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 -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
