Hello,
Currently it is possible for some errors to be detected at write-back
time but not reported to the program as shown by the following script
using the included make_file.c.
---------8<---------8<---------8<---------8<---------8<---------8<---------
#!/bin/sh
# We binary search the size of a file in 40M filesystem that can cause
# the missed error.
MIN=5000000
MAX=50000000
rm fs.40M
dd if=/dev/zero of=fs.40M bs=40M count=0 seek=1 status=noxfer
mkfs.ext2 -F fs.40M
#mkfs.ext3 -F fs.40M
#mkfs.jfs -q fs.40M
#mkfs.reiserfs -fq fs.40M
#mkfs.xfs fs.40M
attempt()
{
SIZE=$1
RES=0
./make_file valid_file $SIZE
mount fs.40M /mnt -o loop
if ! ./make_file /mnt/not_enough_space $SIZE; then
# We could not create the file as the requested size
# was clearly too big
RES=1
fi
umount /mnt
if [ $RES -eq 0 ]; then
mount fs.40M /mnt -o loop
if cmp valid_file /mnt/not_enough_space; then
# The file was too small, it fitted in the filesystem
RES=-1
fi
umount /mnt
fi
if [ $RES -eq 0 ]; then
echo "Undetected ENOSPC with SIZE=$SIZE"
exit
fi
return $RES
}
while [ $((MAX - MIN)) -gt 1 ]; do
SIZE=$(((MIN + MAX) / 2))
attempt $SIZE
RES=$?
if [ $RES -eq 1 ]; then
MAX=$SIZE
else
MIN=$SIZE
fi
done
echo "Could not reproduce the problem"
---------8<---------8<---------8<---------8<---------8<---------8<---------
/* make_file.c */
#include <unistd.h>
#include <sys/fcntl.h>
#include <sys/mman.h>
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char **argv)
{
int size, fd;
char *mapping;
if (argc != 3) {
fprintf(stderr, "Usage: %s FILE SIZE\n", argv[0]);
return 1;
}
size = atoi(argv[2]);
fd = open(argv[1], O_RDWR | O_CREAT, 0600);
if (fd < 0) {
perror(argv[1]);
return 1;
}
if (ftruncate(fd, size) < 0) {
perror("ftruncate");
return 1;
}
mapping = mmap(NULL, size, PROT_WRITE, MAP_SHARED, fd, 0);
if (mapping == MAP_FAILED) {
perror("mmap");
return 1;
}
memset(mapping, 0xFF, size);
sync();
if (msync(mapping, size, MS_SYNC) < 0) {
perror("msync");
return 1;
}
if (close(fd) < 0) {
perror("close");
return 1;
}
printf("%s: successfully written %d bytes\n", argv[1], size);
return 0;
}
---------8<---------8<---------8<---------8<---------8<---------8<---------
make_file.c mmaps a hole, performs some writeback (memset + sync) and
then expects to find some error code in msync(). The script mounts a
40M loopback filesystem and does a binary search to find the size of
a file big enough to provoke a ENOSPC, but small enough to show the
error not being detected at msync() time.
All mmap capable filesystems I tested are affected (ext2, ext3,
jfs, reiserfs, xfs). XFS is special in that it survives the test thanks
to the page_mkwrite() work, i.e. it SIGBUS during memset. Anyway, this
behavious solves ENOSPC but does nothing for EIO.
The offending code is in fs/fs-writeback.c:
sync_sb_inodes(...) ()
{
...
__writeback_single_inode(inode, wbc);
...
}
__writeback_single_inode() gets the error from mapping->flags, clears it
and returns it. But sync_sb_inodes() ignores this return value. In -mm
there is sync_sb_inodes-propagate-errors.patch that propagates the
error from __writeback_single_inode upwards in the call stack. IMHO,
this propagation is useless because:
- the error is combined from the errors in all the synced inodes, so it
just tells that some inode in a specific fs got an error,
- nobody in the call stack is interested in this error: certainly not
pdflush, or 'void sync(2)'.
OTOH, msync() would be interested in finding this error in
mapping->flags, so the patch I proposed in
http://lkml.org/lkml/2006/12/29/136 did:
ret = __writeback_single_inode(inode, wbc);
mapping_set_error(mapping, ret);
But Andrew didn't like it, treating me as a conventional
programmer ;-), hence the detailed explanation to be sure my point
comes across.
With this patch, I think it's important to keep the return values of
sync_sb_inodes() and friends as void, to recall that the errors are
not to be found in the return value but in mapping->flags. That's why
I'd like to see sync_sb_inodes-propagate-errors.patch dropped. Removing
this patch causes some churn in -mm but I can prepare patches to fix
this up if needed.
Thanks for reading my moaning for such a ridiculous diffstat ;-)
Signed-off-by: Guillaume Chazarain <[email protected]>
---
fs/fs-writeback.c | 4 +++-
1 files changed, 3 insertions(+), 1 deletions(-)
diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 16296c7..c44c42f 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -441,6 +441,7 @@ void generic_sync_sb_inodes(struct super_block *sb,
struct address_space *mapping = inode->i_mapping;
struct backing_dev_info *bdi = mapping->backing_dev_info;
long pages_skipped;
+ int err;
if (!bdi_cap_writeback_dirty(bdi)) {
redirty_tail(inode);
@@ -492,7 +493,8 @@ void generic_sync_sb_inodes(struct super_block *sb,
BUG_ON(inode->i_state & I_FREEING);
__iget(inode);
pages_skipped = wbc->pages_skipped;
- __writeback_single_inode(inode, wbc);
+ err = __writeback_single_inode(inode, wbc);
+ mapping_set_error(mapping, ret);
if (wbc->sync_mode == WB_SYNC_HOLD) {
check_dirty_inode(inode);
inode->dirtied_when = jiffies;
--
Guillaume
> + mapping_set_error(mapping, ret);
And of course, s/ret/err/ :-(
--
Guillaume