linux-kernel - Re: MADV_DONTNEED semantics? Was: [RFC PATCH] mm: madvise: Ignore repeated MADV

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Message-ID: <54D2508A.9030804@suse.cz>
Date:	Wed, 04 Feb 2015 18:02:02 +0100
From:	Vlastimil Babka <vbabka@...e.cz>
To:	mtk.manpages@...il.com
CC:	"Kirill A. Shutemov" <kirill@...temov.name>,
	Dave Hansen <dave.hansen@...el.com>,
	Mel Gorman <mgorman@...e.de>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Minchan Kim <minchan@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	lkml <linux-kernel@...r.kernel.org>,
	Linux API <linux-api@...r.kernel.org>,
	linux-man <linux-man@...r.kernel.org>,
	Hugh Dickins <hughd@...gle.com>
Subject: Re: MADV_DONTNEED semantics? Was: [RFC PATCH] mm: madvise: Ignore
 repeated MADV_DONTNEED hints

On 02/04/2015 03:00 PM, Michael Kerrisk (man-pages) wrote:
> Hello Vlastimil,
>
> On 4 February 2015 at 14:46, Vlastimil Babka <vbabka@...e.cz> 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.

>>>>>> - 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.

Thanks,
Vlastimil
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