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Re: [PATCH V4 02/19] physmem: fd-based shared memory

From: Steven Sistare
Subject: Re: [PATCH V4 02/19] physmem: fd-based shared memory
Date: Tue, 17 Dec 2024 16:54:43 -0500
User-agent: Mozilla Thunderbird 
On 12/16/2024 1:19 PM, Peter Xu wrote:

On Fri, Dec 13, 2024 at 11:41:45AM -0500, Steven Sistare wrote:

On 12/12/2024 4:22 PM, Peter Xu wrote:

On Thu, Dec 12, 2024 at 03:38:00PM -0500, Steven Sistare wrote:

On 12/9/2024 2:42 PM, Peter Xu wrote:

On Mon, Dec 02, 2024 at 05:19:54AM -0800, Steve Sistare wrote:

@@ -2089,13 +2154,23 @@ RAMBlock *qemu_ram_alloc_internal(ram_addr_t size, 
ram_addr_t max_size,
        new_block->page_size = qemu_real_host_page_size();
        new_block->host = host;
        new_block->flags = ram_flags;
+
+    if (!host && !xen_enabled()) {


Adding one more xen check is unnecessary.  This patch needed it could mean
that the patch can be refactored.. because we have xen checks in both
ram_block_add() and also in the fd allocation path.

At the meantime, see:

qemu_ram_alloc_from_fd():
       if (kvm_enabled() && !kvm_has_sync_mmu()) {
           error_setg(errp,
                      "host lacks kvm mmu notifiers, -mem-path unsupported");
           return NULL;
       }

I don't think any decent kernel could hit this, but that could be another
sign that this patch duplicated some file allocations.


+        if ((new_block->flags & RAM_SHARED) &&
+            !qemu_ram_alloc_shared(new_block, &local_err)) {
+            goto err;
+        }
+    }
+
        ram_block_add(new_block, &local_err);
-    if (local_err) {
-        g_free(new_block);
-        error_propagate(errp, local_err);
-        return NULL;
+    if (!local_err) {
+        return new_block;
        }
-    return new_block;
+
+err:
+    g_free(new_block);
+    error_propagate(errp, local_err);
+    return NULL;
    }


IIUC we only need to conditionally convert an anon-allocation into an
fd-allocation, and then we don't need to mostly duplicate
qemu_ram_alloc_from_fd(), instead we reuse it.

I do have a few other comments elsewhere, but when I was trying to comment.
E.g., we either shouldn't need to bother caching qemu_memfd_check()
results, or do it in qemu_memfd_check() directly.. and some more.


Someone thought it a good idea to cache the result of qemu_memfd_alloc_check,
and qemu_memfd_check will be called more often.  I'll cache the result inside
qemu_memfd_check for the special case of flags=0.


OK.




Then I think it's easier I provide a patch, and also show that it can be
also smaller changes to do the same thing, with everything fixed up
(e.g. addressing above mmu notifier missing issue).  What do you think as
below?


The key change you make is calling qemu_ram_alloc_from_fd instead of 
file_ram_alloc,
which buys the xen and kvm checks for free.  Sounds good, I will do that in the
context of my patch.

Here are some other changes in your patch, and my responses:

I will drop the "Retrying using MAP_ANON|MAP_SHARED" message, as you did.

However, I am keeping QEMU_VMALLOC_ALIGN, qemu_set_cloexec, and 
trace_qemu_ram_alloc_shared.


I guess no huge deal on these, however since we're talking..  Is that
QEMU_VMALLOC_ALIGN from qemu_anon_ram_alloc()?

A quick dig tells me that it was used to be for anon THPs..

      commit 36b586284e678da28df3af9fd0907d2b16f9311c
      Author: Avi Kivity <avi@redhat.com>
      Date:   Mon Sep 5 11:07:05 2011 +0300

      qemu_vmalloc: align properly for transparent hugepages and KVM

And I'm guessing if at that time was also majorly for guest ram.

Considering that this path won't make an effect until the new aux mem
option is on, I'd think it better to stick without anything special like
QEMU_VMALLOC_ALIGN, until it's justified to be worthwhile.  E.g., Avi used
to explicitly mention this in that commit message:

      Adjust qemu_vmalloc() to honor that requirement.  Ignore it for small 
regions
      to avoid fragmentation.

And this is exactly mostly small regions when it's AUX.. probably except
VGA, but it'll be SHARED on top of shmem not PRIVATE on anon anyway... so
it'll be totally different things.

So I won't worry on that 2M alignment, and I will try to not carry over
that, because then trying to remove it will be harder.. even when we want.


Yes, currently the aux allocations get QEMU_VMALLOC_ALIGN alignment in
qemu_anon_ram_alloc.  I do the same for the shared fd mappings to guarantee
no performance regression,


I don't know how we could guarantee that at all - anon and shmem uses
different knobs to enable/disable THPs after all.. For example:

   $ ls /sys/kernel/mm/transparent_hugepage/*enabled
   /sys/kernel/mm/transparent_hugepage/enabled
   /sys/kernel/mm/transparent_hugepage/shmem_enabled


Yes, but at least shmem_enabled is something the end user can fix.  If
we bake a poor alignment into qemu, the user has no recourse.  By setting
it to QEMU_VMALLOC_ALIGN, I eliminate alignment as a potential performance
issue. There is no practical downside.  We should just do it, especially if
you believe "no huge deal on these" as written above :)


And their default values normally differ too... it means after switching to
fd based we do face the possibility that thp can be gone at least on the
1st 2mb.

When I was suggesting it, I was hoping thp doesn't really matter that lot
on aux mem, even for VGA.

Btw, I don't even think the alignment will affect THP allocations for the
whole vma, anyway?  I mean, it's only about the initial 2MB portion.. IOW,
when not aligned, I think the worst case is we have <2MB at start address
that is not using THP, but later on when it starts to align with 2MB, THPs
will be allocated again.


It depends on the kernel version/implementation.  In 6.13, it is not that
clever for memfd_create + mmap.  An unaligned start means no huge pages anywhere
in the allocation, as shown by the page-types utility.  Add QEMU_VMALLOC_ALIGN,
and I get huge pages.


The challenge is more on the "fd-based" side, where shmem on most distros
will disable THP completely.


as some of them are larger than 2M and would
benefit from using huge pages.  The VA fragmentation is trivial for this small
number of aux blocks in a 64-bit address space, and is no different than it was
for qemu_anon_ram_alloc.


For the 2nd.. Any quick answer on why explicit qemu_set_cloexec() needed?


qemu sets cloexec for all descriptors it opens to prevent them from accidentally
being leaked to another process via fork+exec.


But my question is why this is special?  For example, we don't do that for
"-object memory-backend-memfd", am I right?


We should, the backends also need to set cloexec when they use a cpr fd.
I'll delete the call here and push it into cpr_find_fd.


For 3rd, tracepoint would definitely be fine whenever you feel necessary.


Also, when qemu_memfd_create + qemu_ram_alloc_from_fd fails, qemu should fail 
and exit,
and not fall back, because something unexpected went wrong.  David said the 
same.


Why?  I was trying to rely on such fallback to make it work on e.g. Xen.
In that case, Xen fails there and fallback to xen_ram_alloc() inside the
later call to ram_block_add(), no?


Why -- because something went wrong that should have worked, and we should 
report the
first fault so its cause can be fixed and cpr can be used.


Ahh so it's only about the corner cases where CPR could raise an error?
Can we rely on the failure later on "migrate" command to tell which
ramblock doesn't support it, so the user could be aware as well?


The ramblock migration blocker will indeed tell us which block is a problem.

But, we are throwing away potentially useful information by dropping the
first error message on the floor. We should only fall back for expected
failures.  Unexpected failures mean there is something to fix.

I can compromise and fail on errors from these:
  qemu_memfd_create(name, 0, 0, 0, 0, errp);
  qemu_shm_alloc(0, errp);

but ignore errors from the subsequent call to qemu_ram_alloc_from_fd,
and fall back. That keeps the code simple.


However, to do the above, but still quietly fallback if qemu_ram_alloc_from_fd
fails because of xen or kvm, I would need to return different error codes from
qemu_ram_alloc_from_fd.  Doable, but requires tweaks to all occurrences of
qemu_ram_alloc_from_fd.

And BTW, qemu_ram_alloc_from_fd is defined for CONFIG_POSIX only.  I need
to modify the call site in the patch accordingly.


Yep, I was thinking maybe qemu_ram_alloc_from_fd() had a stub function,
indeed looks not..  "allocating the fd" part definitely has, which I
remember I checked..


Overall, I am not convinced that using qemu_ram_alloc_from_fd in this patch
is better/simpler than my V4 patch using file_ram_alloc, plus adding xen and
kvm_has_sync_mmu checks in qemu_ram_alloc_internal.


As long as you don't need to duplicate these two checks (or duplicate any
such check..) I'm ok.

Reusing qemu_ram_alloc_from_fd() still sounds like the easiest to go.  Yes
we'll need to teach it about resize(), used_length etc. to it, but they all
look sane to me.  We didn't have those simply because we don't have use of
them, now we want to have resizable fd-based mem, that's the right thing to
do to support that on fd allocations.

OTOH, duplicating xen/mmu checks isn't sane to me.. :( It will make the
code harder to maintain because the 3rd qemu_ram_alloc_from_fd() in the
future will need to duplicate it once more (or worse, forget it again until
xen / old kernels reports a failure)..


I'll make the necessary changes to use qemu_ram_alloc_from_fd.

- Steve
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