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

[Minchan Kim <minchan@kernel.org>]

On Tue, Feb 03, 2015 at 12:42:53PM +0100, 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:
> >> [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,
> 

IMHO,

Man page said
"MADV_DONTNEED: 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."

Heap by allocated by malloc(3) is anonymous page so it's a mapping
withtout an underlying file so userspace can expect zero-fill.

Man page said
"EINVAL: The application is attempting to release locked or
shared pages (with MADV_DONTNEED)"

So, user can expect the call on area by allocated by malloc(3)
if he doesn't call mlock will always be successful.

Man page said
"madivse: This call does not influence the semantics of the application
(except in the case of MADV_DONTNEED)"

So, we shouldn't break MADV_DONTNEED's semantic which free pages
instantly. It's a long time semantic and it was one of arguable issues
on MADV_FREE Rik had tried long time ago to replace MADV_DONTNEED
with MADV_FREE.

-- 
Kind regards,
Minchan Kim

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