linux-kernel - Re: [fs/pipe] 5a519c8fe4: WARNING:at_mm/page_alloc.c:#__alloc

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Date:   Wed, 20 Apr 2022 12:07:40 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     kernel test robot <oliver.sang@...el.com>
Cc:     Andrei Vagin <avagin@...il.com>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>, lkp@...ts.01.org,
        kernel test robot <lkp@...el.com>
Subject: Re: [fs/pipe] 5a519c8fe4: WARNING:at_mm/page_alloc.c:#__alloc_pages

On Wed, Apr 20, 2022 at 12:37 AM kernel test robot
<oliver.sang@...el.com> wrote:
>
> commit: 5a519c8fe4d6 ("fs/pipe: use kvcalloc to allocate a pipe_buffer array")
>
> [   32.170781][ T3729] WARNING: The mand mount option has been deprecated and
> [   32.170781][ T3729]          and is ignored by this kernel. Remove the mand
> [   32.170781][ T3729]          option from the mount to silence this warning.

Heh. Not that warning.

This warning:

> [ 224.552771][ T3730] WARNING: CPU: 1 PID: 3730 at mm/page_alloc.c:5364 __alloc_pages (mm/page_alloc.c:5364)

That's just the

  5363          if (unlikely(order >= MAX_ORDER)) {
  5364                  WARN_ON_ONCE(!(gfp & __GFP_NOWARN));
  5365                  return NULL;
  5366          }

so somebody is doing a big allocation that will fail, and doesn't use
__GFP_NOWARN.

That someone being iter_file_splice_write():

> [ 224.567299][ T3730] kmalloc_order (include/linux/gfp.h:572 include/linux/gfp.h:595 include/linux/gfp.h:609 mm/slab_common.c:944)
> [ 224.567707][ T3730] kmalloc_order_trace (mm/slab_common.c:960)
> [ 224.568173][ T3730] __kmalloc (include/linux/slab.h:510 mm/slub.c:4413)
> [ 224.568571][ T3730] iter_file_splice_write (include/linux/slab.h:? include/linux/slab.h:652 fs/splice.c:628)
> [ 224.570060][ T3730] do_splice (fs/splice.c:767 fs/splice.c:1079)
> [ 224.572386][ T3730] __ia32_sys_splice (fs/splice.c:1144 fs/splice.c:1350 fs/splice.c:1332 fs/splice.c:1332)

and that's the

        int nbufs = pipe->max_usage;
        struct bio_vec *array = kcalloc(nbufs, sizeof(struct bio_vec),
                                        GFP_KERNEL);

thing, and no, using __GFP_NOWARN here isn't what we'd want to do,
because the code in question has no fallback (it will just return
-ENOMEM).

Now, technically, returning -ENOMEM is a "fallback", but not really.
It just means the kernel won't crash, it doesn't mean that this is
acceptable behavior.

Basically, that commit 5a519c8fe4d6 made it possible to create a pipe
that is effectively "too large to be used". It used to be that such a
pipe could never be created before, because the 'pipe->bufs' resizing
allocation used to be

        bufs = kcalloc(nr_slots, sizeof(*bufs),
                       GFP_KERNEL_ACCOUNT | __GFP_NOWARN);

and 'sizeof(struct pipe_buffer)' is bigger than 'sizeof(struct
bio_vec)', so if the resizing was successful, then the pipe buffer
count was guaranteed to be smaller than what that file_splice code
would use.

So it really does look like this whole "allow the pipe size to grow
almost unlimited" change was a fundamental mistake. It has these kinds
of subtle issues.

I'm inclined to revert commit 5a519c8fe4d6 - doing multiple iterations
really shouldn't be so expensive, and this shows that the whole "try
to do it in one big go" is fundamentally broken.

Could 'iter_file_splice_write()' be changed to limit it some way? Yes.

Could it be changed to use kvcalloc() too? Yes again.

But I'm not convinced that some odd corner-case CRIU optimization is
worth this kind of pain.

                      Linus