Re: 9p EBADF with cache enabled (Was: 9p fs-cache tests/benchmark (was: 9p fscache Duplicate cookie detected))
From: asmadeus@codewreck.org
Date: 2022-06-12 10:03:17
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Sorry, I had planned on working on this today but the other patchset ended up taking all my time... I think I'm bad at priorities, this is definitely important... David, I think with the latest comments we made it should be relatively straightforward to make netfs use the writeback fid? Could you find some time to have a look? It should be trivial to reproduce, I gave these commands a few mails ago (needs to run as a regular user, on a fscache mount) --- $ dd if=/dev/zero of=test bs=1M count=1 $ chmod 200 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 --- Otherwise I'll try to make some more 9p time again, but it's getting more and more difficult for me... Christian Schoenebeck wrote on Fri, Jun 03, 2022 at 06:46:04PM +0200:
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.This looks good! Thanks for keeping it up.
quoted hunk ↗ jump to hunk
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?
-- Dominique