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. > - 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. -- Kirill A. Shutemov -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>