Re: [PATCH 4/7] nandsim: Don't use PF_MEMALLOC

[Adrian Hunter]

Bityutskiy Artem (Nokia-D/Helsinki) wrote:
> On Tue, 2009-11-17 at 16:19 +0900, KOSAKI Motohiro wrote:
> > Non MM subsystem must not use PF_MEMALLOC. Memory reclaim need few
> > memory, anyone must not prevent it. Otherwise the system cause
> > mysterious hang-up and/or OOM Killer invokation.
> >
> > Cc: David Woodhouse <David.Woodhouse@xxxxxxxxx>
> > Cc: Artem Bityutskiy <Artem.Bityutskiy@xxxxxxxxx>
> > Cc: linux-mtd@xxxxxxxxxxxxxxxxxxx
> > Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
> > ---
> >  drivers/mtd/nand/nandsim.c |   22 ++--------------------
> >  1 files changed, 2 insertions(+), 20 deletions(-)
> >
> > diff --git a/drivers/mtd/nand/nandsim.c b/drivers/mtd/nand/nandsim.c
> > index cd0711b..97a8bbb 100644
> > --- a/drivers/mtd/nand/nandsim.c
> > +++ b/drivers/mtd/nand/nandsim.c
> > @@ -1322,34 +1322,18 @@ static int get_pages(struct nandsim *ns, struct file *file, size_t count, loff_t
> >  	return 0;
> >  }
> >  
> > -static int set_memalloc(void)
> > -{
> > -	if (current->flags & PF_MEMALLOC)
> > -		return 0;
> > -	current->flags |= PF_MEMALLOC;
> > -	return 1;
> > -}
> > -
> > -static void clear_memalloc(int memalloc)
> > -{
> > -	if (memalloc)
> > -		current->flags &= ~PF_MEMALLOC;
> > -}
> > -
> >  static ssize_t read_file(struct nandsim *ns, struct file *file, void *buf, size_t count, loff_t *pos)
> >  {
> >  	mm_segment_t old_fs;
> >  	ssize_t tx;
> > -	int err, memalloc;
> > +	int err;
> >  
> >  	err = get_pages(ns, file, count, *pos);
> >  	if (err)
> >  		return err;
> >  	old_fs = get_fs();
> >  	set_fs(get_ds());
> > -	memalloc = set_memalloc();
> >  	tx = vfs_read(file, (char __user *)buf, count, pos);
> > -	clear_memalloc(memalloc);
> >  	set_fs(old_fs);
> >  	put_pages(ns);
> >  	return tx;
> > @@ -1359,16 +1343,14 @@ static ssize_t write_file(struct nandsim *ns, struct file *file, void *buf, size
> >  {
> >  	mm_segment_t old_fs;
> >  	ssize_t tx;
> > -	int err, memalloc;
> > +	int err;
> >  
> >  	err = get_pages(ns, file, count, *pos);
> >  	if (err)
> >  		return err;
> >  	old_fs = get_fs();
> >  	set_fs(get_ds());
> > -	memalloc = set_memalloc();
> >  	tx = vfs_write(file, (char __user *)buf, count, pos);
> > -	clear_memalloc(memalloc);
> >  	set_fs(old_fs);
> >  	put_pages(ns);
> >  	return tx;PF_MEMALLOC,
>
> I vaguely remember Adrian (CCed) did this on purpose. This is for the
> case when nandsim emulates NAND flash on top of a file. So there are 2
> file-systems involved: one sits on top of nandsim (e.g. UBIFS) and the
> other owns the file which nandsim uses (e.g., ext3).
>
> And I really cannot remember off the top of my head why he needed
> PF_MEMALLOC, but I think Adrian wanted to prevent the direct reclaim
> path to re-enter, say UBIFS, and cause deadlock. But I'd thing that all
> the allocations in vfs_read()/vfs_write() should be GFP_NOFS, so that
> should not be a probelm?

Yes it needs PF_MEMALLOC to prevent deadlock because there can be a
file system on top of nandsim which, in this case, is on top of another
file system.

I do not see how mempools will help here.

Please offer an alternative solution.
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