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Message-Id: <5e67d651-830a-4d99-af37-26f2d0efd640@vates.tech>
Date: Mon, 30 Jun 2025 10:35:01 +0000
From: "Tu Dinh" <ngoc-tu.dinh@...es.tech>
To: "Abinash Singh" <abinashlalotra@...il.com>, jgross@...e.com, sstabellini@...nel.org
Cc: oleksandr_tyshchenko@...m.com, xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org, "Abinash Singh" <abinashsinghlalotra@...il.com>
Subject: Re: [RFC PATCH] xen/gntdev: reduce stack usage by dynamically allocating gntdev_copy_batch
Hi,
On 30/06/2025 06:54, Abinash Singh wrote:
> While building the kernel with LLVM, a warning was reported due to
> excessive stack usage in `gntdev_ioctl`:
>
> drivers/xen/gntdev.c:991: warning: stack frame size (1160) exceeds limit (1024) in function 'gntdev_ioctl'
>
> Further analysis revealed that the large stack frame was caused by
> `struct gntdev_copy_batch`, which was declared on the stack inside
> `gntdev_ioctl_grant_copy()`. Since this function was inlined into
> `gntdev_ioctl`, the stack usage was attributed to the latter.
>
> This patch fixes the issue by dynamically allocating `gntdev_copy_batch`
> using `kmalloc()`, which significantly reduces the stack footprint and
> eliminates the warning.
>
> This approach is consistent with similar fixes upstream, such as:
>
> commit fa26198d30f3 ("iommu/io-pgtable-arm: dynamically allocate selftest device struct")
>
> Fixes: a4cdb556cae0 ("xen/gntdev: add ioctl for grant copy")
> Signed-off-by: Abinash Singh <abinashsinghlalotra@...il.com>
> ---
> The stack usage was confirmed using the `-fstack-usage` flag and mannual analysis, which showed:
>
> drivers/xen/gntdev.c:953: gntdev_ioctl_grant_copy.isra 1048 bytes
> drivers/xen/gntdev.c:826: gntdev_copy 56 bytes
>
> Since `gntdev_ioctl` was calling the inlined `gntdev_ioctl_grant_copy`, the total
> frame size exceeded 1024 bytes, triggering the warning.
>
> This patch addresses the warning and keeps stack usage within acceptable limits.
> Is this patch fine or kmalloc may affect performance ?
> ---
Have you measured the performance impact? gntdev_ioctl_grant_copy is
called quite often especially by the backend. I'm afraid calling the
allocator here may cause even more slowdown than there already is,
especially when memory is tight.
> drivers/xen/gntdev.c | 24 +++++++++++++++---------
> 1 file changed, 15 insertions(+), 9 deletions(-)
>
> diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
> index 61faea1f0663..9811f3d7c87c 100644
> --- a/drivers/xen/gntdev.c
> +++ b/drivers/xen/gntdev.c
> @@ -953,15 +953,19 @@ static int gntdev_grant_copy_seg(struct gntdev_copy_batch *batch,
> static long gntdev_ioctl_grant_copy(struct gntdev_priv *priv, void __user *u)
> {
> struct ioctl_gntdev_grant_copy copy;
> - struct gntdev_copy_batch batch;
> + struct gntdev_copy_batch *batch;
> unsigned int i;
> int ret = 0;
>
> if (copy_from_user(©, u, sizeof(copy)))
> return -EFAULT;
> -
> - batch.nr_ops = 0;
> - batch.nr_pages = 0;
> +
> + batch = kmalloc(sizeof(*batch), GFP_KERNEL);
> + if (!batch)
> + return -ENOMEM;
> +
> + batch->nr_ops = 0;
> + batch->nr_pages = 0;
>
> for (i = 0; i < copy.count; i++) {
> struct gntdev_grant_copy_segment seg;
> @@ -971,18 +975,20 @@ static long gntdev_ioctl_grant_copy(struct gntdev_priv *priv, void __user *u)
> goto out;
> }
>
> - ret = gntdev_grant_copy_seg(&batch, &seg, ©.segments[i].status);
> + ret = gntdev_grant_copy_seg(batch, &seg, ©.segments[i].status);
> if (ret < 0)
> goto out;
>
> cond_resched();
> }
> - if (batch.nr_ops)
> - ret = gntdev_copy(&batch);
> - return ret;
> + if (batch->nr_ops)
> + ret = gntdev_copy(batch);
> + goto free_batch;
>
> out:
> - gntdev_put_pages(&batch);
> + gntdev_put_pages(batch);
> + free_batch:
> + kfree(batch);
> return ret;
> }
>
Ngoc Tu Dinh | Vates XCP-ng Developer
XCP-ng & Xen Orchestra - Vates solutions
web: https://vates.tech
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