[RFC v2 1/3] fs: Add folio_may_straddle_isize helper

Andreas Gruenbacher
2022-12-01T18:11:08Z

Add a folio_may_straddle_isize() helper as a replacement for
pagecache_isize_extended() when we have a locked folio.

Use the new helper in generic_write_end(), iomap_write_end(),
ext4_write_end(), and ext4_journalled_write_end().

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
---
fs/buffer.c | 5 ++---
fs/ext4/inode.c | 13 ++++++-------
fs/iomap/buffered-io.c | 5 +----
include/linux/mm.h | 2 ++
mm/truncate.c | 34 ++++++++++++++++++++++++++++++++++
5 files changed, 45 insertions(+), 14 deletions(-)

diff --git a/fs/buffer.c b/fs/buffer.c
index d9c6d1fbb6dd..bbae1437994b 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -2164,16 +2164,15 @@ int generic_write_end(struct file *file, struct address_space *mapping,
* But it's important to update i_size while still holding page lock:
* page writeout could otherwise come in and zero beyond i_size.
*/
- if (pos + copied > inode->i_size) {
+ if (pos + copied > old_size) {
i_size_write(inode, pos + copied);
i_size_changed = true;
+ folio_may_straddle_isize(inode, page_folio(page), old_size, pos);
}

unlock_page(page);
put_page(page);

- if (old_size < pos)
- pagecache_isize_extended(inode, old_size, pos);
/*
* Don't mark the inode dirty under page lock. First, it unnecessarily
* makes the holding time of page lock longer. Second, it forces lock
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 2b5ef1b64249..48e6b4716415 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -1320,13 +1320,13 @@ static int ext4_write_end(struct file *file,
* If FS_IOC_ENABLE_VERITY is running on this inode, then Merkle tree
* blocks are being written past EOF, so skip the i_size update.
*/
- if (!verity)
+ if (!verity) {
i_size_changed = ext4_update_inode_size(inode, pos + copied);
+ folio_may_straddle_isize(inode, page_folio(page), old_size, pos);
+ }
unlock_page(page);
put_page(page);

- if (old_size < pos && !verity)
- pagecache_isize_extended(inode, old_size, pos);
/*
* Don't mark the inode dirty under page lock. First, it unnecessarily
* makes the holding time of page lock longer. Second, it forces lock
@@ -1432,16 +1432,15 @@ static int ext4_journalled_write_end(struct file *file,
if (!partial)
SetPageUptodate(page);
}
- if (!verity)
+ if (!verity) {
size_changed = ext4_update_inode_size(inode, pos + copied);
+ folio_may_straddle_isize(inode, page_folio(page), old_size, pos);
+ }
ext4_set_inode_state(inode, EXT4_STATE_JDATA);
EXT4_I(inode)->i_datasync_tid = handle->h_transaction->t_tid;
unlock_page(page);
put_page(page);

- if (old_size < pos && !verity)
- pagecache_isize_extended(inode, old_size, pos);
-
if (size_changed) {
ret2 = ext4_mark_inode_dirty(handle, inode);
if (!ret)
diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
index 91ee0b308e13..96e643de32a0 100644
--- a/fs/iomap/buffered-io.c
+++ b/fs/iomap/buffered-io.c
@@ -713,15 +713,12 @@ static size_t iomap_write_end(struct iomap_iter *iter, loff_t pos, size_t len,
if (pos + ret > old_size) {
i_size_write(iter->inode, pos + ret);
iter->iomap.flags |= IOMAP_F_SIZE_CHANGED;
+ folio_may_straddle_isize(iter->inode, folio, old_size, pos);
}
folio_unlock(folio);
-
- if (old_size < pos)
- pagecache_isize_extended(iter->inode, old_size, pos);
if (page_ops && page_ops->page_done)
page_ops->page_done(iter->inode, pos, ret, &folio->page);
folio_put(folio);
-
if (ret < len)
iomap_write_failed(iter->inode, pos + ret, len - ret);
return ret;
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 8bbcccbc5565..e1f03c9ed8df 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1890,6 +1890,8 @@ int generic_access_phys(struct vm_area_struct *vma, unsigned long addr,

extern void truncate_pagecache(struct inode *inode, loff_t new);
extern void truncate_setsize(struct inode *inode, loff_t newsize);
+void folio_may_straddle_isize(struct inode *inode, struct folio *folio,
+ loff_t old_size, loff_t start);
void pagecache_isize_extended(struct inode *inode, loff_t from, loff_t to);
void truncate_pagecache_range(struct inode *inode, loff_t offset, loff_t end);
int generic_error_remove_page(struct address_space *mapping, struct page *page);
diff --git a/mm/truncate.c b/mm/truncate.c
index c0be77e5c008..9a5d3c3c12d0 100644
--- a/mm/truncate.c
+++ b/mm/truncate.c
@@ -779,6 +779,40 @@ void truncate_setsize(struct inode *inode, loff_t newsize)
}
EXPORT_SYMBOL(truncate_setsize);

+/**
+ * folio_may_straddle_isize - update pagecache after extending i_size
+ * @inode: inode for which i_size was extended
+ * @folio: folio to maybe mark read-only
+ * @old_size: original inode size
+ * @start: start of the write
+ *
+ * Handle extending an inode by a write that starts behind the old inode size.
+ * If a block-aligned hole exists between the old inode size and the start of
+ * the write, we mark the folio read-only so that page_mkwrite() is called on
+ * the nearest write access to the page. That way, the filesystem can be sure
+ * that page_mkwrite() is called on the page before a user writes to the page
+ * via mmap.
+ *
+ * This function must be called while we still hold i_rwsem - this not only
+ * makes sure i_size is stable but also that userspace cannot observe the new
+ * i_size value before we are prepared to handle mmap writes there.
+ */
+void folio_may_straddle_isize(struct inode *inode, struct folio *folio,
+ loff_t old_size, loff_t start)
+{
+ unsigned int blocksize = i_blocksize(inode);
+
+ if (round_up(old_size, blocksize) >= round_down(start, blocksize))
+ return;
+
+ /*
+ * See clear_page_dirty_for_io() for details why folio_set_dirty()
+ * is needed.
+ */
+ if (folio_mkclean(folio))
+ folio_set_dirty(folio);
+}
+
/**
* pagecache_isize_extended - update pagecache after extension of i_size
* @inode: inode for which i_size was extended
--
2.38.1