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[99.254.121.117]) by smtp.gmail.com with ESMTPSA id ej6-20020ad45a46000000b00696b1050be8sm2281738qvb.133.2024.04.12.05.39.00 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 12 Apr 2024 05:39:01 -0700 (PDT) Date: Fri, 12 Apr 2024 08:38:59 -0400 From: Peter Xu To: Matthew Wilcox Cc: "Liam R. Howlett" , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Andrew Morton , Suren Baghdasaryan , Lokesh Gidra , Alistair Popple Subject: Re: [PATCH] mm: Always sanity check anon_vma first for per-vma locks Message-ID: References: <20240410170621.2011171-1-peterx@redhat.com> <20240411171319.almhz23xulg4f7op@revolver> MIME-Version: 1.0 In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline X-Rspam-User: X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: 1B98B18000F X-Stat-Signature: q4z1s7n3s5eku61jy15o6ro1buhkyf1b X-HE-Tag: 1712925544-527821 X-HE-Meta: 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 /RY3XY0j IINWxZeK54kK9vHQhhXOVKvH17L+8uafjxfnhF0uHDse0vUmFuh61bo/UfSewVjUXIH3bdh4PBxZxRmrR8r6W8kLK7kEDcGaLyRze+68cmCNXrAMswCzQxs7WmDw+c7DUyuUKVGy1i6ii0x9OD+SkZdgHJOHAl633WeU+FJMYHGi3JyuHM/Azh4gdEFYnMUyPFlFoN1UrTHRPu0z5psyg+9Qu6fNQmvfnf1BxXj8TocZ3+e0W5nG2lrYfPL956bkaPzuzR9oIgCEU0lcvT2dkzm/7y0Ie8qTObLFtjYxbsLvpKxUEm/+GpcT6HWtvymo7k7yiKtzs8PVwQ06y7ykBJ3VXegP8RSMGzjNy9OnIHhG2BmInrGKwF1DHMtO9knY/9uPdic3qY/G+SdeQinYX6wJBVnM8SNqVWtt5 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, Apr 12, 2024 at 04:14:16AM +0100, 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 scalability > > 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 mm_struct *mm, > if (!vma_start_read(vma)) > goto inval; > > - /* > - * find_mergeable_anon_vma uses adjacent vmas which are not locked. > - * 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 VMA */ > if (unlikely(address < vma->vm_start || address >= 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!) > > We'd need some data to be sure it's really a win, but less code is > always good. You at least need two things: (1) don't throw away Jann's comment so easily (2) have a look on whether anon memory has the fallback yet, at all Maybe someone can already comment in a harsh way on this one, but no, I'm not going to be like that. I still don't understand why you don't like so much to not fallback at all if we could, the flags I checked was all in hot cache I think anyway. And since I'm also enough on how you comment in your previous replies, I'll leave the rest comments for others. -- Peter Xu