On Mon 05-10-20 17:08:01, Uladzislau Rezki wrote: > On Fri, Oct 02, 2020 at 11:05:07AM +0200, Michal Hocko wrote: > > On Fri 02-10-20 09:50:14, Mel Gorman wrote: > > > On Fri, Oct 02, 2020 at 09:11:23AM +0200, Michal Hocko wrote: > > > > On Thu 01-10-20 21:26:26, Uladzislau Rezki wrote: > > > > > > > > > > > > No, I meant going back to idea of new gfp flag, but adjust the implementation in > > > > > > the allocator (different from what you posted in previous version) so that it > > > > > > only looks at the flag after it tries to allocate from pcplist and finds out > > > > > > it's empty. So, no inventing of new page allocator entry points or checks such > > > > > > as the one you wrote above, but adding the new gfp flag in a way that it doesn't > > > > > > affect existing fast paths. > > > > > > > > > > > OK. Now i see. Please have a look below at the patch, so we fully understand > > > > > each other. If that is something that is close to your view or not: > > > > > > > > > > <snip> > > > > > t a/include/linux/gfp.h b/include/linux/gfp.h > > > > > index c603237e006c..7e613560a502 100644 > > > > > --- a/include/linux/gfp.h > > > > > +++ b/include/linux/gfp.h > > > > > @@ -39,8 +39,9 @@ struct vm_area_struct; > > > > > #define ___GFP_HARDWALL 0x100000u > > > > > #define ___GFP_THISNODE 0x200000u > > > > > #define ___GFP_ACCOUNT 0x400000u > > > > > +#define ___GFP_NO_LOCKS 0x800000u > > > > > > > > Even if a new gfp flag gains a sufficient traction and support I am > > > > _strongly_ opposed against consuming another flag for that. Bit space is > > > > limited. > > > > > > That is definitely true. I'm not happy with the GFP flag at all, the > > > comment is at best a damage limiting move. It still would be better for > > > a memory pool to be reserved and sized for critical allocations. > > > > Completely agreed. The only existing usecase is so special cased that a > > dedicated pool is not only easier to maintain but it should be also much > > better tuned for the specific workload. Something not really feasible > > with the allocator. > > > > > > Besides that we certainly do not want to allow craziness like > > > > __GFP_NO_LOCK | __GFP_RECLAIM (and similar), do we? > > > > > > That would deserve to be taken to a dumpster and set on fire. The flag > > > combination could be checked in the allocator but the allocator path fast > > > paths are bad enough already. > > > > If a new allocation/gfp mode is absolutely necessary then I believe that > > the most reasoanble way forward would be > > #define GFP_NO_LOCK ((__force gfp_t)0) > > > Agree. Even though i see that some code should be adjusted for it. There are > a few users of the __get_free_page(0); So, need to double check it: Yes, I believe I have pointed that out in the previous discussion. > <snip> > [ 0.650351] BUG: kernel NULL pointer dereference, address: 0000000000000010 > [ 0.651083] #PF: supervisor read access in kernel mode > [ 0.651639] #PF: error_code(0x0000) - not-present page > [ 0.652200] PGD 0 P4D 0 > [ 0.652523] Oops: 0000 [#1] SMP NOPTI > [ 0.652668] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.9.0-rc7-next-20200930+ #140 > [ 0.652668] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-1 04/01/2014 > [ 0.652668] RIP: 0010:__find_event_file+0x21/0x80 > <snip> > > Apart of that. There is a post_alloc_hook(), that gets called from the prep_new_page(). > If "debug page alloc enabled", it maps a page for debug purposes invoking kernel_map_pages(). > __kernel_map_pages() is ARCH specific. For example, powerpc variant uses sleep-able locks > what can be easily converted to raw variant. Yes, there are likely more surprises like that. I am not sure about kasan, page owner (which depens on the stack unwinder) and others which hook into this path. -- Michal Hocko SUSE Labs
