Re: Warning at mm/truncate.c:740

[Jan Kara]

  Hi,

On Sun 09-08-15 10:49:33, Sasha Levin wrote:
> I saw the following warning while fuzzing with trinity:
> 
> [385644.689209] WARNING: CPU: 1 PID: 23536 at mm/truncate.c:740 pagecache_isize_extended+0x124/0x180()
> [385644.691780] Modules linked in:
> [385644.692695] CPU: 1 PID: 23536 Comm: trinity-c242 Not tainted 4.2.0-rc5-next-20150806-sasha-00040-g1b47b00-dirty #2417
> [385644.695636]  ffffffffb21300e0 ffff8800ba3cfc40 ffffffffb1e89dfc 0000000000000000
> [385644.708128]  ffff8800ba3cfc80 ffffffffa8325106 ffffffffa869fdd4 ffff88006bbe1f10
> [385644.710046]  0000000000001007 ffff88006bbe1f60 ffff88006bbe1f10 ffff8803daa965a0
> [385644.722774] Call Trace:
> [385644.723591] dump_stack (lib/dump_stack.c:52)
> [385644.725180] warn_slowpath_common (kernel/panic.c:448)
> [385644.728983] warn_slowpath_null (kernel/panic.c:482)
> [385644.730679] pagecache_isize_extended (mm/truncate.c:740 (discriminator 1))
> [385644.732630] truncate_setsize (mm/truncate.c:710)
> [385644.734469] v9fs_vfs_setattr_dotl (fs/9p/v9fs_vfs.h:81 fs/9p/vfs_inode_dotl.c:593)
> [385644.753009] notify_change (fs/attr.c:270)
> [385644.754303] do_truncate (fs/open.c:64)
> [385644.759181] do_sys_ftruncate.constprop.5 (fs/open.c:193)
> [385644.760669] SyS_ftruncate (fs/open.c:201)
> [385644.761818] entry_SYSCALL_64_fastpath (arch/x86/entry/entry_64.S:186)
> 
> But I'm not really sure how that happens... truncate_setsize() changes the inode
> size before calling pagecache_isize_extended():
> 
> 	i_size_write(inode, newsize);
> 	if (newsize > oldsize)
> 		pagecache_isize_extended(inode, oldsize, newsize);
> 	truncate_pagecache(inode, newsize);
> 
> And notify_change() is verifying that i_mutex is held:
> 
> 	WARN_ON_ONCE(!mutex_is_locked(&inode->i_mutex));
> 
> So it doesn't look like a race either.

Well, looking at the code it can be a race which is specific to 9p
filesystem. It seems to me that 9p can update i_size from
v9fs_refresh_inode_dotl(). That can be called v9fs_lookup_revalidate()
without holding i_mutex. Now I'm not sure d_revalidate() can really race
with truncate on the same inode (whether there isn't something else
protecting this). Al should know better...

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
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