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
