[PATCH v3 0/2] virtiofs: fix the warning for kernel direct IO

[PATCH v3 0/2] virtiofs: fix the warning for kernel direct IO

[Hou Tao]

From: Hou Tao <houtao1@huawei.com>

Hi,

The patch set aims to fix the warning related to an abnormal size
parameter of kmalloc() in virtiofs. Patch #1 fixes it by introducing
use_pages_for_kvec_io option in fuse_conn and enabling it in virtiofs.
Beside the abnormal size parameter for kmalloc, the gfp parameter is
also questionable: GFP_ATOMIC is used even when the allocation occurs
in a kworker context. Patch #2 fixes it by using GFP_NOFS when the
allocation is initiated by the kworker. For more details, please check
the individual patches.

As usual, comments are always welcome.

Change Log:

v3:
 * introduce use_pages_for_kvec_io for virtiofs. When the option is
   enabled, fuse will use iov_iter_extract_pages() to construct a page
   array and pass the pages array instead of a pointer to virtiofs.
   The benefit is twofold: the length of the data passed to virtiofs is
   limited by max_pages, and there is no memory copy compared with v2.

v2: https://lore.kernel.org/linux-fsdevel/20240228144126.2864064-1-houtao@huaweicloud.com/
  * limit the length of ITER_KVEC dio by max_pages instead of the
    newly-introduced max_nopage_rw. Using max_pages make the ITER_KVEC
    dio being consistent with other rw operations.
  * replace kmalloc-allocated bounce buffer by using a bounce buffer
    backed by scattered pages when the length of the bounce buffer for
    KVEC_ITER dio is larger than PAG_SIZE, so even on hosts with
    fragmented memory, the KVEC_ITER dio can be handled normally by
    virtiofs. (Bernd Schubert)
  * merge the GFP_NOFS patch [1] into this patch-set and use
    memalloc_nofs_{save|restore}+GFP_KERNEL instead of GFP_NOFS
    (Benjamin Coddington)

v1: https://lore.kernel.org/linux-fsdevel/20240103105929.1902658-1-houtao@huaweicloud.com/

[1]: https://lore.kernel.org/linux-fsdevel/20240105105305.4052672-1-houtao@huaweicloud.com/

Hou Tao (2):
  virtiofs: use pages instead of pointer for kernel direct IO
  virtiofs: use GFP_NOFS when enqueuing request through kworker

 fs/fuse/file.c      | 12 ++++++++----
 fs/fuse/fuse_i.h    |  3 +++
 fs/fuse/virtio_fs.c | 25 ++++++++++++++++---------
 3 files changed, 27 insertions(+), 13 deletions(-)

-- 
2.29.2

[PATCH v3 1/2] virtiofs: use pages instead of pointer for kernel direct IO

[Hou Tao]

From: Hou Tao <houtao1@huawei.com>

When trying to insert a 10MB kernel module kept in a virtio-fs with cache
disabled, the following warning was reported:

  ------------[ cut here ]------------
  WARNING: CPU: 1 PID: 404 at mm/page_alloc.c:4551 ......
  Modules linked in:
  CPU: 1 PID: 404 Comm: insmod Not tainted 6.9.0-rc5+ #123
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996) ......
  RIP: 0010:__alloc_pages+0x2bf/0x380
  ......
  Call Trace:
   <TASK>
   ? __warn+0x8e/0x150
   ? __alloc_pages+0x2bf/0x380
   __kmalloc_large_node+0x86/0x160
   __kmalloc+0x33c/0x480
   virtio_fs_enqueue_req+0x240/0x6d0
   virtio_fs_wake_pending_and_unlock+0x7f/0x190
   queue_request_and_unlock+0x55/0x60
   fuse_simple_request+0x152/0x2b0
   fuse_direct_io+0x5d2/0x8c0
   fuse_file_read_iter+0x121/0x160
   __kernel_read+0x151/0x2d0
   kernel_read+0x45/0x50
   kernel_read_file+0x1a9/0x2a0
   init_module_from_file+0x6a/0xe0
   idempotent_init_module+0x175/0x230
   __x64_sys_finit_module+0x5d/0xb0
   x64_sys_call+0x1c3/0x9e0
   do_syscall_64+0x3d/0xc0
   entry_SYSCALL_64_after_hwframe+0x4b/0x53
   ......
   </TASK>
  ---[ end trace 0000000000000000 ]---

The warning is triggered as follows:

1) syscall finit_module() handles the module insertion and it invokes
kernel_read_file() to read the content of the module first.

2) kernel_read_file() allocates a 10MB buffer by using vmalloc() and
passes it to kernel_read(). kernel_read() constructs a kvec iter by
using iov_iter_kvec() and passes it to fuse_file_read_iter().

3) virtio-fs disables the cache, so fuse_file_read_iter() invokes
fuse_direct_io(). As for now, the maximal read size for kvec iter is
only limited by fc->max_read. For virtio-fs, max_read is UINT_MAX, so
fuse_direct_io() doesn't split the 10MB buffer. It saves the address and
the size of the 10MB-sized buffer in out_args[0] of a fuse request and
passes the fuse request to virtio_fs_wake_pending_and_unlock().

4) virtio_fs_wake_pending_and_unlock() uses virtio_fs_enqueue_req() to
queue the request. Because virtiofs need DMA-able address, so
virtio_fs_enqueue_req() uses kmalloc() to allocate a bounce buffer for
all fuse args, copies these args into the bounce buffer and passed the
physical address of the bounce buffer to virtiofsd. The total length of
these fuse args for the passed fuse request is about 10MB, so
copy_args_to_argbuf() invokes kmalloc() with a 10MB size parameter and
it triggers the warning in __alloc_pages():

	if (WARN_ON_ONCE_GFP(order > MAX_PAGE_ORDER, gfp))
		return NULL;

5) virtio_fs_enqueue_req() will retry the memory allocation in a
kworker, but it won't help, because kmalloc() will always return NULL
due to the abnormal size and finit_module() will hang forever.

A feasible solution is to limit the value of max_read for virtio-fs, so
the length passed to kmalloc() will be limited. However it will affect
the maximal read size for normal read. And for virtio-fs write initiated
from kernel, it has the similar problem but now there is no way to limit
fc->max_write in kernel.

So instead of limiting both the values of max_read and max_write in
kernel, introducing use_pages_for_kvec_io in fuse_conn and setting it as
true in virtiofs. When use_pages_for_kvec_io is enabled, fuse will use
pages instead of pointer to pass the KVEC_IO data.

Fixes: a62a8ef9d97d ("virtio-fs: add virtiofs filesystem")
Signed-off-by: Hou Tao <houtao1@huawei.com>
---
 fs/fuse/file.c      | 12 ++++++++----
 fs/fuse/fuse_i.h    |  3 +++
 fs/fuse/virtio_fs.c |  1 +
 3 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index b57ce41576407..82b77c5d8c643 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -1471,13 +1471,17 @@ static inline size_t fuse_get_frag_size(const struct iov_iter *ii,
 
 static int fuse_get_user_pages(struct fuse_args_pages *ap, struct iov_iter *ii,
 			       size_t *nbytesp, int write,
-			       unsigned int max_pages)
+			       unsigned int max_pages,
+			       bool use_pages_for_kvec_io)
 {
 	size_t nbytes = 0;  /* # bytes already packed in req */
 	ssize_t ret = 0;
 
-	/* Special case for kernel I/O: can copy directly into the buffer */
-	if (iov_iter_is_kvec(ii)) {
+	/* Special case for kernel I/O: can copy directly into the buffer.
+	 * However if the implementation of fuse_conn requires pages instead of
+	 * pointer (e.g., virtio-fs), use iov_iter_extract_pages() instead.
+	 */
+	if (iov_iter_is_kvec(ii) && !use_pages_for_kvec_io) {
 		unsigned long user_addr = fuse_get_user_addr(ii);
 		size_t frag_size = fuse_get_frag_size(ii, *nbytesp);
 
@@ -1585,7 +1589,7 @@ ssize_t fuse_direct_io(struct fuse_io_priv *io, struct iov_iter *iter,
 		size_t nbytes = min(count, nmax);
 
 		err = fuse_get_user_pages(&ia->ap, iter, &nbytes, write,
-					  max_pages);
+					  max_pages, fc->use_pages_for_kvec_io);
 		if (err && !nbytes)
 			break;
 
diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
index f239196103137..d4f04e19058c1 100644
--- a/fs/fuse/fuse_i.h
+++ b/fs/fuse/fuse_i.h
@@ -860,6 +860,9 @@ struct fuse_conn {
 	/** Passthrough support for read/write IO */
 	unsigned int passthrough:1;
 
+	/* Use pages instead of pointer for kernel I/O */
+	unsigned int use_pages_for_kvec_io:1;
+
 	/** Maximum stack depth for passthrough backing files */
 	int max_stack_depth;
 
diff --git a/fs/fuse/virtio_fs.c b/fs/fuse/virtio_fs.c
index 322af827a2329..36984c0e23d14 100644
--- a/fs/fuse/virtio_fs.c
+++ b/fs/fuse/virtio_fs.c
@@ -1512,6 +1512,7 @@ static int virtio_fs_get_tree(struct fs_context *fsc)
 	fc->delete_stale = true;
 	fc->auto_submounts = true;
 	fc->sync_fs = true;
+	fc->use_pages_for_kvec_io = true;
 
 	/* Tell FUSE to split requests that exceed the virtqueue's size */
 	fc->max_pages_limit = min_t(unsigned int, fc->max_pages_limit,
-- 
2.29.2

[PATCH v3 2/2] virtiofs: use GFP_NOFS when enqueuing request through kworker

[Hou Tao]

From: Hou Tao <houtao1@huawei.com>

When invoking virtio_fs_enqueue_req() through kworker, both the
allocation of the sg array and the bounce buffer still use GFP_ATOMIC.
Considering the size of the sg array may be greater than PAGE_SIZE, use
GFP_NOFS instead of GFP_ATOMIC to lower the possibility of memory
allocation failure and to avoid unnecessarily depleting the atomic
reserves. GFP_NOFS is not passed to virtio_fs_enqueue_req() directly,
GFP_KERNEL and memalloc_nofs_{save|restore} helpers are used instead.

Signed-off-by: Hou Tao <houtao1@huawei.com>
---
 fs/fuse/virtio_fs.c | 24 +++++++++++++++---------
 1 file changed, 15 insertions(+), 9 deletions(-)

diff --git a/fs/fuse/virtio_fs.c b/fs/fuse/virtio_fs.c
index 36984c0e23d14..096b589ed2fcc 100644
--- a/fs/fuse/virtio_fs.c
+++ b/fs/fuse/virtio_fs.c
@@ -91,7 +91,8 @@ struct virtio_fs_req_work {
 };
 
 static int virtio_fs_enqueue_req(struct virtio_fs_vq *fsvq,
-				 struct fuse_req *req, bool in_flight);
+				 struct fuse_req *req, bool in_flight,
+				 gfp_t gfp);
 
 static const struct constant_table dax_param_enums[] = {
 	{"always",	FUSE_DAX_ALWAYS },
@@ -430,6 +431,8 @@ static void virtio_fs_request_dispatch_work(struct work_struct *work)
 
 	/* Dispatch pending requests */
 	while (1) {
+		unsigned int flags;
+
 		spin_lock(&fsvq->lock);
 		req = list_first_entry_or_null(&fsvq->queued_reqs,
 					       struct fuse_req, list);
@@ -440,7 +443,9 @@ static void virtio_fs_request_dispatch_work(struct work_struct *work)
 		list_del_init(&req->list);
 		spin_unlock(&fsvq->lock);
 
-		ret = virtio_fs_enqueue_req(fsvq, req, true);
+		flags = memalloc_nofs_save();
+		ret = virtio_fs_enqueue_req(fsvq, req, true, GFP_KERNEL);
+		memalloc_nofs_restore(flags);
 		if (ret < 0) {
 			if (ret == -ENOMEM || ret == -ENOSPC) {
 				spin_lock(&fsvq->lock);
@@ -545,7 +550,7 @@ static void virtio_fs_hiprio_dispatch_work(struct work_struct *work)
 }
 
 /* Allocate and copy args into req->argbuf */
-static int copy_args_to_argbuf(struct fuse_req *req)
+static int copy_args_to_argbuf(struct fuse_req *req, gfp_t gfp)
 {
 	struct fuse_args *args = req->args;
 	unsigned int offset = 0;
@@ -559,7 +564,7 @@ static int copy_args_to_argbuf(struct fuse_req *req)
 	len = fuse_len_args(num_in, (struct fuse_arg *) args->in_args) +
 	      fuse_len_args(num_out, args->out_args);
 
-	req->argbuf = kmalloc(len, GFP_ATOMIC);
+	req->argbuf = kmalloc(len, gfp);
 	if (!req->argbuf)
 		return -ENOMEM;
 
@@ -1183,7 +1188,8 @@ static unsigned int sg_init_fuse_args(struct scatterlist *sg,
 
 /* Add a request to a virtqueue and kick the device */
 static int virtio_fs_enqueue_req(struct virtio_fs_vq *fsvq,
-				 struct fuse_req *req, bool in_flight)
+				 struct fuse_req *req, bool in_flight,
+				 gfp_t gfp)
 {
 	/* requests need at least 4 elements */
 	struct scatterlist *stack_sgs[6];
@@ -1204,8 +1210,8 @@ static int virtio_fs_enqueue_req(struct virtio_fs_vq *fsvq,
 	/* Does the sglist fit on the stack? */
 	total_sgs = sg_count_fuse_req(req);
 	if (total_sgs > ARRAY_SIZE(stack_sgs)) {
-		sgs = kmalloc_array(total_sgs, sizeof(sgs[0]), GFP_ATOMIC);
-		sg = kmalloc_array(total_sgs, sizeof(sg[0]), GFP_ATOMIC);
+		sgs = kmalloc_array(total_sgs, sizeof(sgs[0]), gfp);
+		sg = kmalloc_array(total_sgs, sizeof(sg[0]), gfp);
 		if (!sgs || !sg) {
 			ret = -ENOMEM;
 			goto out;
@@ -1213,7 +1219,7 @@ static int virtio_fs_enqueue_req(struct virtio_fs_vq *fsvq,
 	}
 
 	/* Use a bounce buffer since stack args cannot be mapped */
-	ret = copy_args_to_argbuf(req);
+	ret = copy_args_to_argbuf(req, gfp);
 	if (ret < 0)
 		goto out;
 
@@ -1309,7 +1315,7 @@ __releases(fiq->lock)
 		 fuse_len_args(req->args->out_numargs, req->args->out_args));
 
 	fsvq = &fs->vqs[queue_id];
-	ret = virtio_fs_enqueue_req(fsvq, req, false);
+	ret = virtio_fs_enqueue_req(fsvq, req, false, GFP_ATOMIC);
 	if (ret < 0) {
 		if (ret == -ENOMEM || ret == -ENOSPC) {
 			/*
-- 
2.29.2

Re: [PATCH v3 1/2] virtiofs: use pages instead of pointer for kernel direct IO

[Hou Tao]

On 4/26/2024 10:39 PM, Hou Tao wrote:
> From: Hou Tao <houtao1@huawei.com>
>
> When trying to insert a 10MB kernel module kept in a virtio-fs with cache
> disabled, the following warning was reported:
>
>   ------------[ cut here ]------------
>   WARNING: CPU: 1 PID: 404 at mm/page_alloc.c:4551 ......
>   Modules linked in:
>   CPU: 1 PID: 404 Comm: insmod Not tainted 6.9.0-rc5+ #123
>   Hardware name: QEMU Standard PC (i440FX + PIIX, 1996) ......
>   RIP: 0010:__alloc_pages+0x2bf/0x380
>   ......
>   Call Trace:
>    <TASK>
>    ? __warn+0x8e/0x150
>    ? __alloc_pages+0x2bf/0x380
>    __kmalloc_large_node+0x86/0x160
>    __kmalloc+0x33c/0x480
>    virtio_fs_enqueue_req+0x240/0x6d0
>    virtio_fs_wake_pending_and_unlock+0x7f/0x190
>    queue_request_and_unlock+0x55/0x60
>    fuse_simple_request+0x152/0x2b0
>    fuse_direct_io+0x5d2/0x8c0
>    fuse_file_read_iter+0x121/0x160
>    __kernel_read+0x151/0x2d0
>    kernel_read+0x45/0x50
>    kernel_read_file+0x1a9/0x2a0
>    init_module_from_file+0x6a/0xe0
>    idempotent_init_module+0x175/0x230
>    __x64_sys_finit_module+0x5d/0xb0
>    x64_sys_call+0x1c3/0x9e0
>    do_syscall_64+0x3d/0xc0
>    entry_SYSCALL_64_after_hwframe+0x4b/0x53
>    ......
>    </TASK>
>   ---[ end trace 0000000000000000 ]---
>
> The warning is triggered as follows:
>

SNIP
> @@ -1585,7 +1589,7 @@ ssize_t fuse_direct_io(struct fuse_io_priv *io, struct iov_iter *iter,
>  		size_t nbytes = min(count, nmax);
>  
>  		err = fuse_get_user_pages(&ia->ap, iter, &nbytes, write,
> -					  max_pages);
> +					  max_pages, fc->use_pages_for_kvec_io);
>  		if (err && !nbytes)
>  			break;

Just find out that flush_kernel_vmap_range() and
invalidate_kernel_vmap_range() should be used before DMA rw operation
and after DMA read operation if the kvec IO is backed by vmalloc() area.
Will update it in v4.
>  
> diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
> index f239196103137..d4f04e19058c1 100644
> --- a/fs/fuse/fuse_i.h
> +++ b/fs/fuse/fuse_i.h
> @@ -860,6 +860,9 @@ struct fuse_conn {
>  	/** Passthrough support for read/write IO */
>  	unsigned int passthrough:1;
>  
> +	/* Use pages instead of pointer for kernel I/O */
> +	unsigned int use_pages_for_kvec_io:1;
> +
>  	/** Maximum stack depth for passthrough backing files */
>  	int max_stack_depth;
>  
> diff --git a/fs/fuse/virtio_fs.c b/fs/fuse/virtio_fs.c
> index 322af827a2329..36984c0e23d14 100644
> --- a/fs/fuse/virtio_fs.c
> +++ b/fs/fuse/virtio_fs.c
> @@ -1512,6 +1512,7 @@ static int virtio_fs_get_tree(struct fs_context *fsc)
>  	fc->delete_stale = true;
>  	fc->auto_submounts = true;
>  	fc->sync_fs = true;
> +	fc->use_pages_for_kvec_io = true;
>  
>  	/* Tell FUSE to split requests that exceed the virtqueue's size */
>  	fc->max_pages_limit = min_t(unsigned int, fc->max_pages_limit,