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SubjectRe: [PATCH] btrfs: remove BUG_ON used as assertions
On Wed, Dec 18, 2019 at 11:38:18AM -0500, Josef Bacik wrote:
> On 12/15/19 12:12 PM, Aditya Pakki wrote:
> > alloc_extent_state_atomic() allocates extents via GFP_ATOMIC flag
> > and cannot fail. There are multiple invocations of BUG_ON on the
> > return value to check for failure. The patch replaces certain
> > invocations of BUG_ON by returning the error upstream.
> >
> > Signed-off-by: Aditya Pakki <pakki001@umn.edu>
>
> I already tried this a few months ago and gave up. There are a few things if
> you want to tackle something like this
>
> 1) use bpf's error injection thing to make sure you handle every path that can
> error out. This is the script I wrote to do just that
>
> https://github.com/josefbacik/debug-scripts/blob/master/error-injection-stress.py
>
> 2) We actually can't fail here. We would need to go back and make _all_ callers
> of lock_extent_bits() handle the allocation error. This is theoretically
> possible, but a giant pain in the ass. In general we can make allocations here
> and we need to be able to make them.
>
> 3) We should probably mark this path with __GFP_NOFAIL because again, this is
> locking and we need locking to succeed.

NOFAIL can introduce loops that could lead to deadlocks, if not used
carefully. __set_extent_bit is not just locking, so if one thread wants
to set bits, allocate, wait, allocator goes to write some memory

eg.

set_extent_bit on some range
alloc state (NOFAIL)
allocator wants to flush dome dirty data
------------------------------>
set_extent_bit
alloc state (NOFAIL)
(wait)

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