[llvm-dev] lld: sigbus error handling

[Rui Ueyama via llvm-dev]

On Mon, Oct 23, 2017 at 3:40 PM, Brian Cain <brian.cain at gmail.com> wrote:

>
>
> On Mon, Oct 23, 2017 at 5:21 PM, Rui Ueyama via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> If your system does not support fallocate(2), we use ftruncate(2) to
>> create an output file. fallocate(2) succeeds even if your disk have less
>> space than the requested size, because it creates a sparse file. If you
>> mmap such sparse file, you'll receive a SIGBUS when the disk actually
>> becomes full.
>>
>> So, lld can die suddenly with SIGBUS when your disk becomes full, and
>> currently we are not doing anything about it. It's sometimes hard to notice
>> that that was caused by the lack of disk space.
>>
>> I wonder if we should print out a hint (e.g. "Bus error -- disk full?")
>> when we receive a SIGBUS. Any opinions?
>>
>>
> What about reading back the file's size with stat() before mapping?
> .st_blocks should give the "real" size.
>

Creating a sparse file is not an error, and a sparse file works fine as
long as your disk has enough space (which is almost always true.) So I
don't know how stat helps.


> BTW, posix_fallocate() might provide better portability and decrease the
> likelihood of falling back on ftruncate().
>

Yes, but we want to avoid that function because when it falls back, it is
very slow. What it does when fallocate(2) is not available is to actually
write 0 to every block to make sure that all disk blocks are allocated.
Doing it every time the linker creates a new file is a bit overkill to
guard against disk full situation which isn't common.


>
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>
>
> --
> -Brian
>
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