Re: [RFC PATCH 0/3] Add hugetlb MADV_DONTNEED support

[Axel Rasmussen <axelrasmussen@google.com>]

On Thu, Jan 27, 2022 at 3:57 AM David Hildenbrand <david@redhat.com> wrote:
>
> On 13.01.22 19:03, Mike Kravetz wrote:
> > Userfaultfd selftests for hugetlb does not perform UFFD_EVENT_REMAP
> > testing.  However, mremap support was recently added in commit
> > 550a7d60bd5e ("mm, hugepages: add mremap() support for hugepage backed
> > vma").  While attempting to enable mremap support in the test, it was
> > discovered that the mremap test indirectly depends on MADV_DONTNEED.
> >
> > hugetlb does not support MADV_DONTNEED.  However, the only thing
> > preventing support is a check in can_madv_lru_vma().  Simply removing
> > the check will enable support.
> >
> > This is sent as a RFC because there is no existing use case calling
> > for hugetlb MADV_DONTNEED support except possibly the userfaultfd test.
> > However, adding support makes sense as it is fairly trivial and brings
> > hugetlb functionality more in line with 'normal' memory.
> >
>
> Just a note:
>
> QEMU doesn't use huge anonymous memory directly (MAP_ANON | MAP_HUGE...)
> but instead always goes either via hugetlbfs or via memfd.
>
> For MAP_PRIVATE hugetlb mappings, fallocate(FALLOC_FL_PUNCH_HOLE) seems
> to get the job done (IOW: also discards private anon pages). See the
> comments in the QEMU code below. I remember that that is somewhat
> inconsistent. For ordinary MAP_PRIVATE mapped files I remember that we
> always need fallocate(FALLOC_FL_PUNCH_HOLE) + madvise(QEMU_MADV_DONTNEED)
> to make sure
>
> a) All file pages are removed
> b) All private anon pages are removed
>
> IIRC hugetlbfs really is different in that regard, but maybe other fs
> behave similarly.
>
> That's why QEMU was able to live for now without MADV_DONTNEED support
> for hugetlbfs and most probably won't ever need it.

Agreed, all of the production use cases I'm aware of use hugetlbfs,
not MAP_HUGE...

But, I would say this is convenient for testing purposes. It's
slightly more convenient to not have to mount hugetlbfs / perform the
associated setup for tests.

Perhaps that's only a small motivation for enabling this, but then
again Mike's patch to do so is likewise very small. :)


>
>
> ...
>        /* The logic here is messy;
>          *    madvise DONTNEED fails for hugepages
>          *    fallocate works on hugepages and shmem
>          *    shared anonymous memory requires madvise REMOVE
>          */
>         need_madvise = (rb->page_size == qemu_host_page_size);
>         need_fallocate = rb->fd != -1;
>         if (need_fallocate) {
>             /* For a file, this causes the area of the file to be zero'd
>              * if read, and for hugetlbfs also causes it to be unmapped
>              * so a userfault will trigger.
>              */
> #ifdef CONFIG_FALLOCATE_PUNCH_HOLE
>             ret = fallocate(rb->fd, FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE,
>                             start, length);
>             if (ret) {
>                 ret = -errno;
>                 error_report("ram_block_discard_range: Failed to fallocate "
>                              "%s:%" PRIx64 " +%zx (%d)",
>                              rb->idstr, start, length, ret);
>                 goto err;
>             }
> #else
>             ret = -ENOSYS;
>             error_report("ram_block_discard_range: fallocate not available/file"
>                          "%s:%" PRIx64 " +%zx (%d)",
>                          rb->idstr, start, length, ret);
>             goto err;
> #endif
>         }
>         if (need_madvise) {
>             /* For normal RAM this causes it to be unmapped,
>              * for shared memory it causes the local mapping to disappear
>              * and to fall back on the file contents (which we just
>              * fallocate'd away).
>              */
> #if defined(CONFIG_MADVISE)
>             if (qemu_ram_is_shared(rb) && rb->fd < 0) {
>                 ret = madvise(host_startaddr, length, QEMU_MADV_REMOVE);
>             } else {
>                 ret = madvise(host_startaddr, length, QEMU_MADV_DONTNEED);
>             }
>             if (ret) {
>                 ret = -errno;
>                 error_report("ram_block_discard_range: Failed to discard range "
>                              "%s:%" PRIx64 " +%zx (%d)",
>                              rb->idstr, start, length, ret);
>                 goto err;
>             }
> #else
> ...
>
> --
> Thanks,
>
> David / dhildenb
>