On Wed 15-11-17 17:33:32, Will Deacon wrote: > Hi Michal, > > On Fri, Nov 10, 2017 at 01:26:35PM +0100, Michal Hocko wrote: > > From 7f0fcd2cab379ddac5611b2a520cdca8a77a235b Mon Sep 17 00:00:00 2001 > > From: Michal Hocko <mhocko@xxxxxxxx> > > Date: Fri, 10 Nov 2017 11:27:17 +0100 > > Subject: [PATCH] arch, mm: introduce arch_tlb_gather_mmu_lazy > > > > 5a7862e83000 ("arm64: tlbflush: avoid flushing when fullmm == 1") has > > introduced an optimization to not flush tlb when we are tearing the > > whole address space down. Will goes on to explain > > > > : Basically, we tag each address space with an ASID (PCID on x86) which > > : is resident in the TLB. This means we can elide TLB invalidation when > > : pulling down a full mm because we won't ever assign that ASID to > > : another mm without doing TLB invalidation elsewhere (which actually > > : just nukes the whole TLB). > > > > This all is nice but tlb_gather users are not aware of that and this can > > actually cause some real problems. E.g. the oom_reaper tries to reap the > > whole address space but it might race with threads accessing the memory [1]. > > It is possible that soft-dirty handling might suffer from the same > > problem [2]. > > > > Introduce an explicit lazy variant tlb_gather_mmu_lazy which allows the > > behavior arm64 implements for the fullmm case and replace it by an > > explicit lazy flag in the mmu_gather structure. exit_mmap path is then > > turned into the explicit lazy variant. Other architectures simply ignore > > the flag. > > > > [1] http://lkml.kernel.org/r/20171106033651.172368-1-wangnan0@xxxxxxxxxx > > [2] http://lkml.kernel.org/r/20171110001933.GA12421@bbox > > Signed-off-by: Michal Hocko <mhocko@xxxxxxxx> > > --- > > arch/arm/include/asm/tlb.h | 3 ++- > > arch/arm64/include/asm/tlb.h | 2 +- > > arch/ia64/include/asm/tlb.h | 3 ++- > > arch/s390/include/asm/tlb.h | 3 ++- > > arch/sh/include/asm/tlb.h | 2 +- > > arch/um/include/asm/tlb.h | 2 +- > > include/asm-generic/tlb.h | 6 ++++-- > > include/linux/mm_types.h | 2 ++ > > mm/memory.c | 17 +++++++++++++++-- > > mm/mmap.c | 2 +- > > 10 files changed, 31 insertions(+), 11 deletions(-) > > > > diff --git a/arch/arm/include/asm/tlb.h b/arch/arm/include/asm/tlb.h > > index d5562f9ce600..fe9042aee8e9 100644 > > --- a/arch/arm/include/asm/tlb.h > > +++ b/arch/arm/include/asm/tlb.h > > @@ -149,7 +149,8 @@ static inline void tlb_flush_mmu(struct mmu_gather *tlb) > > > > static inline void > > arch_tlb_gather_mmu(struct mmu_gather *tlb, struct mm_struct *mm, > > - unsigned long start, unsigned long end) > > + unsigned long start, unsigned long end, > > + bool lazy) > > { > > tlb->mm = mm; > > tlb->fullmm = !(start | (end+1)); > > diff --git a/arch/arm64/include/asm/tlb.h b/arch/arm64/include/asm/tlb.h > > index ffdaea7954bb..7adde19b2bcc 100644 > > --- a/arch/arm64/include/asm/tlb.h > > +++ b/arch/arm64/include/asm/tlb.h > > @@ -43,7 +43,7 @@ static inline void tlb_flush(struct mmu_gather *tlb) > > * The ASID allocator will either invalidate the ASID or mark > > * it as used. > > */ > > - if (tlb->fullmm) > > + if (tlb->lazy) > > return; > > This looks like the right idea, but I'd rather make this check: > > if (tlb->fullmm && tlb->lazy) > > since the optimisation doesn't work for anything than tearing down the > entire address space. OK, that makes sense. > Alternatively, I could actually go check MMF_UNSTABLE in tlb->mm, which > would save you having to add an extra flag in the first place, e.g.: > > if (tlb->fullmm && !test_bit(MMF_UNSTABLE, &tlb->mm->flags)) > > which is a nice one-liner. But that would make it oom_reaper specific. What about the softdirty case Minchan has mentioned earlier? -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx"> email@xxxxxxxxx </a>