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SubjectRe: [PATCH 4/7] nandsim: Don't use PF_MEMALLOC
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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@intel.com>
> Cc: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
> Cc: linux-mtd@lists.infradead.org
> Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.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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