On Mon, 2010-09-20 at 18:18 +0200, Fubo Chen wrote:
> On Mon, Aug 30, 2010 at 11:22 AM, Nicholas A. Bellinger
> <nab@xxxxxxxxxxxxxxx> wrote:
> > From: Nicholas Bellinger <nab@xxxxxxxxxxxxxxx>
> >
> > This patch adds the O_SYNC FILEIO subsystem plugin for accessing struct file
> > using Linux/VFS struct file_operations. It also includes WriteCache and FUA
> > Write Emulation support to struct file using fs/sync.c/vfs_fsync_range()
> >
> > Signed-off-by: Nicholas A. Bellinger <nab@xxxxxxxxxxxxxxx>
> >
> > [ ... ]
> >
> > +static int fd_do_writev(struct fd_request *req, struct se_task *task)
> > +{
> > + struct file *fd = req->fd_dev->fd_file;
> > + struct scatterlist *sg = task->task_sg;
> > + struct iovec iov[req->fd_sg_count];
>
> Kernel stack size is 8 KB. On a 64-bit system struct iovec takes 16
> bytes. What happens if req->fd_sg_count is large ? Kernel crash ?
>
Greetings Fubo,
So wrt to a potential 8K stack over here for the local scope iov[]
arrays in fd_do_writev() and fd_do_read()..
Currently target_core_file.h defines a FD_MAX_SECTORS of 1024, which
means that each struct se_task *task here will be containing at most a
1024 sector request.
In scatterlist terms for a TCM fabric module like LIO-Target that does
1:1 struct scatterlist w/ PAGE_SIZE allocations, this would be:
((1024 * block_size) / PAGE_SIZE) * sizeof(struct iovec);
which for 64-bit uses 2048 bytes of stack for a FD_MAX_SECTORS task.
For a fabric module like TCM_Loop that maps incoming Linux/SCSI
scatterlist memory to internal TCM memory points and down to FILEIO
code, the maximum here is a scatterlist + offset for every block_size
sector for certain cases like mkfs* and friends, which means:
((1024 * block_size) / block_size) * sizeof(struct iovec);
which for 64-bit uses 16384 bytes for a FD_MAX_SECTORS task with a
scatterlist entry per 512-byte logical block.
For at least for the latter case this is a problem with 8K stacks, so it
looks these will need to convert to dynamic allocation within the local
scope. I will push a patch to fix this shortly.
Many thanks for your astute observation!
--nab
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