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Re: [Qemu-devel] [PATCH v3 for-2.2 2/3] raw-posix: SEEK_HOLE suffices, g


From: Max Reitz
Subject: Re: [Qemu-devel] [PATCH v3 for-2.2 2/3] raw-posix: SEEK_HOLE suffices, get rid of FIEMAP
Date: Mon, 17 Nov 2014 11:30:44 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.2.0

On 2014-11-17 at 11:18, Markus Armbruster wrote:
Commit 5500316 (May 2012) implemented raw_co_is_allocated() as
follows:

1. If defined(CONFIG_FIEMAP), use the FS_IOC_FIEMAP ioctl

2. Else if defined(SEEK_HOLE) && defined(SEEK_DATA), use lseek()

3. Else pretend there are no holes

Later on, raw_co_is_allocated() was generalized to
raw_co_get_block_status().

Commit 4f11aa8 (May 2014) changed it to try the three methods in order
until success, because "there may be implementations which support
[SEEK_HOLE/SEEK_DATA] but not [FIEMAP] (e.g., NFSv4.2) as well as vice
versa."

Unfortunately, we used FIEMAP incorrectly: we lacked FIEMAP_FLAG_SYNC.
Commit 38c4d0a (Sep 2014) added it.  Because that's a significant
speed hit, the next commit 7c159037 put SEEK_HOLE/SEEK_DATA first.

As you see, the obvious use of FIEMAP is wrong, and the correct use is
slow.  I guess this puts it somewhere between -7 "The obvious use is
wrong" and -10 "It's impossible to get right" on Rusty Russel's Hard
to Misuse scale[*].

"Fortunately", the FIEMAP code is used only when

* SEEK_HOLE/SEEK_DATA aren't defined, but CONFIG_FIEMAP is

   Uncommon.  SEEK_HOLE had no XFS implementation between 2011 (when it
   was introduced for ext4 and btrfs) and 2012.

* SEEK_HOLE/SEEK_DATA and CONFIG_FIEMAP are defined, but lseek() fails

   Unlikely.

Thus, the FIEMAP code executes rarely.  Makes it a nice hidey-hole for
bugs.  Worse, bugs hiding there can theoretically bite even on a host
that has SEEK_HOLE/SEEK_DATA.

I don't want to worry about this crap, not even theoretically.  Get
rid of it.

[*] http://ozlabs.org/~rusty/index.cgi/tech/2008-04-01.html

Signed-off-by: Markus Armbruster <address@hidden>
Reviewed-by: Max Reitz <address@hidden>
Reviewed-by: Eric Blake <address@hidden>
---
  block/raw-posix.c | 60 ++++---------------------------------------------------
  1 file changed, 4 insertions(+), 56 deletions(-)

I only just now realized that you're not getting rid of FIEMAP completely; there's still the skip_fiemap flag in BDRVRawState. I don't care for now, though, thus my R-b stands.

Max



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