hi,
fscache cannot currently be used with btrfs as the backing store for the
cache (managed by cachefilesd).
This is because cachefiles needs the ->bmap address_space_operation, and
btrfs doesn't provide it.
cachefiles only uses this to find out if a particular page is a 'hole' or
not. For btrfs, this can be done with 'SEEK_DATA'.
Unfortunately it doesn't seem to be possible to query a filesystem or a file
to see if SEEK_DATA is reliable or not, so we cannot simply use SEEK_DATA
when reliable, else ->bmap if available.
The following patch make fscache work for me on btrfs. It explicitly checks
for BTRFS_SUPER_MAGIC. Not really a nice solution, but all I could think of.
Is there a better way? Could a better way be created? Maybe
SEEK_DATA_RELIABLE ??
Comments, suggestions welcome.
Also, if you do try to use fscache on btrfs with 3.19, then nothing gets
cached (as expected) and with a heavy load you can lose a race and get an
asserting fail in fscache_enqueue_operation
ASSERT(fscache_object_is_available(op->object));
It looks like the object is being killed before it is available...
[ 859.700765] kernel BUG at ../fs/fscache/operation.c:38!
...
[ 859.703124] Call Trace:
[ 859.703193] [<ffffffffa0448044>] fscache_run_op.isra.4+0x34/0x80 [fscache]
[ 859.703260] [<ffffffffa0448160>] fscache_start_operations+0xa0/0xf0 [fscache]
[ 859.703388] [<ffffffffa0446cd8>] fscache_kill_object+0x98/0xc0 [fscache]
[ 859.703455] [<ffffffffa04475c1>] fscache_object_work_func+0x151/0x210 [fscache]
[ 859.703578] [<ffffffff81078b07>] process_one_work+0x147/0x3c0
[ 859.703642] [<ffffffff8107929c>] worker_thread+0x20c/0x470
I haven't figured out the cause of that yet.
Thanks,
NeilBrown
diff --git a/fs/cachefiles/namei.c b/fs/cachefiles/namei.c
index 1e51714eb33e..1389d8483d5d 100644
--- a/fs/cachefiles/namei.c
+++ b/fs/cachefiles/namei.c
@@ -20,6 +20,7 @@
#include <linux/namei.h>
#include <linux/security.h>
#include <linux/slab.h>
+#include <linux/magic.h>
#include "internal.h"
#define CACHEFILES_KEYBUF_SIZE 512
@@ -647,7 +648,8 @@ lookup_again:
ret = -EPERM;
aops = object->dentry->d_inode->i_mapping->a_ops;
- if (!aops->bmap)
+ if (!aops->bmap &&
+ object->dentry->d_sb->s_magic != BTRFS_SUPER_MAGIC)
goto check_error;
object->backer = object->dentry;
diff --git a/fs/cachefiles/rdwr.c b/fs/cachefiles/rdwr.c
index c6cd8d7a4eef..49fb330c0ab8 100644
--- a/fs/cachefiles/rdwr.c
+++ b/fs/cachefiles/rdwr.c
@@ -410,11 +410,11 @@ 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 */
- if (inode->i_sb->s_blocksize > PAGE_SIZE)
+ if (inode->i_mapping->a_ops->bmap &&
+ inode->i_sb->s_blocksize > PAGE_SIZE)
goto enobufs;
shift = PAGE_SHIFT - inode->i_sb->s_blocksize_bits;
@@ -423,20 +423,36 @@ int cachefiles_read_or_alloc_page(struct fscache_retrieval *op,
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 (inode->i_mapping->a_ops->bmap) {
+ /* 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);
+ } else {
+ /* Use llseek */
+ struct path path;
+ struct file *file;
+ path.mnt = cache->mnt;
+ path.dentry = object->backer;
+ file = dentry_open(&path, O_RDONLY, cache->cache_cred);
+ if (IS_ERR(file))
+ goto enobufs;
+ block = vfs_llseek(file, page->index << PAGE_SHIFT, SEEK_DATA);
+ filp_close(file, NULL);
+ if (block != page->index << PAGE_SHIFT)
+ block = 0;
+ else
+ block = 1;
+ }
if (block) {
/* submit the apparently valid page to the backing fs to be
* read from disk */
@@ -707,11 +723,11 @@ 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 */
- if (inode->i_sb->s_blocksize > PAGE_SIZE)
+ if (inode->i_mapping->a_ops->bmap &&
+ inode->i_sb->s_blocksize > PAGE_SIZE)
goto all_enobufs;
shift = PAGE_SHIFT - inode->i_sb->s_blocksize_bits;
@@ -729,21 +745,38 @@ int cachefiles_read_or_alloc_pages(struct fscache_retrieval *op,
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 (inode->i_mapping->a_ops->bmap) {
+ /* 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);
+ } else {
+ /* Use llseek */
+ struct path path;
+ struct file *file;
+ path.mnt = cache->mnt;
+ path.dentry = object->backer;
+ file = dentry_open(&path, O_RDONLY, cache->cache_cred);
+ if (IS_ERR(file))
+ goto all_enobufs;
+ block = vfs_llseek(file, page->index << PAGE_SHIFT, SEEK_DATA);
+ filp_close(file, NULL);
+ if (block != page->index << PAGE_SHIFT)
+ block = 0;
+ else
+ block = 1;
+ }
if (block) {
/* we have data - add it to the list to give to the
* backing fs */
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