Date:   Sat, 18 Feb 2023 21:15:33 +0900
From:   asmadeus@codewreck.org
To:     Eric Van Hensbergen <ericvh@kernel.org>
Cc:     v9fs-developer@lists.sourceforge.net, rminnich@gmail.com,
        lucho@ionkov.net, linux-kernel@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, linux_oss@crudebyte.com
Subject: Re: [PATCH v4 10/11] fs/9p: writeback mode fixes
Message-ID: <Y/DBZSaAsRiNR2WV@codewreck.org>
References: <20230124023834.106339-1-ericvh@kernel.org>
 <20230218003323.2322580-1-ericvh@kernel.org>
 <20230218003323.2322580-11-ericvh@kernel.org>
 <Y/Ch8o/6HVS8Iyeh@codewreck.org>
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In-Reply-To: <Y/Ch8o/6HVS8Iyeh@codewreck.org>
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asmadeus@codewreck.org wrote on Sat, Feb 18, 2023 at 07:01:22PM +0900:
> > diff --git a/fs/9p/vfs_super.c b/fs/9p/vfs_super.c
> > index 5fc6a945bfff..797f717e1a91 100644
> > --- a/fs/9p/vfs_super.c
> > +++ b/fs/9p/vfs_super.c
> 
> > @@ -323,16 +327,17 @@ static int v9fs_write_inode_dotl(struct inode *inode,
> >  	 */
> >  	v9inode = V9FS_I(inode);
> >  	p9_debug(P9_DEBUG_VFS, "%s: inode %p, writeback_fid %p\n",
> > -		 __func__, inode, v9inode->writeback_fid);
> > -	if (!v9inode->writeback_fid)
> > -		return 0;
> > +		 __func__, inode, fid);
> > +	if (!fid)
> > +		return -EINVAL;
> 
> Hmm, what happens if we return EINVAL here?
> Might want a WARN_ONCE or something?

Answering myself on this: No idea what happens, but it's fairly
common...
(I saw it from wb_writeback which considers any non-zero return value as
'progress', so the error is progress as well... Might make more sense to
return 0 here actually? need more thorough checking, didn't take time to
dig through this either...)

That aside I ran with my comments addressed and cache=fscache, and
things blew up during ./configure of coreutils-9.1 in qemu:
(I ran it as root without setting the env var so it failed, that much is
expected -- the evict_inode blowing up isn't)
-------
checking whether mknod can create fifo without root privileges... configure: error: in `/mnt/coreutils-9.1':
configure: error: you should not run configure as root (set FORCE_UNSAFE_CONFIGURE=1 in environment to bypass this check)
See `config.log' for more details
FS-Cache:
FS-Cache: Assertion failed
FS-Cache: 2 == 0 is false
------------[ cut here ]------------
kernel BUG at fs/fscache/cookie.c:985!
invalid opcode: 0000 [#3] SMP PTI
CPU: 0 PID: 9707 Comm: rm Tainted: G      D            6.2.0-rc2+ #37
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.0-debian-1.16.0-5 04/01/2014
RIP: 0010:__fscache_relinquish_cookie.cold+0x5a/0x8f
Code: 48 c7 c7 21 5e b8 81 e8 34 87 ff ff 48 c7 c7 2f 5e b8 81 e8 28 87 ff ff 48 63 73 04 31 d2 48 c7 c7 00 61 b8 81 e8 16 87 ff ff <0f> 0b 44 8b 47 04 8b 4f 0c 45 0f b8
RSP: 0018:ffffc90002697e08 EFLAGS: 00010286
RAX: 0000000000000019 RBX: ffff8880077de210 RCX: 00000000ffffefff
RDX: 00000000ffffffea RSI: 00000000ffffefff RDI: 0000000000000001
RBP: ffffc90002697e18 R08: 0000000000004ffb R09: 00000000ffffefff
R10: ffffffff8264ea20 R11: ffffffff8264ea20 R12: 0000000000000000
R13: ffffffffc00870e0 R14: ffff88800308cd20 R15: ffff8880046a0020
FS:  00007fec5aa33000(0000) GS:ffff88807cc00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00000000009af4d8 CR3: 0000000007490000 CR4: 00000000000006b0
Call Trace:
 <TASK>
 v9fs_evict_inode+0x78/0x90 [9p]
 evict+0xc0/0x160
 iput+0x171/0x220
 do_unlinkat+0x197/0x280
 __x64_sys_unlinkat+0x37/0x60
 do_syscall_64+0x3c/0x80
 entry_SYSCALL_64_after_hwframe+0x63/0xcd
RIP: 0033:0x7fec5ab33fdb
Code: 73 01 c3 48 8b 0d 55 9e 0f 00 f7 d8 64 89 01 48 83 c8 ff c3 66 2e 0f 1f 84 00 00 00 00 00 90 f3 0f 1e fa b8 07 01 00 00 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 08
RSP: 002b:00007ffd460b1858 EFLAGS: 00000246 ORIG_RAX: 0000000000000107
RAX: ffffffffffffffda RBX: 00000000009af830 RCX: 00007fec5ab33fdb
RDX: 0000000000000000 RSI: 00000000009ae3d0 RDI: 00000000ffffff9c
RBP: 00000000009ae340 R08: 0000000000000003 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007ffd460b1a40 R14: 0000000000000000 R15: 00000000009af830
 </TASK>
Modules linked in: 9pnet_virtio 9p 9pnet siw ib_core
---[ end trace 0000000000000000 ]---
RIP: 0010:__fscache_relinquish_cookie.cold+0x5a/0x8f
Code: 48 c7 c7 21 5e b8 81 e8 34 87 ff ff 48 c7 c7 2f 5e b8 81 e8 28 87 ff ff 48 63 73 04 31 d2 48 c7 c7 00 61 b8 81 e8 16 87 ff ff <0f> 0b 44 8b 47 04 8b 4f 0c 45 0f b8
RSP: 0018:ffffc90002237e08 EFLAGS: 00010286
RAX: 0000000000000019 RBX: ffff8880077de9a0 RCX: 00000000ffffefff
RDX: 00000000ffffffea RSI: 00000000ffffefff RDI: 0000000000000001
RBP: ffffc90002237e18 R08: 0000000000004ffb R09: 00000000ffffefff
R10: ffffffff8264ea20 R11: ffffffff8264ea20 R12: 0000000000000000
R13: ffffffffc00870e0 R14: ffff888003a6b500 R15: ffff8880046a0020
FS:  00007fec5aa33000(0000) GS:ffff88807cc00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00000000009af4d8 CR3: 0000000007490000 CR4: 00000000000006b0
./configure: line 88:  9707 Segmentation fault      exit $1
-----------

I don't have time to investigate but I'm afraid this needs a bit more
time as well, sorry :/


For reference, here's how I addressed my comments. I don't think that's
related to the problem at hand but can retry later without it if you
think something's fishy:
---------
diff --git a/fs/9p/vfs_dir.c b/fs/9p/vfs_dir.c
index 44918c60357f..c16c39ba55d6 100644
--- a/fs/9p/vfs_dir.c
+++ b/fs/9p/vfs_dir.c
@@ -215,7 +215,7 @@ int v9fs_dir_release(struct inode *inode, struct file *filp)
 	p9_debug(P9_DEBUG_VFS, "inode: %p filp: %p fid: %d\n",
 		 inode, filp, fid ? fid->fid : -1);
 	if (fid) {
-		if ((fid->qid.type == P9_QTFILE) && (filp->f_mode & FMODE_WRITE))
+		if ((S_ISREG(inode->i_mode)) && (filp->f_mode & FMODE_WRITE))
 			v9fs_flush_inode_writeback(inode);
 
 		spin_lock(&inode->i_lock);
diff --git a/fs/9p/vfs_file.c b/fs/9p/vfs_file.c
index 936daff9f948..e322d4196be6 100644
--- a/fs/9p/vfs_file.c
+++ b/fs/9p/vfs_file.c
@@ -60,7 +60,7 @@ int v9fs_file_open(struct inode *inode, struct file *file)
 			return PTR_ERR(fid);
 
 		if ((v9ses->cache >= CACHE_WRITEBACK) && (omode & P9_OWRITE)) {
-			int writeback_omode = (omode & !P9_OWRITE) | P9_ORDWR;
+			int writeback_omode = (omode & ~P9_OWRITE) | P9_ORDWR;
 
 			p9_debug(P9_DEBUG_CACHE, "write-only file with writeback enabled, try opening O_RDWR\n");
 			err = p9_client_open(fid, writeback_omode);
diff --git a/fs/9p/vfs_inode.c b/fs/9p/vfs_inode.c
index d53475e1ba27..062c34524b1f 100644
--- a/fs/9p/vfs_inode.c
+++ b/fs/9p/vfs_inode.c
@@ -230,22 +230,7 @@ v9fs_blank_wstat(struct p9_wstat *wstat)
 
 int v9fs_flush_inode_writeback(struct inode *inode)
 {
-	struct writeback_control wbc = {
-		.nr_to_write = LONG_MAX,
-		.sync_mode = WB_SYNC_ALL,
-		.range_start = 0,
-		.range_end = -1,
-	};
-
-	int retval = filemap_fdatawrite_wbc(inode->i_mapping, &wbc);
-
-	if (retval != 0) {
-		p9_debug(P9_DEBUG_ERROR,
-			"trying to flush inode %p failed with error code %d\n",
-			inode, retval);
-	}
-
-	return retval;
+	return filemap_write_and_wait(inode->i_mapping);
 }
 
 /**
------
-- 
Dominique