Re: [PATCH v2 4/6] virtiofs: support bounce buffer backed by scattered pages

[Brian Foster <bfoster@redhat.com>]

On Sat, Mar 09, 2024 at 12:14:23PM +0800, Hou Tao wrote:
> Hi,
> 
> On 2/29/2024 11:01 PM, Brian Foster wrote:
> > On Wed, Feb 28, 2024 at 10:41:24PM +0800, Hou Tao wrote:
> >> From: Hou Tao <houtao1@huawei.com>
> >>
> >> When reading a file kept in virtiofs from kernel (e.g., insmod a kernel
> >> module), if the cache of virtiofs is disabled, the read buffer will be
> >> passed to virtiofs through out_args[0].value instead of pages. Because
> >> virtiofs can't get the pages for the read buffer, virtio_fs_argbuf_new()
> >> will create a bounce buffer for the read buffer by using kmalloc() and
> >> copy the read buffer into bounce buffer. If the read buffer is large
> >> (e.g., 1MB), the allocation will incur significant stress on the memory
> >> subsystem.
> >>
> >> So instead of allocating bounce buffer by using kmalloc(), allocate a
> >> bounce buffer which is backed by scattered pages. The original idea is
> >> to use vmap(), but the use of GFP_ATOMIC is no possible for vmap(). To
> >> simplify the copy operations in the bounce buffer, use a bio_vec flex
> >> array to represent the argbuf. Also add an is_flat field in struct
> >> virtio_fs_argbuf to distinguish between kmalloc-ed and scattered bounce
> >> buffer.
> >>
> >> Signed-off-by: Hou Tao <houtao1@huawei.com>
> >> ---
> >>  fs/fuse/virtio_fs.c | 163 ++++++++++++++++++++++++++++++++++++++++----
> >>  1 file changed, 149 insertions(+), 14 deletions(-)
> >>
> >> diff --git a/fs/fuse/virtio_fs.c b/fs/fuse/virtio_fs.c
> >> index f10fff7f23a0f..ffea684bd100d 100644
> >> --- a/fs/fuse/virtio_fs.c
> >> +++ b/fs/fuse/virtio_fs.c
> > ...
> >> @@ -408,42 +425,143 @@ static void virtio_fs_request_dispatch_work(struct work_struct *work)
> >>  	}
> >>  }
> >>  
> > ...  
> >>  static void virtio_fs_argbuf_copy_from_in_arg(struct virtio_fs_argbuf *argbuf,
> >>  					      unsigned int offset,
> >>  					      const void *src, unsigned int len)
> >>  {
> >> -	memcpy(argbuf->buf + offset, src, len);
> >> +	struct iov_iter iter;
> >> +	unsigned int copied;
> >> +
> >> +	if (argbuf->is_flat) {
> >> +		memcpy(argbuf->f.buf + offset, src, len);
> >> +		return;
> >> +	}
> >> +
> >> +	iov_iter_bvec(&iter, ITER_DEST, argbuf->s.bvec,
> >> +		      argbuf->s.nr, argbuf->s.size);
> >> +	iov_iter_advance(&iter, offset);
> > Hi Hou,
> >
> > Just a random comment, but it seems a little inefficient to reinit and
> > readvance the iter like this on every copy/call. It looks like offset is
> > already incremented in the callers of the argbuf copy helpers. Perhaps
> > iov_iter could be lifted into the callers and passed down, or even just
> > include it in the argbuf structure and init it at alloc time?
> 
> Sorry for the late reply. Being busy with off-site workshop these days.
> 

No worries.

> I have tried a similar idea before in which iov_iter was saved directly
> in argbuf struct, but it didn't work out well. The reason is that for
> copy both in_args and out_args, an iov_iter is needed, but the direction
> is different. Currently the bi-directional io_vec is not supported, so
> the code have to initialize the iov_iter twice: one for copy from
> in_args and another one for copy to out_args.
> 

Ok, seems reasonable enough.

> For dio read initiated from kernel, both of its in_numargs and
> out_numargs is 1, so there will be only one iov_iter_advance() in
> virtio_fs_argbuf_copy_to_out_arg() and the offset is 64, so I think the
> overhead will be fine. For dio write initiated from kernel, its
> in_numargs is 2 and out_numargs is 1, so there will be two invocations
> of iov_iter_advance(). The first one with offset=64, and the another one
> with offset=round_up_page_size(64 + write_size), so the later one may
> introduce extra overhead. But compared with the overhead of data copy, I
> still think the overhead of calling iov_iter_advance() is fine.
> 

I'm not claiming the overhead is some practical problem here, but rather
that we shouldn't need to be concerned with it in the first place with
some cleaner code. It's been a bit since I first looked at this. I was
originally wondering about defining the iov_iter in the caller and pass
as a param, but on another look, do the lowest level helpers really need
to exist?

If you don't anticipate further users, IMO something like the diff below
is a bit more clean (compile tested only, but no reinits and less code
overall). But that's just my .02, feel free to use it or not..

Brian

--- 8< ---

diff --git a/fs/fuse/virtio_fs.c b/fs/fuse/virtio_fs.c
index 9ee71051c89f..9de477e9ccd5 100644
--- a/fs/fuse/virtio_fs.c
+++ b/fs/fuse/virtio_fs.c
@@ -544,26 +544,6 @@ static unsigned int virtio_fs_argbuf_setup_sg(struct virtio_fs_argbuf *argbuf,
 	return cur - sg;
 }
 
-static void virtio_fs_argbuf_copy_from_in_arg(struct virtio_fs_argbuf *argbuf,
-					      unsigned int offset,
-					      const void *src, unsigned int len)
-{
-	struct iov_iter iter;
-	unsigned int copied;
-
-	if (argbuf->is_flat) {
-		memcpy(argbuf->f.buf + offset, src, len);
-		return;
-	}
-
-	iov_iter_bvec(&iter, ITER_DEST, argbuf->s.bvec,
-		      argbuf->s.nr, argbuf->s.size);
-	iov_iter_advance(&iter, offset);
-
-	copied = _copy_to_iter(src, len, &iter);
-	WARN_ON_ONCE(copied != len);
-}
-
 static unsigned int
 virtio_fs_argbuf_out_args_offset(struct virtio_fs_argbuf *argbuf,
 				 const struct fuse_args *args)
@@ -577,26 +557,6 @@ virtio_fs_argbuf_out_args_offset(struct virtio_fs_argbuf *argbuf,
 	return round_up(offset, PAGE_SIZE);
 }
 
-static void virtio_fs_argbuf_copy_to_out_arg(struct virtio_fs_argbuf *argbuf,
-					     unsigned int offset, void *dst,
-					     unsigned int len)
-{
-	struct iov_iter iter;
-	unsigned int copied;
-
-	if (argbuf->is_flat) {
-		memcpy(dst, argbuf->f.buf + offset, len);
-		return;
-	}
-
-	iov_iter_bvec(&iter, ITER_SOURCE, argbuf->s.bvec,
-		      argbuf->s.nr, argbuf->s.size);
-	iov_iter_advance(&iter, offset);
-
-	copied = _copy_from_iter(dst, len, &iter);
-	WARN_ON_ONCE(copied != len);
-}
-
 /*
  * Returns 1 if queue is full and sender should wait a bit before sending
  * next request, 0 otherwise.
@@ -684,23 +644,41 @@ static void virtio_fs_hiprio_dispatch_work(struct work_struct *work)
 static void copy_args_to_argbuf(struct fuse_req *req)
 {
 	struct fuse_args *args = req->args;
+	struct virtio_fs_argbuf *argbuf = req->argbuf;
+	struct iov_iter iter;
+	unsigned int copied;
 	unsigned int offset = 0;
 	unsigned int num_in;
 	unsigned int i;
 
+	if (!argbuf->is_flat) {
+		iov_iter_bvec(&iter, ITER_DEST, argbuf->s.bvec, argbuf->s.nr,
+			argbuf->s.size);
+	}
+
 	num_in = args->in_numargs - args->in_pages;
 	for (i = 0; i < num_in; i++) {
-		virtio_fs_argbuf_copy_from_in_arg(req->argbuf, offset,
-						  args->in_args[i].value,
-						  args->in_args[i].size);
-		offset += args->in_args[i].size;
+		const void *src = args->in_args[i].value;
+		unsigned int len = args->in_args[i].size;
+
+		if (argbuf->is_flat) {
+			memcpy(argbuf->f.buf + offset, src, len);
+			offset += len;
+			continue;
+		}
+
+		iov_iter_advance(&iter, len);
+		copied = _copy_to_iter(src, len, &iter);
+		WARN_ON_ONCE(copied != len);
 	}
 }
 
 /* Copy args out of req->argbuf */
 static void copy_args_from_argbuf(struct fuse_args *args, struct fuse_req *req)
 {
-	struct virtio_fs_argbuf *argbuf;
+	struct virtio_fs_argbuf *argbuf = req->argbuf;
+	struct iov_iter iter;
+	unsigned int copied;
 	unsigned int remaining;
 	unsigned int offset;
 	unsigned int num_out;
@@ -711,10 +689,14 @@ static void copy_args_from_argbuf(struct fuse_args *args, struct fuse_req *req)
 	if (!num_out)
 		goto out;
 
-	argbuf = req->argbuf;
+	if (!argbuf->is_flat)
+		iov_iter_bvec(&iter, ITER_SOURCE, argbuf->s.bvec,
+		      argbuf->s.nr, argbuf->s.size);
+
 	offset = virtio_fs_argbuf_out_args_offset(argbuf, args);
 	for (i = 0; i < num_out; i++) {
 		unsigned int argsize = args->out_args[i].size;
+		void *dst = args->out_args[i].value;
 
 		if (args->out_argvar &&
 		    i == args->out_numargs - 1 &&
@@ -722,10 +704,14 @@ static void copy_args_from_argbuf(struct fuse_args *args, struct fuse_req *req)
 			argsize = remaining;
 		}
 
-		virtio_fs_argbuf_copy_to_out_arg(argbuf, offset,
-						 args->out_args[i].value,
-						 argsize);
-		offset += argsize;
+		if (argbuf->is_flat) {
+			memcpy(dst, argbuf->f.buf + offset, argsize);
+			offset += argsize;
+		} else {
+			iov_iter_advance(&iter, argsize);
+			copied = _copy_from_iter(dst, argsize, &iter);
+			WARN_ON_ONCE(copied != argsize);
+		}
 
 		if (i != args->out_numargs - 1)
 			remaining -= argsize;