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, -- 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
