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

[Michal Hocko <mhocko@kernel.org>]

On Tue 22-10-19 11:13:20, Mike Christie wrote:
> On 10/22/2019 06:24 AM, Michal Hocko wrote:
> > On Mon 21-10-19 16:41:37, Mike Christie wrote:
> >> There are several storage drivers like dm-multipath, iscsi, tcmu-runner,
> >> amd nbd that have userspace components that can run in the IO path. For
> >> example, iscsi and nbd's userspace deamons may need to recreate a socket
> >> and/or send IO on it, and dm-multipath's daemon multipathd may need to
> >> send IO to figure out the state of paths and re-set them up.
> >>
> >> In the kernel these drivers have access to GFP_NOIO/GFP_NOFS and the
> >> memalloc_*_save/restore functions to control the allocation behavior,
> >> but for userspace we would end up hitting a allocation that ended up
> >> writing data back to the same device we are trying to allocate for.
> > 
> > Which code paths are we talking about here? Any ioctl or is this a
> > general syscall path? Can we mark the process in a more generic way?
> 
> It depends on the daemon. The common one for example are iscsi and nbd
> need network related calls like sendmsg, recvmsg, socket, etc.
> tcmu-runner could need the network ones and also read and write when it
> does IO to a FS or device. dm-multipath needs the sg io ioctls.

OK, so there is not a clear kernel entry point that could be explicitly
annotated. This would imply a per task context. This is an important
information. And I am wondering how those usecases ever worked in the
first place. This is not a minor detail.
 
> > E.g. we have PF_LESS_THROTTLE (used by nfsd). It doesn't affect the
> > reclaim recursion but it shows a pattern that doesn't really exhibit
> > too many internals. Maybe we need PF_IO_FLUSHER or similar?
> 
> I am not familiar with PF_IO_FLUSHER. If it prevents the recursion
> problem then please send me details and I will look into it for the next
> posting.

PF_IO_FLUSHER doesn't exist. I just wanted to point out that similarly
to PF_LESS_THROTTLE it should be a more high level per task flag rather
than something as low level as a direct control of gfp allocation
context. PF_LESS_THROTTLE simply tells that the task is a part of the
reclaim process and therefore it shouldn't be a subject of a normal
throttling - whatever that means. PF_IO_FLUSHER would mean that the user
context is a part of the IO path and therefore there are certain reclaim
recursion restrictions.
 
> >> 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?
-- 
Michal Hocko
SUSE Labs