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AJvYcCWoj9UNGjDvsJcGtX0g9oTfbgdp/zvBR8VjVLR7Et7vzy1ZQQBiJKv3CkYb598fJXS+5aVHVYfdi6ylyuwzTzOlFpah/JJBhbN7XtgV X-Gm-Message-State: AOJu0Yw+dv4JfqALhxWQOaEqgM4t06YIqLoS/YNqE0LRcVMiNjI+Smsm ng4BGA3umV1f4q3ujijU31b8UqDkICK4zBrC4cjIqLiiRZZ4PR7m X-Google-Smtp-Source: AGHT+IGq8WDJvVMPjAUIosp0LtnPy4A/AiNvUulsnBTh8qtM5J2fgL5a3aAC1JEdhMqyWFZ/rB/2cg== X-Received: by 2002:a05:6870:9689:b0:24f:f282:241c with SMTP id 586e51a60fabf-2512214b611mr1830733fac.59.1717568057375; Tue, 04 Jun 2024 23:14:17 -0700 (PDT) Received: from [192.168.255.10] ([43.132.141.20]) by smtp.gmail.com with ESMTPSA id d2e1a72fcca58-702423c7bf0sm7951204b3a.42.2024.06.04.23.14.14 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 04 Jun 2024 23:14:16 -0700 (PDT) Message-ID: <5bb3bbf6-6a22-449f-96f1-b9476357f284@gmail.com> Date: Wed, 5 Jun 2024 14:14:13 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 02/10] mm/ksm: skip subpages of compound pages To: Matthew Wilcox , alexs@kernel.org Cc: Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org, izik.eidus@ravellosystems.com, aarcange@redhat.com, chrisw@sous-sol.org, hughd@google.com, david@redhat.com References: <20240604042454.2012091-1-alexs@kernel.org> <20240604042454.2012091-3-alexs@kernel.org> Content-Language: en-US From: Alex Shi In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 6/5/24 11:52 AM, Matthew Wilcox wrote: > On Tue, Jun 04, 2024 at 12:24:44PM +0800, alexs@kernel.org wrote: >> From: "Alex Shi (tencent)" >> >> When a folio isn't fit for KSM, the subpages are unlikely to be good, >> So let's skip the rest page checking to save some actions. > > Why would you say that is true? We have plenty of evidence that > userspace allocators can allocate large folios, then use only the first > few bytes, leaving many tail pages full of zeroes. Um, that do need tail pages... Is there some way to use more folio in ksm? > >> @@ -2660,6 +2662,9 @@ static struct ksm_rmap_item *scan_get_next_rmap_item(struct page **page) >> cond_resched(); >> continue; >> } >> + >> + VM_WARN_ON(PageTail(*page)); >> + nr = compound_nr(*page); > > And this is simply wrong. *page can obviously be a tail page. > Got it. Thanks a lot! Alex