[Patch] tmpfs: add fallocate support

[Amerigo Wang <amwang@redhat.com>]

This patch adds fallocate support to tmpfs. I tested this patch
with the following test case,

	% sudo mount -t tmpfs -o size=100 tmpfs /mnt
	% touch /mnt/foobar
	% echo hi > /mnt/foobar
	% fallocate -o 3 -l 5000 /mnt/foobar          
	fallocate: /mnt/foobar: fallocate failed: No space left on device
	% fallocate -o 3 -l 3000 /mnt/foobar
	% ls -l /mnt/foobar
	-rw-rw-r-- 1 wangcong wangcong 3003 Nov 15 16:10 /mnt/foobar
	% dd if=/dev/zero of=/mnt/foobar seek=3 bs=1 count=3000
	3000+0 records in
	3000+0 records out
	3000 bytes (3.0 kB) copied, 0.0153224 s, 196 kB/s
	% hexdump -C /mnt/foobar                                   
	00000000  68 69 0a 00 00 00 00 00  00 00 00 00 00 00 00 00  |hi..............|
	00000010  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
	*
	00000bb0  00 00 00 00 00 00 00 00  00 00 00                 |...........|
	00000bbb
	% cat /mnt/foobar
	hi

Signed-off-by: WANG Cong <amwang@redhat.com>

---
diff --git a/mm/shmem.c b/mm/shmem.c
index d672250..438b7b8 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -30,6 +30,7 @@
 #include <linux/mm.h>
 #include <linux/export.h>
 #include <linux/swap.h>
+#include <linux/falloc.h>
 
 static struct vfsmount *shm_mnt;
 
@@ -1431,6 +1432,102 @@ static ssize_t shmem_file_splice_read(struct file *in, loff_t *ppos,
 	return error;
 }
 
+static long shmem_fallocate(struct file *file, int mode,
+			    loff_t offset, loff_t len)
+{
+	struct inode *inode = file->f_path.dentry->d_inode;
+	struct address_space *mapping = inode->i_mapping;
+	struct shmem_inode_info *info = SHMEM_I(inode);
+	struct shmem_sb_info *sbinfo = SHMEM_SB(inode->i_sb);
+	pgoff_t start = DIV_ROUND_UP(offset, PAGE_CACHE_SIZE);
+	pgoff_t end = DIV_ROUND_UP((offset + len), PAGE_CACHE_SIZE);
+	pgoff_t index = start;
+	gfp_t gfp = mapping_gfp_mask(mapping);
+	loff_t i_size = i_size_read(inode);
+	struct page *page = NULL;
+	int ret;
+
+	if ((offset + len) <= i_size)
+		return 0;
+
+	if (!(mode & FALLOC_FL_KEEP_SIZE)) {
+		ret = inode_newsize_ok(inode, (offset + len));
+		if (ret)
+			return ret;
+	}
+
+	if (start == end) {
+		if (!(mode & FALLOC_FL_KEEP_SIZE))
+			i_size_write(inode, offset + len);
+		return 0;
+	}
+
+	if (shmem_acct_block(info->flags))
+		return -ENOSPC;
+
+	if (sbinfo->max_blocks) {
+		unsigned long blocks = (end - index) * BLOCKS_PER_PAGE;
+		if (blocks + percpu_counter_sum(&sbinfo->used_blocks)
+				>= sbinfo->max_blocks) {
+			ret = -ENOSPC;
+			goto unacct;
+		}
+	}
+
+	while (index < end) {
+		if (sbinfo->max_blocks)
+			percpu_counter_add(&sbinfo->used_blocks, BLOCKS_PER_PAGE);
+
+		page = shmem_alloc_page(gfp, info, index);
+		if (!page) {
+			ret = -ENOMEM;
+			goto decused;
+		}
+
+		SetPageSwapBacked(page);
+		__set_page_locked(page);
+		ret = mem_cgroup_cache_charge(page, current->mm,
+						gfp & GFP_RECLAIM_MASK);
+		if (!ret)
+			ret = shmem_add_to_page_cache(page, mapping, index,
+						gfp, NULL);
+		if (ret)
+			goto unlock;
+		lru_cache_add_anon(page);
+
+		spin_lock(&info->lock);
+		info->alloced++;
+		inode->i_blocks += BLOCKS_PER_PAGE;
+		inode->i_ctime = inode->i_mtime = CURRENT_TIME;
+		shmem_recalc_inode(inode);
+		spin_unlock(&info->lock);
+
+		clear_highpage(page);
+		flush_dcache_page(page);
+		SetPageUptodate(page);
+		unlock_page(page);
+		page_cache_release(page);
+		cond_resched();
+		index++;
+		if (!(mode & FALLOC_FL_KEEP_SIZE))
+			i_size_write(inode, index << PAGE_CACHE_SHIFT);
+	}
+
+	goto unacct;
+
+unlock:
+	if (page) {
+		unlock_page(page);
+		page_cache_release(page);
+	}
+decused:
+	if (sbinfo->max_blocks)
+		percpu_counter_sub(&sbinfo->used_blocks, BLOCKS_PER_PAGE);
+unacct:
+	shmem_unacct_blocks(info->flags, 1);
+	return ret;
+}
+
 static int shmem_statfs(struct dentry *dentry, struct kstatfs *buf)
 {
 	struct shmem_sb_info *sbinfo = SHMEM_SB(dentry->d_sb);
@@ -2286,6 +2383,7 @@ static const struct file_operations shmem_file_operations = {
 	.fsync		= noop_fsync,
 	.splice_read	= shmem_file_splice_read,
 	.splice_write	= generic_file_splice_write,
+	.fallocate	= shmem_fallocate,
 #endif
 };
 

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