On 4 February 2015 at 18:02, Vlastimil Babka <vbabka@xxxxxxx> wrote:
> On 02/04/2015 03:00 PM, Michael Kerrisk (man-pages) wrote:
>>
>> Hello Vlastimil,
>>
>> On 4 February 2015 at 14:46, Vlastimil Babka <vbabka@xxxxxxx> wrote:
>>>>>
>>>>> - that covers mlocking ok, not sure if the rest fits the "shared pages"
>>>>> case
>>>>> though. I dont see any check for other kinds of shared pages in the
>>>>> code.
>>>>
>>>>
>>>> Agreed. "shared" here seems confused. I've removed it. And I've
>>>> added mention of "Huge TLB pages" for this error.
>>>
>>>
>>> Thanks.
>>
>>
>> I also added those cases for MADV_REMOVE, BTW.
>
>
> Right. There's also the following for MADV_REMOVE that needs updating:
>
> "Currently, only shmfs/tmpfs supports this; other filesystems return with
> the error ENOSYS."
>
> - it's not just shmem/tmpfs anymore. It should be best to refer to
> fallocate(2) option FALLOC_FL_PUNCH_HOLE which seems to be (more) up to
> date.
>
> - AFAICS it doesn't return ENOSYS but EOPNOTSUPP. Also neither error code is
> listed in the ERRORS section.
Yup, I recently added that as well, based on a patch from Jan Chaloupka.
>>>>>>> - The word "will result" did sound as a guarantee at least to me. So
>>>>>>> here it
>>>>>>> could be changed to "may result (unless the advice is ignored)"?
>>>>>>
>>>>>> It's too late to fix documentation. Applications already depends on
>>>>>> the
>>>>>> beheviour.
>>>>>
>>>>> Right, so as long as they check for EINVAL, it should be safe. It
>>>>> appears
>>>>> that
>>>>> jemalloc does.
>>>>
>>>> So, first a brief question: in the cases where the call does not error
>>>> out,
>>>> are we agreed that in the current implementation, MADV_DONTNEED will
>>>> always result in zero-filled pages when the region is faulted back in
>>>> (when we consider pages that are not backed by a file)?
>>>
>>> I'd agree at this point.
>>
>> Thanks for the confirmation.
>>
>>> Also we should probably mention anonymously shared pages (shmem). I think
>>> they behave the same as file here.
>>
>> You mean tmpfs here, right? (I don't keep all of the synonyms straight.)
>
> shmem is tmpfs (that by itself would fit under "files" just fine), but also
> sys V segments created by shmget(2) and also mappings created by mmap with
> MAP_SHARED | MAP_ANONYMOUS. I'm not sure if there's a single manpage to
> refer to the full list.
So, how about this text:
After a successful MADV_DONTNEED operation, the seman‐
tics of memory access in the specified region are
changed: subsequent accesses of pages in the range
will succeed, but will result in either reloading of
the memory contents from the underlying mapped file
(for shared file mappings, shared anonymous mappings,
and shmem-based techniques such as System V shared
memory segments) or zero-fill-on-demand pages for
anonymous private mappings.
Thanks,
Michael
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