obj: allocation is partially outside the pool

[igchor]

ISSUE: obj: allocation is partially outside the pool

Environment Information

• PMDK package version(s): 1.7, 1.6, 1.5, 1.4
• OS(es) version(s):
• ndctl version(s):
• kernel version(s):
• compiler, libraries, packaging and other related tools version(s):

Please provide a reproduction of the bug:

Reproduction is available here: https://github.com/igchor/obj_repro

Accessing memory returned by pmemobj_tx_xalloc causes segfault.

How often bug is revealed: (always, often, rare):

rare - only if pool has certain size and with specific allocation sizes

Actual behavior:

Segfault when accessing memory

Expected behavior:

No segfault, memory is within the pool

Details

Allocator did not take the size of all possible metadata when calculating total available heap size for user allocations.

Additional information about Priority and Help Requested:

Are you willing to submit a pull request with a proposed change? (Yes, No)

Requested priority: (Showstopper, High, Medium, Low)

[marcinslusarz]

Quoting Piotr (#4522):

The calculations for total available heap size for user
allocations didn't take all possible metadata into account,
leading to situations where for certain heap sizes
the zone size was too large by a chunk.

To hit this problem the pool has to be almost full. The heap size calculation can be off by 1kB. There's 1/256 chance of hitting this problem for randomly sized pool. However, for pools whose size is a multiple of 1MB (1048576 bytes), the problem doesn't occur.

PMDK 1.8 fixes this problem for newly created pools. The fix for pools created with older versions will be part of PMDK 1.8.1, along with patches for previous stable branches.

For now, the workaround for this problem in the case of pools on FSDAX is to manually extend the pool by 1kB. Pools on Device DAX with default alignment (2MB) are not affected because theirs sizes must be a multiple of 1MB. For pools on Device DAX with 4kB alignment, there's no known workaround if the problem has been hit.