Re: [PATCH v2] regset: use kvzalloc() for regset_get_alloc()

Hi,

On Mon, Feb 26, 2024 at 3:55 PM Doug Anderson [off-list ref] wrote:

Hi,

On Mon, Feb 5, 2024 at 9:27 AM Douglas Anderson [off-list ref] wrote:

quoted

While browsing through ChromeOS crash reports, I found one with an
allocation failure that looked like this:

  chrome: page allocation failure: order:7,
          mode:0x40dc0(GFP_KERNEL|__GFP_COMP|__GFP_ZERO),
          nodemask=(null),cpuset=urgent,mems_allowed=0
  CPU: 7 PID: 3295 Comm: chrome Not tainted
          5.15.133-20574-g8044615ac35c #1 (HASH:1162 1)
  Hardware name: Google Lazor (rev3 - 8) with KB Backlight (DT)
  Call trace:
  ...
  warn_alloc+0x104/0x174
  __alloc_pages+0x5f0/0x6e4
  kmalloc_order+0x44/0x98
  kmalloc_order_trace+0x34/0x124
  __kmalloc+0x228/0x36c
  __regset_get+0x68/0xcc
  regset_get_alloc+0x1c/0x28
  elf_core_dump+0x3d8/0xd8c
  do_coredump+0xeb8/0x1378
  get_signal+0x14c/0x804
  ...

An order 7 allocation is (1 << 7) contiguous pages, or 512K. It's not
a surprise that this allocation failed on a system that's been running
for a while.

More digging showed that it was fairly easy to see the order 7
allocation by just sending a SIGQUIT to chrome (or other processes) to
generate a core dump. The actual amount being allocated was 279,584
bytes and it was for "core_note_type" NT_ARM_SVE.

There was quite a bit of discussion [1] on the mailing lists in
response to my v1 patch attempting to switch to vmalloc. The overall
conclusion was that we could likely reduce the 279,584 byte allocation
by quite a bit and Mark Brown has sent a patch to that effect [2].
However even with the 279,584 byte allocation gone there are still
65,552 byte allocations. These are just barely more than the 65,536
bytes and thus would require an order 5 allocation.

An order 5 allocation is still something to avoid unless necessary and
nothing needs the memory here to be contiguous. Change the allocation
to kvzalloc() which should still be efficient for small allocations
but doesn't force the memory subsystem to work hard (and maybe fail)
at getting a large contiguous chunk.

[1] https://lore.kernel.org/r/20240201171159.1.Id9ad163b60d21c9e56c2d686b0cc9083a8ba7924@changeid (local)
[2] https://lore.kernel.org/r/20240203-arm64-sve-ptrace-regset-size-v1-1-2c3ba1386b9e@kernel.org (local)

Signed-off-by: Douglas Anderson <dianders@chromium.org>
---

Changes in v2:
- Use kvzalloc() instead of vmalloc().
- Update description based on v1 discussion.

 fs/binfmt_elf.c | 2 +-
 kernel/regset.c | 6 +++---
 2 files changed, 4 insertions(+), 4 deletions(-)

Just wanted to check in to see if there's anything else that I need to
do here. Mark's patch to avoid the order 7 allocations [1] has landed,
but we still want this kvzalloc() because the order 5 allocations
can't really be avoided. I'm happy to sit tight for longer but just
wanted to make sure it was clear that we still want my patch _in
addition_ to Mark's patch and to see if there was anything else you
needed me to do.

Thanks!

[1] https://lore.kernel.org/r/20240213-arm64-sve-ptrace-regset-size-v2-1-c7600ca74b9b@kernel.org (local)

I'm not trying to be a pest here, so if this is on someone's todo list
and they'll get to it eventually then feel free to tell me to go away
and I'll snooze this for another few months. I just want to make sure
it's not forgotten.

I've been assuming that someone like Al Viro or Christian Brauner
would land this patch eventually and I know Al responded rather
quickly to my v1 [2]. I think all of Al's issues were resolved by Mark
Brown's patch [1] (which has landed in the arm64 tree) and my updating
of the patch description in v2. I see that Al and Christian are
flagged as maintainers of "fs/binfmt_elf.c" which is one of the two
files I'm touching, so that's mostly why I was assuming they would
land it.

...but I realize that perhaps my assumptions are wrong and this needs
to go through a different maintainer. In this case (if I'm reading it
correctly) Al and Christian are listed because the file is under "fs"
even though this isn't _really_ much of a filesystem-related patch.
Perhaps this needs to go through something like Andrew Morton's tree
since he often picks up patches that have nowhere else to land? If
someone else has suggestions, I'm all ears. I'm also happy to repost
this patch in case it helps with a maintainer applying it.

Thanks!

-Doug

[1] https://lore.kernel.org/r/20240213-arm64-sve-ptrace-regset-size-v2-1-c7600ca74b9b@kernel.org (local)
[2] https://lore.kernel.org/r/20240202012249.GU2087318@ZenIV/ (local)

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