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

[Vlastimil Babka]

On 02/03/2015 11:53 AM, Kirill A. Shutemov wrote:
> On Tue, Feb 03, 2015 at 09:19:15AM +0100, Vlastimil Babka wrote:
>> [CC linux-api, man pages]
>> 
>> On 02/02/2015 11:22 PM, Dave Hansen wrote:
>> > On 02/02/2015 08:55 AM, Mel Gorman wrote:
>> >> This patch identifies when a thread is frequently calling MADV_DONTNEED
>> >> on the same region of memory and starts ignoring the hint. On an 8-core
>> >> single-socket machine this was the impact on ebizzy using glibc 2.19.
>> > 
>> > The manpage, at least, claims that we zero-fill after MADV_DONTNEED is
>> > called:
>> > 
>> >>      MADV_DONTNEED
>> >>               Do  not  expect  access in the near future.  (For the time being, the application is finished with the given range, so the kernel can free resources
>> >>               associated with it.)  Subsequent accesses of pages in this range will succeed, but will result either in reloading of the memory contents  from  the
>> >>               underlying mapped file (see mmap(2)) or zero-fill-on-demand pages for mappings without an underlying file.
>> > 
>> > So if we have anything depending on the behavior that it's _always_
>> > zero-filled after an MADV_DONTNEED, this will break it.
>> 
>> OK, so that's a third person (including me) who understood it as a zero-fill
>> guarantee. I think the man page should be clarified (if it's indeed not
>> guaranteed), or we have a bug.
>> 
>> The implementation actually skips MADV_DONTNEED for
>> VM_LOCKED|VM_HUGETLB|VM_PFNMAP vma's.
> 
> 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).

- 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.

>> - 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.

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,

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