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From: Roman Gushchin To: Peter Zijlstra Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Andrew Morton , Jann Horn , Will Deacon , "Aneesh Kumar K.V" , Nick Piggin , Hugh Dickins , linux-arch@vger.kernel.org Subject: Re: [PATCH v2] mmu_gather: move tlb flush for VM_PFNMAP/VM_MIXEDMAP vmas into free_pgtables() Message-ID: References: <20250122232716.1321171-1-roman.gushchin@linux.dev> <20250123214531.GA969@noisy.programming.kicks-ass.net> <20250124082250.GA13226@noisy.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20250124082250.GA13226@noisy.programming.kicks-ass.net> X-Migadu-Flow: FLOW_OUT X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: C3E5318000F X-Stat-Signature: 1y799c9dxgks381i81npuouixkptjr7k X-Rspam-User: X-HE-Tag: 1737768241-234206 X-HE-Meta: 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 o8494N42 /rmp+alBy9RbxycB0+FeCSdYk/5o02Obq1p71Az1/aK40zcbQ45MnGNulp9r0TTanO2jYDeS6s3km0l1ynlSbbzo1XJult5WplQVfW9+CM+NnXFY4Wf4debfI79eMknF2dfhIsgm+UiWm4Dde3AuISUJMNGk4ARQ21zJ8Wq8Sq7FmgUJ7ueWftRyi1nhXXXrcYIGtg4cUwP2K72xXy+uOUjWN6j1kVNt3E9BAkfz7HwFONEXE7dg/GQuDrihq6NRENiAmUmnYRKXG0IQ= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Fri, Jan 24, 2025 at 09:22:50AM +0100, Peter Zijlstra wrote: > On Thu, Jan 23, 2025 at 11:12:33PM +0000, Roman Gushchin wrote: > > > > +static inline void tlb_free_vma(struct mmu_gather *tlb, struct vm_area_struct *vma) > > > { > > > if (tlb->fullmm) > > > return; > > > > > > /* > > > * VM_PFNMAP is more fragile because the core mm will not track the > > > + * page mapcount -- there might not be page-frames for these PFNs > > > + * after all. > > > + * > > > + * Specifically() there is a race between munmap() and > > > + * unmap_mapping_range(), where munmap() will unlink the VMA, such > > > + * that unmap_mapping_range() will no longer observe the VMA and > > > + * no-op, without observing the TLBI, returning prematurely. > > > + * > > > + * So if we're about to unlink such a VMA, and we have pending > > > + * TLBI for such a vma, flush things now. > > > */ > > > + if ((vma->vm_flags & (VM_PFNMAP|VM_MIXEDMAP)) && tlb->vma_pfn) > > > tlb_flush_mmu_tlbonly(tlb); > > > > Why do we need to re-check vma->vm_flags & (VM_PFNMAP|VM_MIXEDMAP) here? > > No need, but an opportunity. > > > In free_pgtables() we're iterating over multiple vma's. What if the first has > > no VM_PFNMAP set, but some other do? Idk if it's even possible, but it's not > > obvious that it's not possible either. > > If we only need to flush PFN entries before unlinking PFN VMAs, then: > > - if there are no PFNs pending (vma_pfn), we don't need to flush; > - if no PFN vma is being freed (vm_flags), we don't need to flush. Right, but if I understand the code correctly, more than one vma can be freed by a single free_pgtables() invocation. Should we then check each vma's flags in the while loop in free_pgtables()? But then we're back to where we're now with multiple flushes. Do I misunderstand this? Thanks