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

[Long Li <longli@microsoft.com>]

> Subject: Re: [Patch v2 13/15] CIFS: Add support for direct I/O read
> 
> 
> 
> On 5/30/2018 3:48 PM, Long Li wrote:
> > 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
> +++++++++++++++++++++++++++++++++++++++++++++++++++++++
> >   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);
> >   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)
> >   	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)
> >   		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");
> > +		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"?
> 
> > +
> > +	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"
> > +				" 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.
> 
> > +		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.
> 
> > +			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?
> 
> > +		}
> > +
> > +		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.

> 
> > +		}
> > +
> > +		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;
> >