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Fri, 12 Apr 2024 05:47:05 -0700 (PDT) MIME-Version: 1.0 References: <20240410170621.2011171-1-peterx@redhat.com> <20240411171319.almhz23xulg4f7op@revolver> In-Reply-To: From: Suren Baghdasaryan Date: Fri, 12 Apr 2024 05:46:52 -0700 Message-ID: Subject: Re: [PATCH] mm: Always sanity check anon_vma first for per-vma locks To: Matthew Wilcox Cc: Peter Xu , "Liam R. Howlett" , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Andrew Morton , Lokesh Gidra , Alistair Popple Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspam-User: X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 778A040016 X-Stat-Signature: tu5juxg8hcps7ffp4tx6z31rrnsg4q5q X-HE-Tag: 1712926026-312909 X-HE-Meta: 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 lSPw1MAr fB+J8AUoLUcrlB5ZiH8z98acTwYtj77YyAwvRr/RSSCQ5ndbAMAexfxYGW2FYN8oge2iC4F7+CUXUnDv2m3/U6plOx7WpET/2RoGJsFdB5ZnNIcwfTJg3lXumgm2LCS+z+XxYCkP8DQZTsWPgZ2uKCHtPUqvzc1uTKrSFUCwbnrU0cXgkPUysiebdgicSKdlSEHr6NP3ORubD4Ynuc/QCMZ8IKXKJQ3zyHw5P1nOM6o8QkxrPPxkGj6GUyUsbAz7XJJ8Z/7K3ALGz7n399Yu7LhCenOyBphYZHmb7E1BitY+7psokMAZYz9XaX40IIXZrCXUp/p0B6VBglzI= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000001, 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 Thu, Apr 11, 2024 at 8:14=E2=80=AFPM Matthew Wilcox wrote: > > On Thu, Apr 11, 2024 at 11:02:32PM +0100, Matthew Wilcox wrote: > > > How many instructions it takes for a late RETRY for WRITEs to private= file > > > mappings, fallback to mmap_sem? > > > > Doesn't matter. That happens _once_ per VMA, and it's dwarfed by the > > cost of allocating and initialising the COWed page. You're adding > > instructions to every single page fault. I'm not happy that we had to > > add extra instructions to the fault path for single-threaded programs, > > but we at least had the justification that we were improving scalabilit= y > > on large systems. Your excuse is "it makes the code cleaner". And > > honestly, I don't think it even does that. > > Suren, what would you think to this? > > diff --git a/mm/memory.c b/mm/memory.c > index 6e2fe960473d..e495adcbe968 100644 > --- a/mm/memory.c > +++ b/mm/memory.c > @@ -5821,15 +5821,6 @@ struct vm_area_struct *lock_vma_under_rcu(struct m= m_struct *mm, > if (!vma_start_read(vma)) > goto inval; > > - /* > - * find_mergeable_anon_vma uses adjacent vmas which are not locke= d. > - * This check must happen after vma_start_read(); otherwise, a > - * concurrent mremap() with MREMAP_DONTUNMAP could dissociate the= VMA > - * from its anon_vma. > - */ > - if (unlikely(vma_is_anonymous(vma) && !vma->anon_vma)) > - goto inval_end_read; > - > /* Check since vm_start/vm_end might change before we lock the VM= A */ > if (unlikely(address < vma->vm_start || address >=3D vma->vm_end)= ) > goto inval_end_read; > > That takes a few insns out of the page fault path (good!) at the cost > of one extra trip around the fault handler for the first fault on an > anon vma. It makes the file & anon paths more similar to each other > (good!) I see what you mean. The impact would depend on the workload but in my earlier tests when developing per-VMA locks there were on average less than 1% faults which were for anonymous pages and had vma->anon_vma=3D=3DNULL. I recorded that after using my desktop for a day or so and running a series of benchmark tests. Again, that number might be drastically different on some other workloads. About the code, I'll take a closer look once I'm back from vacation this weekend but I think you will also have to modify do_anonymous_page() to use vmf_anon_prepare() instead of anon_vma_prepare(). > > We'd need some data to be sure it's really a win, but less code is > always good. > > We could even eagerly initialise vma->anon_vma for anon vmas. I don't > know why we don't do that. You found the answer to that question a long time ago and IIRC it was because in many cases we end up not needing to set vma->anon_vma at all. So, this is an optimization to try avoiding extra operations whenever we can. I'll try to find your comment on this.