Re: [PATCH v4] mm: Add PM_HUGE_THP_MAPPING to /proc/pid/pagemap

[Mina Almasry <almasrymina@google.com>]

On Mon, Nov 15, 2021 at 5:41 PM Andrew Morton <akpm@linux-foundation.org> wrote:
>
> On Wed, 10 Nov 2021 14:11:20 -0800 Mina Almasry <almasrymina@google.com> wrote:
>
> > Add PM_HUGE_THP MAPPING to allow userspace to detect whether a given virt
> > address is currently mapped by a transparent huge page or not.  Example
> > use case is a process requesting THPs from the kernel (via a huge tmpfs
> > mount for example), for a performance critical region of memory.  The
> > userspace may want to query whether the kernel is actually backing this
> > memory by hugepages or not.
> >
> > PM_HUGE_THP_MAPPING bit is set if the virt address is mapped at the PMD
> > level and the underlying page is a transparent huge page.
> >
> > A few options were considered:
> > 1. Add /proc/pid/pageflags that exports the same info as
> >    /proc/kpageflags.  This is not appropriate because many kpageflags are
> >    inappropriate to expose to userspace processes.
> > 2. Simply get this info from the existing /proc/pid/smaps interface.
> >    There are a couple of issues with that:
> >    1. /proc/pid/smaps output is human readable and unfriendly to
> >       programatically parse.
> >    2. /proc/pid/smaps is slow.  The cost of reading /proc/pid/smaps into
> >       userspace buffers is about ~800us per call, and this doesn't
> >       include parsing the output to get the information you need. The
> >       cost of querying 1 virt address in /proc/pid/pagemaps however is
> >       around 5-7us.
> >
> > Tested manually by adding logging into transhuge-stress, and by
> > allocating THP and querying the PM_HUGE_THP_MAPPING flag at those
> > virtual addresses.
> >
> > --- a/tools/testing/selftests/vm/transhuge-stress.c
> > +++ b/tools/testing/selftests/vm/transhuge-stress.c
> > @@ -16,6 +16,12 @@
> >  #include <string.h>
> >  #include <sys/mman.h>
> >
> > +/*
> > + * We can use /proc/pid/pagemap to detect whether the kernel was able to find
> > + * hugepages or no. This can be very noisy, so is disabled by default.
> > + */
> > +#define NO_DETECT_HUGEPAGES
> > +
> >
> > ...
> >
> > +#ifndef NO_DETECT_HUGEPAGES
> > +             if (!PAGEMAP_THP(ent[0]))
> > +                     fprintf(stderr, "WARNING: detected non THP page\n");
> > +#endif
>
> This looks like a developer thing.  Is there any point in leaving it in
> the mainline code?

I used this to test locally and I thought it may be useful, but on
second thought probably not worth it. Removed in v6 I just sent.

On Mon, Nov 15, 2021 at 5:59 PM Peter Xu <peterx@redhat.com> wrote:
>
> On Mon, Nov 15, 2021 at 02:50:26PM -0800, Mina Almasry wrote:
> > PM_THP_MAPPED sounds good to me.
> >
> > TBH I think I still prefer this approach because it's a very simple 2
> > line patch which addresses the concrete use case I have well. I'm not
> > too familiar with the smaps code to be honest but I think adding a
> > range-based smaps API will be a sizeable patch to add a syscall,
> > handle a stable interface, and handle cases where the memory range
> > doesn't match a VMA boundary. I'm not sure the performance benefit
> > would justify this patch and I'm not sure the extra info from smaps
> > would be widely useful. However if you insist and folks believe this
> > is the better approach I can prototype a range-based smaps and test
> > its performance to see if it works for us as well, just let me know
> > what kind of API you're envisioning.
>
> Yeah indeed I haven't yet thought enough on such a new interface, it's just
> that I think it'll be something that solves a broader range of requests
> including the thp-aware issue, so I raised it up.
>
> That shouldn't require a lot code change either afaiu, as smap_gather_stats()
> already takes a "start" and I think what's missing is another end where we just
> pass in 0 when we want the default vma->vm_end as the end of range.
>
> I don't have a solid clue on other use case to ask for that more generic
> interface, so please feel free to move on with it.  If you'll need a repost to
> address the comment from Andrew on removing the debugging lines, please also
> consider using the shorter PM_THP_MAPPED then it looks good to me too.
>

Awesome, thanks! PM_THP_MAPPED sounds good to me and I just sent v6
with these changes.

> Thanks!
>
> --
> Peter Xu
>