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

[Mike Christie <mchristi@redhat.com>]

On 10/23/2019 02:11 AM, Michal Hocko wrote:
> 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).

Do you mean we would do something like:

prctl()
....
case PF_SET_IO_FLUSHER:
        current->flags |= PF_MEMALLOC_NOIO;
....

or are you saying we would add a new PF_IO_FLUSHER flag and then modify
PF_MEMALLOC_NOIO uses like in current_gfp_context:

if (current->flags & (PF_MEMALLOC_NOIO | PF_IO_FLUSHER)
      flags &= ~(__GFP_IO | __GFP_FS);

?

> 
>>>>>> 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.


I am not sure. I will have to defer to general FS experts like Dave or
Martin and Damien for the specific fuse case. There do not seem to be a
lot of places where we check for __GFP_IO so configs with fuse and
bcache for example are probably not a big deal. However, I am not very
familiar with some of the other code paths in the mm layer and how FSs
interact with them.