https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=247276
Bug ID: 247276
Summary: [fusefs] lockup when using mmap w/ direct_io
Product: Base System
Version: 12.1-RELEASE
Hardware: Any
OS: Any
Status: New
Severity: Affects Some People
Priority: ---
Component: kern
Assignee: [email protected]
Reporter: [email protected]
Attachment #215581 text/plain
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Created attachment 215581
--> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=215581&action=edit
example mmap usage
I'm the author of the FUSE based filesystem mergerfs. A user recently reported
that after updating to 12.1 some client software to mergerfs was locking up.
rtorrent in particular.
rtorrent uses mmap. Linux's FUSE implementation is unable to handle mmap when
direct_io is enabled so I recommend to users not enable direct_io who need any
software leveraging mmap. The user hadn't read the docs and was using direct_io
on FreeBSD. It appears that mmap does work on FreeBSD's FUSE implementation
when direct_io is used so he had no issues.
Once updated his setup started blocking and the apps became unkillable (waiting
on IO). I wrote a simple mmap example to read and write to a shared mapped file
and could reproduce the issue.
When direct_io is off everything works fine. If I enable direct_io I will see
read requests come into the mergerfs server and so long as I only read from the
mapped memory it works fine. When writing however as soon as it needs to flush
it seems to lock up. I don't see any write commands come in, the client app
blocks and is unkillable, other calls into the filesystem seem to work briefly
and then block. A stack trace of mergerfs seems to indicate that it is working
as normal and could take requests. If mergerfs is killed none of the clients
receive an error from the syscalls they are blocked on.
I've been able to reproduce this with sshfs by simply adding `direct_io` to the
mount options.
Attached is an example that triggers it.
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