Re: [BUG] trigger BUG_ON in mas_store_prealloc when low memory

[Liam R. Howlett]

* John Hsu (許永翰) <John.Hsu@xxxxxxxxxxxx> [230609 04:37]:
> Hi reviewers and author liam.howlett@xxxxxxxxxx,
> Kindly ping.
> 

Hello!

Thanks for reporting this issue.

> We met BUG_ON in mas_store_prealloc with kernel-6.1 stress testing
> environment.

version 6.1 or 6.1.x?  Which exact version (git id or version number)

> According to coredump, BUG_ON is triggered by mas->node with error
> number -ENOMEM(0xffffffffffffffd2).
> There are some mas_node_count functions in mas_wr_store_entry, and it
> seems that mas->node may be set to error node with -ENOMEM if there was
> no enough memory spcace for maple tree operations.
> We think that return -ENOMEM instead of directly triggering BUG_ON when
> memory is not available is suitable,

This BUG_ON() is necessary since this function should _never_ run out of
memory; this function does not return an error code. mas_preallocate()
should have gotten you the memory necessary (or returned an -ENOMEM)
prior to the call to mas_store_prealloc(), so this is probably an
internal tree problem.

>because in reality the tree
> operation shouldn't be performed in this situation.

There is a tree operation being performed here.  mprotect is merging a
vma by the looks of the call stack.  Why do you think no tree operation
is necessary?

> 
> following are the backtrace:
> mas_store_prealloc+0x23c/0x484
> vma_mas_store+0xe4/0x2d0
> __vma_adjust+0xab0/0x1470
> vma_merge+0x5b8/0x5d4
> mprotect_fixup+0x1f4/0x478
> __arm64_sys_mprotect+0x6b0/0x8f0
> invoke_syscall+0x84/0x264
> el0_svc_common+0x118/0x1f0
> do_el0_svc+0x5c/0x184
> el0_svc+0x38/0x98

I see this is arm64.  Do you have a reproducer?  If you don't have a
reproducer, I can try stress-ng on amr64 to simulate your workload using
mprotect, but I need to know the exact kernel version as this issue may
have been fixed in a later stable release.

Thanks,
Liam