Re: [PATCH 4/4] mm: make every pte dirty on do_swap_page

[Minchan Kim]

Hello Andrew,

On Thu, Apr 09, 2015 at 01:59:39PM -0700, Andrew Morton wrote:
> On Thu, 9 Apr 2015 08:50:25 +0900 Minchan Kim <minchan@xxxxxxxxxx> wrote:
> 
> > Bump.
> 
> I'm getting the feeling that MADV_FREE is out of control.
> 
> Below is the overall rollup of
> 
> mm-support-madvisemadv_free.patch
> mm-support-madvisemadv_free-fix.patch
> mm-support-madvisemadv_free-fix-2.patch
> mm-dont-split-thp-page-when-syscall-is-called.patch
> mm-dont-split-thp-page-when-syscall-is-called-fix.patch
> mm-dont-split-thp-page-when-syscall-is-called-fix-2.patch
> mm-free-swp_entry-in-madvise_free.patch
> mm-move-lazy-free-pages-to-inactive-list.patch
> mm-move-lazy-free-pages-to-inactive-list-fix.patch
> mm-move-lazy-free-pages-to-inactive-list-fix-fix.patch
> mm-move-lazy-free-pages-to-inactive-list-fix-fix-fix.patch
> mm-make-every-pte-dirty-on-do_swap_page.patch
> 
> 
> It's pretty large and has its sticky little paws in all sorts of places.
> 
> 
> The feature would need to be pretty darn useful to justify a mainline
> merge.  Has any such usefulness been demonstrated?

Jemalloc has used MADV_FREE instead of MADV_DONTNEED for a long time
in MADV_FREE supporting OSes(FreeBSD, Solaris, Darwin, Windows).
It used MADV_DONTNEED on only Linux because there was no the feature.

========================== &< ===========================

jemalloc:

/*
 * Methods for purging unused pages differ between operating systems.
 *
 *   madvise(..., MADV_DONTNEED) : On Linux, this immediately discards pages,
 *                                 such that new pages will be demand-zeroed if
 *                                 the address region is later touched.
 *   madvise(..., MADV_FREE) : On FreeBSD and Darwin, this marks pages as being
 *                             unused, such that they will be discarded rather
 *                             than swapped out.
 */
...

bool
pages_purge(void *addr, size_t length)
{
        bool unzeroed;

#ifdef _WIN32
        VirtualAlloc(addr, length, MEM_RESET, PAGE_READWRITE);
        unzeroed = true;
#elif defined(JEMALLOC_HAVE_MADVISE)
#  ifdef JEMALLOC_PURGE_MADVISE_DONTNEED
#    define JEMALLOC_MADV_PURGE MADV_DONTNEED
#    define JEMALLOC_MADV_ZEROS true
#  elif defined(JEMALLOC_PURGE_MADVISE_FREE)
#    define JEMALLOC_MADV_PURGE MADV_FREE
#    define JEMALLOC_MADV_ZEROS false
#  else
#    error "No madvise(2) flag defined for purging unused dirty pages."
#  endif
        int err = madvise(addr, length, JEMALLOC_MADV_PURGE);
        unzeroed = (!JEMALLOC_MADV_ZEROS || err != 0); 
#  undef JEMALLOC_MADV_PURGE
#  undef JEMALLOC_MADV_ZEROS
#else
        /* Last resort no-op. */
        unzeroed = true;
#endif
        return (unzeroed);
}


Tcmalloc is same page.

========================== &< ===========================

// MADV_FREE is specifically designed for use by malloc(), but only
// FreeBSD supports it; in linux we fall back to the somewhat inferior
// MADV_DONTNEED.
#if !defined(MADV_FREE) && defined(MADV_DONTNEED)
# define MADV_FREE  MADV_DONTNEED
#endif

..

bool TCMalloc_SystemRelease(void* start, size_t length) {
#ifdef MADV_FREE
  if (FLAGS_malloc_devmem_start) {
    // It's not safe to use MADV_FREE/MADV_DONTNEED if we've been
    // mapping /dev/mem for heap memory.
    return false;
  }
  if (FLAGS_malloc_disable_memory_release) return false;
  if (pagesize == 0) pagesize = getpagesize();
  const size_t pagemask = pagesize - 1;

  size_t new_start = reinterpret_cast<size_t>(start);
  size_t end = new_start + length;
  size_t new_end = end;

  // Round up the starting address and round down the ending address
  // to be page aligned:
  new_start = (new_start + pagesize - 1) & ~pagemask;
  new_end = new_end & ~pagemask;

  ASSERT((new_start & pagemask) == 0);
  ASSERT((new_end & pagemask) == 0);
  ASSERT(new_start >= reinterpret_cast<size_t>(start));
  ASSERT(new_end <= end);

  if (new_end > new_start) {
    int result;
    do {
      result = madvise(reinterpret_cast<char*>(new_start),
          new_end - new_start, MADV_FREE);
    } while (result == -1 && errno == EAGAIN);

    return result != -1;
  }
#endif
  return false;
}

glibc want it, too.
https://sourceware.org/ml/libc-alpha/2015-02/msg00197.html


-- 
Kind regards,
Minchan Kim
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