Re: [PATCH] Add prctl support for controlling PF_MEMALLOC V2

[Michal Hocko]

On Wed 23-10-19 07:43:44, Dave Chinner wrote:
> On Tue, Oct 22, 2019 at 06:33:10PM +0200, Michal Hocko wrote:

Thanks for more clarifiation regarding PF_LESS_THROTTLE.

[...]
> > PF_IO_FLUSHER would mean that the user
> > context is a part of the IO path and therefore there are certain reclaim
> > recursion restrictions.
> 
> If PF_IO_FLUSHER just maps to PF_LESS_THROTTLE|PF_MEMALLOC_NOIO,
> then I'm not sure we need a new definition. Maybe that's the ptrace
> flag name, but in the kernel we don't need a PF_IO_FLUSHER process
> flag...

Yes, the internal implementation would do something like that. I was
more interested in the user space visible API at this stage. Something
generic enough because exporting MEMALLOC flags is just a bad idea IMHO
(especially PF_MEMALLOC).

> > > >> This patch allows the userspace deamon to set the PF_MEMALLOC* flags
> > > >> with prctl during their initialization so later allocations cannot
> > > >> calling back into them.
> > > > 
> > > > TBH I am not really happy to export these to the userspace. They are
> > > > an internal implementation detail and the userspace shouldn't really
> > > 
> > > They care in these cases, because block/fs drivers must be able to make
> > > forward progress during writes. To meet this guarantee kernel block
> > > drivers use mempools and memalloc/GFP flags.
> > > 
> > > For these userspace components of the block/fs drivers they already do
> > > things normal daemons do not to meet that guarantee like mlock their
> > > memory, disable oom killer, and preallocate resources they have control
> > > over. They have no control over reclaim like the kernel drivers do so
> > > its easy for us to deadlock when memory gets low.
> > 
> > OK, fair enough. How much of a control do they really need though. Is a
> > single PF_IO_FLUSHER as explained above (essentially imply GPF_NOIO
> > context) sufficient?
> 
> I think some of these usrspace processes work at the filesystem
> level and so really only need GFP_NOFS allocation (fuse), while
> others work at the block device level (iscsi, nbd) so need GFP_NOIO
> allocation. So there's definitely an argument for providing both...

The main question is whether giving more APIs is really necessary. Is
there any real problem to give them only PF_IO_FLUSHER and let both
groups use this one? It will imply more reclaim restrictions for solely
FS based ones but is this a practical problem? If yes we can always add
PF_FS_$FOO later on.
-- 
Michal Hocko
SUSE Labs