Re: [PATCH] mm/vmpressure.c: Include GFP_KERNEL flag to vmpressure

[Shaju Abraham]

On Mon, Mar 9, 2020 at 9:42 PM Michal Hocko <mhocko@xxxxxxxxxx> wrote:
>
> On Mon 09-03-20 21:02:50, Shaju Abraham wrote:
> > On Mon, Mar 9, 2020 at 5:28 PM Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> >
> > > On Mon 09-03-20 11:31:41, Shaju Abraham wrote:
> > > > The VM pressure notification flags have excluded GFP_KERNEL with the
> > > > reasoning that user land will not be able to take any action in case of
> > > > kernel memory being low. This is not true always. Consider the case of
> > > > a user land program managing all the huge memory pages. By including
> > > > GFP_KERNEL flag whenever the kernel memory is low, pressure notification
> > > > can be send, and the manager process can split huge pages to satisfy
> > > kernel
> > > > memory requirement.
> > >
> > > Are you sure about this reasoning? GFP_KERNEL = __GFP_FS | __GFP_IO |
> > > __GFP_RECLAIM
> > > Two of the flags mentioned there are already listed so we are talking
> > > about __GFP_RECLAIM here. Including it here would be a more appropriate
> > > change than GFP_KERNEL btw.
> > >
> > > But still I do not really understand what is the actual problem and how
> > > is this patch meant to fix it. vmpressure is triggered only from the
> > > reclaim path which inherently requires to have __GFP_RECLAIM present
> > > so I fail to see how this can make any change at all. How have you
> > > tested it?
> > >
> > >    We have a user space application which waits on memory pressure events.
>
> > Upon receiving the event, the user space program will free up huge
> > pages to make more memory available in the system.  This mechanism
> > works fine if the memory is being consumed by other user space
> > applications. To test this, we wrote a test program which will
> > allocate all the memory available in the system using malloc() and
> > touch the allocated pages. When the free memory level becomes low,
> > the pressure event is fired and the process gets notified about it .
> > The same test is repeated with kmalloc() instead of malloc(). A test
> > kernel module is developed, which will allocate all the available
> > memory with kmalloc(GFP_KERNEL) flag.  The OOM killer gets invoked in
> > this case. The memory pressure event is not fired.  After modifying
> > the vmpressure.c with the attached patch, the pressure event gets
> > triggered.  Swap is disabled in the system we were testing.
>
> Are you sure this is really the case? I am either missing something here
> or your test might simply be timing specific because
>
>         GFP_KERNEL & (__GFP_FS | __GFP_IO) = true
>
> so I really do not see how the current code could bail out on the test
> you are patching so that the patch would make any change. The only real
> difference this patch makes is to trigger events for __GFP_RECLAIM
> allocations which could be GFP_NOIO. All non-sleepable allocations would
> wake kswapd and that would in turn reclaim with _GFP_FS | __GFP_IO set
> so the check doesn't change anything.
>
> Am I missing something?
 No . You are right. The pressure event does get generated from kernel
but before the
 user space gets time to act, OOM killer is invoked.

Regards
Shaju



> --
> Michal Hocko
> SUSE Labs