block_device *bdev; /* block device for I/O */
+};
+
+/*
+ * Flags for iomap_begin / iomap_end. No flag implies a read.
+ */
+#define IOMAP_WRITE (1 << 0)
+#define IOMAP_ZERO (1 << 1)
+
+struct iomap_ops {
+ /*
+ * Return the existing mapping at pos, or reserve space starting at
+ * pos for up to length, as long as we can do it as a single mapping.
+ * The actual length is returned in iomap->length.
+ */
+ int (*iomap_begin)(struct inode *inode, loff_t pos, loff_t length,
+ unsigned flags, struct iomap *iomap);
+
+ /*
+ * Commit and/or unreserve space previous allocated using iomap_begin.
+ * Written indicates the length of the successful write operation which
+ * needs to be commited, while the rest needs to be unreserved.
+ * Written might be zero if no data was written.
+ */
+ int (*iomap_end)(struct inode *inode, loff_t pos, loff_t length,
+ ssize_t written, unsigned flags, struct iomap *iomap);
};

+ssize_t iomap_file_buffered_write(struct kiocb *iocb, struct iov_iter *from,
+ struct iomap_ops *ops);
+int iomap_zero_range(struct inode *inode, loff_t pos, loff_t len,
+ bool *did_zero, struct iomap_ops *ops);
+int iomap_truncate_page(struct inode *inode, loff_t pos, bool *did_zero,
+ struct iomap_ops *ops);
+int iomap_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf,
+ struct iomap_ops *ops);
+
#endif /* LINUX_IOMAP_H */