linux-kernel - Re: [PATCH 4/7] nandsim: Don't use PF

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Message-Id: <1258988417.18407.44.camel@localhost>
Date:	Mon, 23 Nov 2009 17:00:17 +0200
From:	Artem Bityutskiy <Artem.Bityutskiy@...ia.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	LKML <linux-kernel@...r.kernel.org>, linux-mm <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Woodhouse <David.Woodhouse@...el.com>,
	linux-mtd@...ts.infradead.org,
	Adrian Hunter <adrian.hunter@...ia.com>
Subject: Re: [PATCH 4/7] nandsim: Don't use PF_MEMALLOC

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@...el.com>
> Cc: Artem Bityutskiy <Artem.Bityutskiy@...ia.com>
> Cc: linux-mtd@...ts.infradead.org
> Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
> ---
>  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;

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?

-- 
Best Regards,
Artem Bityutskiy (Артём Битюцкий)

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