RE: [Patch v2 13/15] CIFS: Add support for direct I/O read

Subject: Re: [Patch v2 13/15] CIFS: Add support for direct I/O read



On 5/30/2018 3:48 PM, Long Li wrote:

quoted

From: Long Li <longli@microsoft.com>

Implement the function for direct I/O read. It doesn't support AIO,
which will be implemented in a follow up patch.

Signed-off-by: Long Li <longli@microsoft.com>
---
  fs/cifs/cifsfs.h |   1 +
  fs/cifs/file.c   | 149

+++++++++++++++++++++++++++++++++++++++++++++++++++++++

quoted

  2 files changed, 150 insertions(+)

diff --git a/fs/cifs/cifsfs.h b/fs/cifs/cifsfs.h index

5f02318..7fba9aa 100644

--- a/fs/cifs/cifsfs.h
+++ b/fs/cifs/cifsfs.h

@@ -102,6 +102,7 @@ extern int cifs_open(struct inode *inode, struct file

*file);

quoted

  extern int cifs_close(struct inode *inode, struct file *file);
  extern int cifs_closedir(struct inode *inode, struct file *file);
  extern ssize_t cifs_user_readv(struct kiocb *iocb, struct iov_iter
*to);
+extern ssize_t cifs_direct_readv(struct kiocb *iocb, struct iov_iter
+*to);
  extern ssize_t cifs_strict_readv(struct kiocb *iocb, struct iov_iter *to);
  extern ssize_t cifs_user_writev(struct kiocb *iocb, struct iov_iter *from);
  extern ssize_t cifs_strict_writev(struct kiocb *iocb, struct
iov_iter *from); diff --git a/fs/cifs/file.c b/fs/cifs/file.c index
87eece6..e6e6f24 100644

--- a/fs/cifs/file.c
+++ b/fs/cifs/file.c

@@ -2955,6 +2955,18 @@ cifs_read_allocate_pages(struct cifs_readdata

*rdata, unsigned int nr_pages)

quoted

  	return rc;
  }

+static void cifs_direct_readdata_release(struct kref *refcount) {
+	struct cifs_readdata *rdata = container_of(refcount,
+					struct cifs_readdata, refcount);
+	unsigned int i;
+
+	for (i = 0; i < rdata->nr_pages; i++)
+		put_page(rdata->pages[i]);
+
+	cifs_readdata_release(refcount);
+}
+
  static void
  cifs_uncached_readdata_release(struct kref *refcount)
  {

@@ -3267,6 +3279,143 @@ collect_uncached_read_data(struct

cifs_aio_ctx *ctx)

quoted

  		complete(&ctx->done);
  }

+static void cifs_direct_readv_complete(struct work_struct *work) {
+	struct cifs_readdata *rdata =
+		container_of(work, struct cifs_readdata, work);
+
+	complete(&rdata->done);
+	kref_put(&rdata->refcount, cifs_direct_readdata_release); }
+
+ssize_t cifs_direct_readv(struct kiocb *iocb, struct iov_iter *to) {
+	size_t len, cur_len, start;
+	unsigned int npages, rsize, credits;
+	struct file *file;
+	struct cifs_sb_info *cifs_sb;
+	struct cifsFileInfo *cfile;
+	struct cifs_tcon *tcon;
+	struct page **pagevec;
+	ssize_t rc, total_read = 0;
+	struct TCP_Server_Info *server;
+	loff_t offset = iocb->ki_pos;
+	pid_t pid;
+	struct cifs_readdata *rdata;
+
+	/*
+	 * iov_iter_get_pages_alloc() doesn't work with ITER_KVEC,
+	 * fall back to data copy read path
+	 */
+	if (to->type & ITER_KVEC) {
+		cifs_dbg(FYI, "use non-direct cifs_user_readv for kvec

I/O\n");

quoted

+		return cifs_user_readv(iocb, to);
+	}
+
+	len = iov_iter_count(to);
+	if (!len)
+		return 0;
+
+	file = iocb->ki_filp;
+	cifs_sb = CIFS_FILE_SB(file);
+	cfile = file->private_data;
+	tcon = tlink_tcon(cfile->tlink);
+	server = tcon->ses->server;
+
+	if (!server->ops->async_readv)
+		return -ENOSYS;
+
+	if (cifs_sb->mnt_cifs_flags & CIFS_MOUNT_RWPIDFORWARD)
+		pid = cfile->pid;
+	else
+		pid = current->tgid;
+
+	if ((file->f_flags & O_ACCMODE) == O_WRONLY)
+		cifs_dbg(FYI, "attempting read on write only file instance\n");

Confusing. Maybe "attempting read on write-only filehandle"?

quoted

+
+	do {
+		rc = server->ops->wait_mtu_credits(server, cifs_sb->rsize,
+					&rsize, &credits);
+		if (rc)
+			break;
+
+		cur_len = min_t(const size_t, len, rsize);
+
+		rc = iov_iter_get_pages_alloc(to, &pagevec, cur_len, &start);
+		if (rc < 0) {
+			cifs_dbg(VFS,
+				"couldn't get user pages (rc=%zd) iter

type %d"

quoted

+				" iov_offset %lu count %lu\n",
+				rc, to->type, to->iov_offset, to->count);
+			dump_stack();
+			break;
+		}
+
+		rdata = cifs_readdata_direct_alloc(
+				pagevec, cifs_direct_readv_complete);
+		if (!rdata) {
+			add_credits_and_wake_if(server, credits, 0);
+			rc = -ENOMEM;
+			break;
+		}
+
+		npages = (rc + start + PAGE_SIZE-1) / PAGE_SIZE;
+		rdata->nr_pages = npages;
+		rdata->page_offset = start;
+		rdata->pagesz = PAGE_SIZE;
+		rdata->tailsz = npages > 1 ?
+				rc-(PAGE_SIZE-start)-(npages-2)*PAGE_SIZE :
+				rc;

This expression makes my head hurt. Surely it can be simplified, or expressed
in a clearer way.

quoted

+		cur_len = rc;
+
+		rdata->cfile = cifsFileInfo_get(cfile);
+		rdata->offset = offset;
+		rdata->bytes = rc;
+		rdata->pid = pid;
+		rdata->read_into_pages = cifs_uncached_read_into_pages;
+		rdata->copy_into_pages = cifs_uncached_copy_into_pages;
+		rdata->credits = credits;
+
+		rc = 0;
+		if (rdata->cfile->invalidHandle)
+			rc = cifs_reopen_file(rdata->cfile, true);
+
+		if (!rc)
+			rc = server->ops->async_readv(rdata);
+
+		if (rc) {

This whole rc thing is messy. Initializing to zero, setting only in one case, then
testing the result, then setting it again, is twisted.
I actually think a goto or two would read much more clearly.

quoted

+			add_credits_and_wake_if(server, rdata->credits, 0);
+			kref_put(&rdata->refcount,
+				 cifs_direct_readdata_release);
+			if (rc == -EAGAIN)
+				continue;
+			break;

It's worth a comment here that this either breaks or continues the entire do {}
while (); and btw when it breaks it does *not* return "rc".
Again, maybe a goto instead of a break?

quoted

+		}
+
+		wait_for_completion(&rdata->done);
+		rc = rdata->result;
+		if (rc) {
+			kref_put(
+				&rdata->refcount,
+				cifs_direct_readdata_release);
+			if (rc == -EAGAIN)
+				continue;
+			break;

Ditto.

I will re-work this patch.

quoted

+		}
+
+		total_read += rdata->got_bytes;
+		kref_put(&rdata->refcount, cifs_direct_readdata_release);
+
+		iov_iter_advance(to, cur_len);
+		len -= cur_len;
+		offset += cur_len;
+	} while (len);
+
+	iocb->ki_pos += total_read;
+
+	return total_read;
+}
+
  ssize_t cifs_user_readv(struct kiocb *iocb, struct iov_iter *to)
  {
  	struct file *file = iocb->ki_filp;