Re: [v2 PATCH] mm: shmem: allow split THP when truncating THP partially

From: David Hildenbrand
Date: Fri Feb 21 2020 - 04:36:37 EST

On 21.02.20 10:07, Michael S. Tsirkin wrote:
> On Thu, Feb 20, 2020 at 10:16:54AM -0800, Alexander Duyck wrote:
>> On Tue, Dec 3, 2019 at 4:43 PM Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx> wrote:
>>> Currently when truncating shmem file, if the range is partial of THP
>>> (start or end is in the middle of THP), the pages actually will just get
>>> cleared rather than being freed unless the range cover the whole THP.
>>> Even though all the subpages are truncated (randomly or sequentially),
>>> the THP may still be kept in page cache. This might be fine for some
>>> usecases which prefer preserving THP.
>>> But, when doing balloon inflation in QEMU, QEMU actually does hole punch
>>> or MADV_DONTNEED in base page size granulairty if hugetlbfs is not used.
>>> So, when using shmem THP as memory backend QEMU inflation actually doesn't
>>> work as expected since it doesn't free memory. But, the inflation
>>> usecase really needs get the memory freed. Anonymous THP will not get
>>> freed right away too but it will be freed eventually when all subpages are
>>> unmapped, but shmem THP would still stay in page cache.
>>> Split THP right away when doing partial hole punch, and if split fails
>>> just clear the page so that read to the hole punched area would return
>>> zero.
>>> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
>>> Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
>>> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
>>> Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
>> One question I would have is if this is really the desired behavior we
>> are looking for?
>> By proactively splitting the THP you are likely going to see a
>> performance regression with the virtio-balloon driver enabled in QEMU.
>> I would suspect the response to that would be to update the QEMU code
>> to identify the page size of the shared memory ramblock. At that
>> point I suspect it would start behaving the same as how it currently
>> handles anonymous memory, and the work done here would essentially
>> have been wasted other than triggering the desire to resolve this in
>> QEMU to avoid a performance regression.
>> The code for inflating a the balloon in virtio-balloon in QEMU can be
>> found here:
>> If there is a way for us to just populate the value obtained via
>> qemu_ram_pagesize with the THP page size instead of leaving it at 4K,
>> which is the size I am assuming it is at since you indicated that it
>> is just freeing the base page size, then we could address the same
>> issue and likely get the desired outcome of freeing the entire THP
>> page when it is no longer used.
>> - Alex
> Well that would be racy right? It could be THP when you call
> the function, by the time you try to free it, it's already
> split up ...
> Two more points:
> 1. we can probably teach QEMU to always use the pbp
> machinery - will be helpful to reduce number of madvise calls too.

The pbp machinery only works in the special case where the target page
size > 4k and the guest is nice enough to send the 4k chunks of a target
page sequentially. If the guest sends random pages, it is not of any use.

> 2. Something we should do is teach balloon to
> inflate using address/length pairs instead of PFNs.
> This way we can pass a full THP in one go.

The balloon works on 4k pages only. It is expected to break up THP and
harm performance. Or if that's not possible *do nothing*. Similar to
when balloon inflation is inhibited (e.g., vfio).

There was some work on huge page ballooning in a paper I read. But once
the guest is out of huge pages to report, it would want to fallback to
smaller granularity (down to 4k, to create real memory pressure), where
you would end up in the very same situation you are right now. So it's -
IMHO - only of limited used.

With what you suggest, you'll harm performance to reuse more memory.
IMHO, ballooning can be expected to harm performance. (after all, if you
inflate a 4k page in your guest, the guest won't be able to use a huge
page around that page anymore as well - until it compacts balloon
memory, resulting in new deflate/inflate steps). But I guess, it depends
on the use case ...


David / dhildenb