Re: [v4] Update mmap() flags and errors lists

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

On 2024-06-21 12:17, DJ Delorie wrote:
> Mathieu Desnoyers <mathieu.desnoyers@efficios.com> writes:
>>>>> +@item MAP_SYNC
>>>>> +This flag is used to map persistent memory devices into the running
>>>>> +program in such a way that writes to the mapping are immediately
>>>>> +written to the device as well.  Unlike most other flags, this one will
>>>>> +fail unless @code{MAP_SHARED_VALIDATE} is also given.
>>>>
>>>> Note that this wording is misleading. Users of persistent memory devices
>>>> need to issue explicit "flush" instructions to ensure that writes are
>>>> made persistent to the device. The MAP_SYNC merely guarantees that
>>>> memory mappings within a file on a dax-enabled filesystem will appear
>>>> at the same file offset after a crash/reboot. It goes not guarantee
>>>> anything about write persistence.
>>>
>>>     "This flag is supported only for files supporting DAX (direct mapping
>>>      of persistent memory)"
>>>
>>>     "it will be visible in the same file at the same offset even after the
>>>      system crashes or is rebooted."
>>>
>>> That sounds like persistence to me?
>>
>> AFAIU it states persistence of the mapping location within the file, but
>> it does not guarantee anything about persistence of the writes to that
>> mapping.
>>
>> In many cases, persistence of the writes to the mapping are only made
>> persistent after specific cache-flushing instructions have been issued.
>>
>> See this man page for reference:
>>
>> https://manpages.debian.org/experimental/libpmem-dev/pmem_flush.3
> 
> How about if I just say "This is a special flag for DAX devices." and
> not bother trying to be helpful?  ;-)
> 

That works for me.

> All the kernel does is write out dirty metadata whenever dirty data is
> written out, when the flag is set.

Exactly, it's affecting how the page fault handler populates the file
metadata before allowing user-space to see the newly populated page
AFAIU.

Thanks,

Mathieu


-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com