Messages in this thread Patch in this message | | | From | Christian Schoenebeck <> | Subject | Re: 9p EBADF with cache enabled (Was: 9p fs-cache tests/benchmark (was: 9p fscache Duplicate cookie detected)) | Date | Fri, 03 Jun 2022 18:46:04 +0200 |
| |
On Freitag, 6. Mai 2022 21:14:52 CEST Christian Schoenebeck wrote: > On Mittwoch, 4. Mai 2022 23:48:47 CEST asmadeus@codewreck.org wrote: > > Christian Schoenebeck wrote on Wed, May 04, 2022 at 08:33:36PM +0200: > > > On Dienstag, 3. Mai 2022 12:21:23 CEST asmadeus@codewreck.org wrote: > > > > - add some complex code to track the exact byte range that got > > > > updated > > > > > > > > in some conditions e.g. WRONLY or read fails? > > > > That'd still be useful depending on how the backend tracks file mode, > > > > qemu as user with security_model=mapped-file keeps files 600 but with > > > > passthrough or none qemu wouldn't be able to read the file regardless > > > > of > > > > what we do on client... > > > > Christian, if you still have an old kernel around did that use to > > > > work? > > > > > > Sorry, what was the question, i.e. what should I test / look for > > > precisely? :) > > > > I was curious if older kernel does not issue read at all, or issues read > > on writeback fid correctly opened as root/RDRW > > > > You can try either the append.c I pasted a few mails back or the dd > > commands, as regular user. > > > > $ dd if=/dev/zero of=test bs=1M count=1 > > $ chmod 400 test > > # drop cache or remount > > $ dd if=/dev/urandom of=test bs=102 seek=2 count=1 conv=notrunc > > dd: error writing 'test': Bad file descriptor > > Seems you were right, the old kernel opens the file with O_RDWR. > > The following was taken with cache=loose, pre-netfs kernel version, using > your append code and file to be appended already containing 34 bytes, > relevant file is fid 7: > > v9fs_open tag 0 id 12 fid 7 mode 2 > v9fs_open_return tag 0 id 12 qid={type 0 version 1651854932 path 3108899} > iounit 4096 v9fs_xattrwalk tag 0 id 30 fid 5 newfid 8 name > security.capability v9fs_rerror tag 0 id 30 err 95 > v9fs_read tag 0 id 116 fid 7 off 0 max_count 4096 > v9fs_read_return tag 0 id 116 count 34 err 45 > v9fs_read tag 0 id 116 fid 7 off 34 max_count 4062 > v9fs_read_return tag 0 id 116 count 0 err 11 > v9fs_clunk tag 0 id 120 fid 6 > v9fs_clunk tag 0 id 120 fid 4 > [delay] > v9fs_write tag 0 id 118 fid 7 off 0 count 39 cnt 1 > v9fs_write_return tag 0 id 118 total 39 err 11 > v9fs_fsync tag 0 id 50 fid 7 datasync 0 > > BTW to see this protocol debug output with QEMU: > > cd qemu/build > ../configure --enable-trace-backends=log ... > make -jN > ./qemu-system-x86_64 -trace 'v9fs*' ...
I had another time slice on this issue today. As Dominique pointed out before, the writeback_fid was and still is opened with O_RDWR [fs/9p/fid.c]:
struct p9_fid *v9fs_writeback_fid(struct dentry *dentry) { int err; struct p9_fid *fid, *ofid;
ofid = v9fs_fid_lookup_with_uid(dentry, GLOBAL_ROOT_UID, 0); fid = clone_fid(ofid); if (IS_ERR(fid)) goto error_out; p9_client_clunk(ofid); /* * writeback fid will only be used to write back the * dirty pages. We always request for the open fid in read-write * mode so that a partial page write which result in page * read can work. */ err = p9_client_open(fid, O_RDWR); if (err < 0) { p9_client_clunk(fid); fid = ERR_PTR(err); goto error_out; } error_out: return fid; }
The problem rather seems to be that the new netfs code does not use the writeback_fid when doing an implied read before the actual partial writeback.
As I showed in my previous email, the old pre-netfs kernel versions also did a read before partial writebacks, but apparently used the special writeback_fid for that.
I added some trap code to recent netfs kernel version:
diff --git a/net/9p/client.c b/net/9p/client.c index 8bba0d9cf975..11ff1ee2130e 100644 --- a/net/9p/client.c +++ b/net/9p/client.c @@ -1549,12 +1549,21 @@ int p9_client_unlinkat(struct p9_fid *dfid, const char *name, int flags) } EXPORT_SYMBOL(p9_client_unlinkat); +void p9_bug(void) { + BUG_ON(true); +} +EXPORT_SYMBOL(p9_bug); + int p9_client_read(struct p9_fid *fid, u64 offset, struct iov_iter *to, int *err) { int total = 0; *err = 0; + if ((fid->mode & O_ACCMODE) == O_WRONLY) { + p9_bug(); + } + while (iov_iter_count(to)) { int count; @@ -1648,6 +1657,10 @@ p9_client_write(struct p9_fid *fid, u64 offset, struct iov_iter *from, int *err) p9_debug(P9_DEBUG_9P, ">>> TWRITE fid %d offset %llu count %zd\n", fid->fid, offset, iov_iter_count(from)); + if ((fid->mode & O_ACCMODE) == O_RDONLY) { + p9_bug(); + } + while (iov_iter_count(from)) { int count = iov_iter_count(from); int rsize = fid->iounit; Which triggers the trap in p9_client_read() with cache=loose. Here is the backtrace [based on d615b5416f8a1afeb82d13b238f8152c572d59c0]:
[ 139.365314] p9_client_read (net/9p/client.c:1553 net/9p/client.c:1564) 9pnet [ 139.148806] v9fs_issue_read (fs/9p/vfs_addr.c:45) 9p [ 139.149268] netfs_begin_read (fs/netfs/io.c:91 fs/netfs/io.c:579 fs/netfs/io.c:625) netfs [ 139.149725] ? xas_load (lib/xarray.c:211 lib/xarray.c:242) [ 139.150057] ? xa_load (lib/xarray.c:1469) [ 139.150398] netfs_write_begin (fs/netfs/buffered_read.c:407) netfs [ 139.150883] v9fs_write_begin (fs/9p/vfs_addr.c:279 (discriminator 2)) 9p [ 139.151293] generic_perform_write (mm/filemap.c:3789) [ 139.151721] ? generic_update_time (fs/inode.c:1858) [ 139.152112] ? file_update_time (fs/inode.c:2089) [ 139.152504] __generic_file_write_iter (mm/filemap.c:3916) [ 139.152943] generic_file_write_iter (./include/linux/fs.h:753 mm/filemap.c:3948) [ 139.153348] new_sync_write (fs/read_write.c:505 (discriminator 1)) [ 139.153754] vfs_write (fs/read_write.c:591) [ 139.154090] ksys_write (fs/read_write.c:644) [ 139.154417] do_syscall_64 (arch/x86/entry/common.c:50 arch/x86/entry/common.c:80) [ 139.154776] entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:115)
I still had not time to read the netfs code part yet, but I assume netfs falls back to a generic 9p read on the O_WRONLY opened fid here, instead of using the special O_RDWR opened 'writeback_fid'.
Is there already some info available in the netfs API that the read is actually part of a writeback task, so that we could force on 9p driver level to use the special writeback_fid for the read in this case instead?
Best regards, Christian Schoenebeck
| |