Messages in this thread Patch in this message | | | Date | Sun, 12 Jun 2022 19:02:27 +0900 | From | asmadeus@codewrec ... | Subject | Re: 9p EBADF with cache enabled (Was: 9p fs-cache tests/benchmark (was: 9p fscache Duplicate cookie detected)) |
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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.
> > 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
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