From mboxrd@z Thu Jan 1 00:00:00 1970 From: Michal Hocko Subject: Re: [PATCH 2/5] kernel.h: Add non_block_start/end() Date: Fri, 16 Aug 2019 10:10:29 +0200 Message-ID: <20190816081029.GA27790@dhcp22.suse.cz> References: <20190815132127.GI9477@dhcp22.suse.cz> <20190815141219.GF21596@ziepe.ca> <20190815155950.GN9477@dhcp22.suse.cz> <20190815165631.GK21596@ziepe.ca> <20190815174207.GR9477@dhcp22.suse.cz> <20190815182448.GP21596@ziepe.ca> <20190815190525.GS9477@dhcp22.suse.cz> <20190815191810.GR21596@ziepe.ca> <20190815193526.GT9477@dhcp22.suse.cz> <20190815201323.GU21596@ziepe.ca> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20190815201323.GU21596@ziepe.ca> Sender: linux-kernel-owner@vger.kernel.org To: Jason Gunthorpe Cc: LKML , linux-mm@kvack.org, DRI Development , Intel Graphics Development , Peter Zijlstra , Ingo Molnar , Andrew Morton , David Rientjes , Christian =?iso-8859-1?Q?K=F6nig?= , =?iso-8859-1?B?Suly9G1l?= Glisse , Masahiro Yamada , Wei Wang , Andy Shevchenko , Thomas Gleixner , Jann Horn , Feng Tang , Kees Cook , Randy Dunlap , Daniel Vetter List-Id: intel-gfx@lists.freedesktop.org On Thu 15-08-19 17:13:23, Jason Gunthorpe wrote: > On Thu, Aug 15, 2019 at 09:35:26PM +0200, Michal Hocko wrote: > > > > The last detail is I'm still unclear what a GFP flags a blockable > > > invalidate_range_start() should use. Is GFP_KERNEL OK? > > > > I hope I will not make this muddy again ;) > > invalidate_range_start in the blockable mode can use/depend on any sleepable > > allocation allowed in the context it is called from. > > 'in the context is is called from' is the magic phrase, as > invalidate_range_start is called while holding several different mm > related locks. I know at least write mmap_sem and i_mmap_rwsem > (write?) > > Can GFP_KERNEL be called while holding those locks? i_mmap_rwsem would be problematic because it is taken during the reclaim. > This is the question of indirect dependency on reclaim via locks you > raised earlier. > > > So in other words it is no different from any other function in the > > kernel that calls into allocator. As the API is missing gfp context > > then I hope it is not called from any restricted contexts (except > > from the oom which we have !blockable for). > > Yes, the callers are exactly my concern. > > > > Lockdep has > > > complained on that in past due to fs_reclaim - how do you know if it > > > is a false positive? > > > > I would have to see the specific lockdep splat. > > See below. I found it when trying to understand why the registration > of the mmu notififer was so oddly coded. > > The situation was: > > down_write(&mm->mmap_sem); > mm_take_all_locks(mm); > kmalloc(GFP_KERNEL); <--- lockdep warning Ugh. mm_take_all_locks :/ > I understood Daniel said he saw this directly on a recent kernel when > working with his lockdep patch? > > Checking myself, on todays kernel I see a call chain: > > shrink_all_memory > fs_reclaim_acquire(sc.gfp_mask); > [..] > do_try_to_free_pages > shrink_zones > shrink_node > shrink_node_memcg > shrink_list > shrink_active_list > page_referenced > rmap_walk > rmap_walk_file > i_mmap_lock_read > down_read(i_mmap_rwsem) > > So it is possible that the down_read() above will block on > i_mmap_rwsem being held in the caller of invalidate_range_start which > is doing kmalloc(GPF_KERNEL). > > Is this OK? The lockdep annotation says no.. It's not as per the above code patch which is easily possible because mm_take_all_locks will lock all file vmas. -- Michal Hocko SUSE Labs