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Tue, 10 Nov 2026 20:44:54 -0800 (PST) Received: from google.com ([2a00:79e0:2031:6:ad6a:b446:df00:78dc]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-3b17db5cc73sm2535622a91.10.2026.11.10.20.44.51 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 10 Nov 2026 20:44:53 -0800 (PST) Date: Wed, 11 Nov 2026 13:44:49 +0900 From: Sergey Senozhatsky To: Yosry Ahmed Cc: Sergey Senozhatsky , Andrew Morton , Nhat Pham , Yosry Ahmed , Johannes Weiner , Brian Geffon , Minchan Kim , Richard Chang , linux-kernel@vger.kernel.org, linux-block@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH v2 08/11] zsmalloc: encode 1-based obj_idx so valid obj is never 0 Message-ID: References: <20261110054836.1515030-1-senozhatsky@chromium.org> <20261110054836.1515030-9-senozhatsky@chromium.org> X-Mailing-List: linux-block@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On (26/11/10 16:44), Yosry Ahmed wrote: [..] > > Store obj_idx + 1 in obj, so that a valid obj is never 0, and adjust > > ZS_MIN_ALLOC_SIZE so that the largest index still fits. Unlike a > > separate tag bit, this doesn't take any bits from obj. zsmalloc did > > the same before handles moved to handle_cachep, see > > commit 67296874eb1c ("staging: zsmalloc: Ensure handle is never 0 on > > success"). > > > > Signed-off-by: Sergey Senozhatsky > > --- > > mm/zsmalloc.c | 19 +++++++++++++------ > > 1 file changed, 13 insertions(+), 6 deletions(-) > > > > diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c > > index 3a6a7253fd01..aa436df0cc4d 100644 > > --- a/mm/zsmalloc.c > > +++ b/mm/zsmalloc.c > > @@ -61,7 +61,8 @@ > > * Object location (, ) is encoded as > > * a single (unsigned long) handle value. > > * > > - * Note that object index starts from 0. > > + * Note that object index starts from 0, and is encoded as > > + * + 1 so that a valid encoded obj value is never 0. > > Well, this may be a stupid question, but why not make obj_idx start at > 1? I didn't write that code! I just resurrected 67296874eb1c! Just joking. We use idx to calculate offset, we need it to start from 0: off = offset_in_page(class->size * obj_idx)