Re: MADV_DONTNEED semantics? Was: [RFC PATCH] mm: madvise: Ignore repeated MADV_DONTNEED hints

["Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>]

Hello Vlastimil,

On 4 February 2015 at 14:46, Vlastimil Babka <vbabka@suse.cz> wrote:
> On 02/03/2015 05:20 PM, Michael Kerrisk (man-pages) wrote:
>>
>> On 02/03/2015 12:42 PM, Vlastimil Babka wrote:
>>>
>>> On 02/03/2015 11:53 AM, Kirill A. Shutemov wrote:
>>>>
>>>> On Tue, Feb 03, 2015 at 09:19:15AM +0100, Vlastimil Babka wrote:
>>>>
>>>> It doesn't skip. It fails with -EINVAL. Or I miss something.
>>>
>>>
>>> No, I missed that. Thanks for pointing out. The manpage also explains
>>> EINVAL in
>>> this case:
>>>
>>> *  The application is attempting to release locked or shared pages (with
>>> MADV_DONTNEED).
>>
>> Yes, there is that. But the page could be more explicit when discussing
>> MADV_DONTNEED in the main text. I've done that.
>>
>>> - 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.

>>>>> - 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.)

>>> I still wouldnt be sure just by reading the man page that the clearing is
>>> guaranteed whenever I dont get an error return value, though,
>>
>> I'm not quite sure what you want here. I mean: if there's an error,
>
> I was just reiterating that the guarantee is not clear from if you consider
> all the statements in the man page.
>
>> then the DONTNEED action didn't occur, right? Therefore, there won't
>> be zero-filled pages. But, for what it's worth, I added "If the
>> operation succeeds" at the start of that sentence beginning "Subsequent
>> accesses...".
>
> Yes, that should clarify it. Thanks!

Okay.

>> Now, some history, explaining why the page is a bit of a mess,
>> and for that matter why I could really use more help on it from MM
>> folk (especially in the form of actual patches [1], rather than notes
>> about deficiencies in the documentation), because:
>>
>>      ***I simply cannot keep up with all of the details***.
>
> I see, and expected it would be like this. I would just send patch if the
> situation was clear, but here we should agree first, and I thought you
> should be involved from the beginning.

Sorry -- I should have made it clearer, this statement was not
targeted at you personally, or even necessarily at this particular
thread. It was a general comment, that came up sharply to me as I
looked at how much cruft there is in the madvise() page.

>> Once upon a time (Linux 2.4), there was madvise() with just 5 flags:
>>
>>         MADV_NORMAL
>>         MADV_RANDOM
>>         MADV_SEQUENTIAL
>>         MADV_WILLNEED
>>         MADV_DONTNEED
>>
>> And already a dozen years ago, *I* added the text about MADV_DONTNEED.
>> Back then, I believe it was true. I'm not sure if it's still true now,
>> but I assume for the moment that it is, and await feedback. And the
>> text saying that the call does not affect the semantics of memory
>> access dates back even further (and was then true, MADV_DONTNEED aside).
>>
>> Those 5 flags have analogs in POSIX's posix_madvise() (albeit, there
>> is a semantic mismatch between the destructive MADV_DONTNEED and
>> POSIX's nondestructive POSIX_MADV_DONTNEED). They also appear
>> on most other implementations.
>>
>> Since the original implementation, numerous pieces of cruft^W^W^W
>> excellent new flags have been overloaded into this one system call.
>> Some of those certainly violated the "does not change the semantics
>> of the application" statement, but, sadly, the kernel developers who
>> implemented MADV_REMOVE or MADV_DONTFORK did not think to send a
>> patch to the man page for those new flags, one that might have noted
>> that the semantics of the application are changed by such flags. Equally
>> sadly, I did overlook to scan the bigger page when *I* added
>> documentation of these flags to those pages, otherwise I might have
>> caught that detail.
>>
>> So, just to repeat, I  could really use more help on it from MM
>> folk in the form of actual patches to the man page.
>
> Thanks for the background. I'll try to remember to check for man-pages part
> when I review some api changing patch.

That would be great.

Thanks,

Michael



-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/

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