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Subject[PATCH 2/3] fscache/cachefiles: optionally use SEEK_DATA instead of ->bmap.
cachefiles currently uses 'bmap' to determine if a given block
in a file has been cached, or not.
Not all filesystems support bmap, particularly BTRFS.

SEEK_DATA can be used to determine if a block in a file has
been allocated, but not all filesystems support this reliably.
On filesystems without explicit report, SEEK_DATA will report anything
below i_size to be valid data.

So:
- add a file_system_type flag which confirms that SEEK_DATA and
SEEK_HOLE will reliably report holes,
- change cachefiles to use vfs_lseek if FS_SUPPORTS_SEEK_HOLE is
set, and only use ->bmap if it isn't.

Subsequent patch will set flag for btrfs. Other filesystems could
usefully have FS_SUPPORTS_SEEK_HOLE set, but I'll leave that to the
relevant maintainers to decide.

Signed-off-by: NeilBrown <neilb@suse.de>
---
fs/cachefiles/namei.c | 15 ++++--
fs/cachefiles/rdwr.c | 119 +++++++++++++++++++++++++++++++------------------
include/linux/fs.h | 1
3 files changed, 86 insertions(+), 49 deletions(-)

diff --git a/fs/cachefiles/namei.c b/fs/cachefiles/namei.c
index 5404afcdee98..5d5e56c645ec 100644
--- a/fs/cachefiles/namei.c
+++ b/fs/cachefiles/namei.c
@@ -643,14 +643,17 @@ lookup_again:
/* open a file interface onto a data file */
if (object->type != FSCACHE_COOKIE_TYPE_INDEX) {
if (d_is_reg(object->dentry)) {
- const struct address_space_operations *aops;

ret = -EPERM;
- aops = object->dentry->d_inode->i_mapping->a_ops;
- if (!aops->bmap)
- goto check_error;
- if (object->dentry->d_sb->s_blocksize > PAGE_SIZE)
- goto check_error;
+ if (!(object->dentry->d_sb->s_type->fs_flags
+ & FS_SUPPORTS_SEEK_HOLE)) {
+ const struct address_space_operations *aops;
+ aops = object->dentry->d_inode->i_mapping->a_ops;
+ if (!aops->bmap)
+ goto check_error;
+ if (object->dentry->d_sb->s_blocksize > PAGE_SIZE)
+ goto check_error;
+ }

object->backer = object->dentry;
} else {
diff --git a/fs/cachefiles/rdwr.c b/fs/cachefiles/rdwr.c
index 9bd44ff48cc0..7c7cbfae7b19 100644
--- a/fs/cachefiles/rdwr.c
+++ b/fs/cachefiles/rdwr.c
@@ -394,9 +394,8 @@ int cachefiles_read_or_alloc_page(struct fscache_retrieval *op,
struct cachefiles_object *object;
struct cachefiles_cache *cache;
struct inode *inode;
- sector_t block0, block;
- unsigned shift;
int ret;
+ bool have_data;

object = container_of(op->op.object,
struct cachefiles_object, fscache);
@@ -410,31 +409,47 @@ int cachefiles_read_or_alloc_page(struct fscache_retrieval *op,

inode = object->backer->d_inode;
ASSERT(S_ISREG(inode->i_mode));
- ASSERT(inode->i_mapping->a_ops->bmap);
ASSERT(inode->i_mapping->a_ops->readpages);

- /* calculate the shift required to use bmap */
- shift = PAGE_SHIFT - inode->i_sb->s_blocksize_bits;
-
op->op.flags &= FSCACHE_OP_KEEP_FLAGS;
op->op.flags |= FSCACHE_OP_ASYNC;
op->op.processor = cachefiles_read_copier;

- /* we assume the absence or presence of the first block is a good
- * enough indication for the page as a whole
- * - TODO: don't use bmap() for this as it is _not_ actually good
- * enough for this as it doesn't indicate errors, but it's all we've
- * got for the moment
- */
- block0 = page->index;
- block0 <<= shift;
-
- block = inode->i_mapping->a_ops->bmap(inode->i_mapping, block0);
- _debug("%llx -> %llx",
- (unsigned long long) block0,
- (unsigned long long) block);
-
- if (block) {
+ if (inode->i_sb->s_type->fs_flags & FS_SUPPORTS_SEEK_HOLE) {
+ /* Use llseek */
+ struct path path;
+ struct file *file;
+ loff_t addr;
+ path.mnt = cache->mnt;
+ path.dentry = object->backer;
+ file = dentry_open(&path, O_RDONLY, cache->cache_cred);
+ if (IS_ERR(file))
+ goto enobufs;
+ addr = page->index;
+ addr <<= PAGE_SHIFT;
+ have_data = (addr == vfs_llseek(file, addr, SEEK_DATA));
+ filp_close(file, NULL);
+ } else {
+ /* we assume the absence or presence of the first block is a good
+ * enough indication for the page as a whole
+ * - TODO: don't use bmap() for this as it is _not_ actually good
+ * enough for this as it doesn't indicate errors, but it's all we've
+ * got for the moment
+ */
+ /* calculate the shift required to use bmap */
+ unsigned shift = PAGE_SHIFT - inode->i_sb->s_blocksize_bits;
+ sector_t block0, block;
+
+ block0 = page->index;
+ block0 <<= shift;
+
+ block = inode->i_mapping->a_ops->bmap(inode->i_mapping, block0);
+ _debug("%llx -> %llx",
+ (unsigned long long) block0,
+ (unsigned long long) block);
+ have_data = (block != 0);
+ }
+ if (have_data) {
/* submit the apparently valid page to the backing fs to be
* read from disk */
ret = cachefiles_read_backing_file_one(object, op, page);
@@ -683,7 +698,7 @@ int cachefiles_read_or_alloc_pages(struct fscache_retrieval *op,
struct pagevec pagevec;
struct inode *inode;
struct page *page, *_n;
- unsigned shift, nrbackpages;
+ unsigned nrbackpages;
int ret, ret2, space;

object = container_of(op->op.object,
@@ -704,11 +719,8 @@ int cachefiles_read_or_alloc_pages(struct fscache_retrieval *op,

inode = object->backer->d_inode;
ASSERT(S_ISREG(inode->i_mode));
- ASSERT(inode->i_mapping->a_ops->bmap);
ASSERT(inode->i_mapping->a_ops->readpages);

- /* calculate the shift required to use bmap */
- shift = PAGE_SHIFT - inode->i_sb->s_blocksize_bits;

pagevec_init(&pagevec, 0);

@@ -721,24 +733,45 @@ int cachefiles_read_or_alloc_pages(struct fscache_retrieval *op,

ret = space ? -ENODATA : -ENOBUFS;
list_for_each_entry_safe(page, _n, pages, lru) {
- sector_t block0, block;
-
- /* we assume the absence or presence of the first block is a
- * good enough indication for the page as a whole
- * - TODO: don't use bmap() for this as it is _not_ actually
- * good enough for this as it doesn't indicate errors, but
- * it's all we've got for the moment
- */
- block0 = page->index;
- block0 <<= shift;
-
- block = inode->i_mapping->a_ops->bmap(inode->i_mapping,
- block0);
- _debug("%llx -> %llx",
- (unsigned long long) block0,
- (unsigned long long) block);
-
- if (block) {
+ bool have_data;
+
+ if (inode->i_sb->s_type->fs_flags & FS_SUPPORTS_SEEK_HOLE) {
+ /* Use llseek */
+ struct path path;
+ struct file *file;
+ loff_t addr;
+
+ path.mnt = cache->mnt;
+ path.dentry = object->backer;
+ file = dentry_open(&path, O_RDONLY, cache->cache_cred);
+ if (IS_ERR(file))
+ goto all_enobufs;
+ addr = page->index;
+ addr <<= PAGE_SHIFT;
+ have_date = (addr == vfs_llseek(file, addr, SEEK_DATA));
+ filp_close(file, NULL);
+ } else {
+ /* we assume the absence or presence of the first block is a
+ * good enough indication for the page as a whole
+ * - TODO: don't use bmap() for this as it is _not_ actually
+ * good enough for this as it doesn't indicate errors, but
+ * it's all we've got for the moment
+ */
+ /* calculate the shift required to use bmap */
+ unsigned shift = PAGE_SHIFT - inode->i_sb->s_blocksize_bits;
+ sector_t block0, block;
+
+ block0 = page->index;
+ block0 <<= shift;
+
+ block = inode->i_mapping->a_ops->bmap(inode->i_mapping,
+ block0);
+ _debug("%llx -> %llx",
+ (unsigned long long) block0,
+ (unsigned long long) block);
+ have_data = (block != 0);
+ }
+ if (have_data) {
/* we have data - add it to the list to give to the
* backing fs */
list_move(&page->lru, &backpages);
diff --git a/include/linux/fs.h b/include/linux/fs.h
index f4131e8ead74..ae28d175eeb4 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1863,6 +1863,7 @@ struct file_system_type {
#define FS_HAS_SUBTYPE 4
#define FS_USERNS_MOUNT 8 /* Can be mounted by userns root */
#define FS_USERNS_DEV_MOUNT 16 /* A userns mount does not imply MNT_NODEV */
+#define FS_SUPPORTS_SEEK_HOLE 32 /* SEEK_HOLE/SEEK_DATA reliably detect holes */
#define FS_RENAME_DOES_D_MOVE 32768 /* FS will handle d_move() during rename() internally. */
struct dentry *(*mount) (struct file_system_type *, int,
const char *, void *);



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