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

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

> ext KOSAKI Motohiro wrote:
> > Hi
> > 
> > Thank you for this useful comments.
> > 
> >>> 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.
> > 
> > I have few questions.
> > 
> > Can you please explain more detail? Another stackable filesystam
> > (e.g. ecryptfs) don't have such problem. Why nandsim have its issue?
> > What lock cause deadlock?
> 
> The file systems are not stacked.  One is over nandsim, which nandsim
> does not know about because it is just a lowly NAND device, and, with
> the file cache option, one file system below to provide the file cache.
> 
> The deadlock is the kernel writing out dirty pages to the top file system
> which writes to nandsim which writes to the bottom file system which
> allocates memory which causes dirty pages to be written out to the top
> file system, which tries to write to nandsim => deadlock.

You mean you want to prevent pageout() instead reclaim itself?
Dropping filecache seems don't make recursive call, right?