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Date: Tue, 21 May 2019 09:17:32 -0700 From: Andy Lutomirski <luto@...nel.org> To: Rick Edgecombe <rick.p.edgecombe@...el.com> Cc: LKML <linux-kernel@...r.kernel.org>, Peter Zijlstra <peterz@...radead.org>, sparclinux <sparclinux@...r.kernel.org>, Linux-MM <linux-mm@...ck.org>, Network Development <netdev@...r.kernel.org>, Dave Hansen <dave.hansen@...el.com>, Nadav Amit <namit@...are.com>, "David S. Miller" <davem@...emloft.net>, Rick Edgecombe <redgecombe.lkml@...il.com>, Meelis Roos <mroos@...ux.ee>, Borislav Petkov <bp@...en8.de>, Andy Lutomirski <luto@...nel.org>, Ingo Molnar <mingo@...hat.com> Subject: Re: [PATCH v2 2/2] vmalloc: Remove work as from vfree path On Mon, May 20, 2019 at 4:39 PM Rick Edgecombe <rick.p.edgecombe@...el.com> wrote: > > From: Rick Edgecombe <redgecombe.lkml@...il.com> > > Calling vm_unmap_alias() in vm_remove_mappings() could potentially be a > lot of work to do on a free operation. Simply flushing the TLB instead of > the whole vm_unmap_alias() operation makes the frees faster and pushes > the heavy work to happen on allocation where it would be more expected. > In addition to the extra work, vm_unmap_alias() takes some locks including > a long hold of vmap_purge_lock, which will make all other > VM_FLUSH_RESET_PERMS vfrees wait while the purge operation happens. > > Lastly, page_address() can involve locking and lookups on some > configurations, so skip calling this by exiting out early when > !CONFIG_ARCH_HAS_SET_DIRECT_MAP. Hmm. I would have expected that the major cost of vm_unmap_aliases() would be the flush, and at least informing the code that the flush happened seems valuable. So would guess that this patch is actually a loss in throughput. --Andy
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