[Bug 205932] [panic] Kernel panic when copying from ext2fs partition to UFS partition

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=205932

--- Comment #6 from Damjan Jovanovic <damjan.jov at gmail.com> ---
(In reply to Will B from comment #5)

In the "cp" tool, function copy_file() in file /usr/src/bin/cp/utils.c has 2
ways of copying a file: mmap + write, and read + write.

mmap + write is used only when all of the following are true:
* VM_AND_BUFFER_CACHE_SYNCHRONIZED is defined (it is, in Makefile)
* It's a regular file (according to S_ISREG())
* The file's size is > 0
* The file's size is <= 8 MiB
* Calling mmap() succeeds

You say it happens on different files each time. My guess is all of the files
that it happens on, are regular non-empty files <= 8 MiB in size.

There's 2 known bugs where mmap() succeeds but reading the mapped contents can
cause problems:

1. The serious bug 205938 where struct buf leaks from ext4_bmapext(), causing a
panic on every attempt to read even 1 byte of the mapped data. Because your
trace shows the panic happens with lbn=0 (the very first block of the file)
this is probably the problem here. This bug was fixed in CURRENT yesterday,
which is newer than your 10.2-RELEASE. You could try the patch from that bug,
it does apply and work for me (for a different mmap problem) on 10.2-RELEASE.

2. Even with that bug fixed, sparse blocks will at best contain garbage data
read from the wrong disk blocks instead of zeroes, corrupting the copied file
you create (maybe a panic is also possible?). This is bug 205816 which I
previously mentioned. This isn't even fixed in CURRENT, but there is a patch on
that bug you can try.

(BTW you could also use ext4fuse from ports instead of the in-kernel ext2fs
driver since it doesn't suffer from problem 1, but it does suffer from problem
2, and I don't know/have a patch for it)

With both those patches, copying files from EXT4 should be reliable and
correct, but very slow in the mmap + write case (about 250 kB/second with 100%
CPU usage) which is unbearably slow if you have lots of files <= 8 MiB. I have
a 1 line patch to speed it up about 100-fold, but it only works on CURRENT -
10.2 panics with it. Will keep trying.

This is the sad state of current EXT4 support outside Linux...

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