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 [off-list ref] wrote:

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- 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.

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- 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.

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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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