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Tue, 06 Jan 2026 18:20:26 -0800 (PST) Received: from google.com ([2a00:79e0:2031:6:9f6a:2617:8891:93ff]) by smtp.gmail.com with ESMTPSA id d2e1a72fcca58-819bab0da0bsm3256759b3a.13.2026.01.06.18.20.23 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 06 Jan 2026 18:20:25 -0800 (PST) Date: Wed, 7 Jan 2026 11:20:20 +0900 From: Sergey Senozhatsky To: Yosry Ahmed Cc: Sergey Senozhatsky , Andrew Morton , Nhat Pham , Minchan Kim , Johannes Weiner , Brian Geffon , linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH] zsmalloc: use actual object size to detect spans Message-ID: References: <20260106042507.2579150-1-senozhatsky@chromium.org> <5smqbald5bollibqjsvqw2tfngdoiiucurikdgqtz6xjb7u7vz@7p6hskoixaak> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspam-User: X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: 5AEC8160005 X-Stat-Signature: nhp3pkqgz9zxgddjkpm1wm9s9jbieqdg X-HE-Tag: 1767752427-298095 X-HE-Meta: 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 1cMbFjqf bSAgDpdGGcD9E5wrSXmrOdvK7pLOBsPTxZIeQ9/pktu4itisfgozuLZorytpOlEXmSvYj2DnglBVlGZZgGh6T4yZohWrQEcBbPskHsBpfVHZvreX4trDNQz+qqHIQixYQ9dIG6GsBXwqS/zRlyr5K/83XuQmXO659nfLeiYN1fwqM2/oQPf8bCY+HH4796oJCG4/d5JJQgznyz0pm06i8cNKdaV4l9h/g+o3+btTDUrm05ocVgTNjZYuyxFwBRUm1cT8y3iIQxzLDNcvPkp/r7Q6KlzboFB3GNwpUjfp35PHf9HWKCLkaELN+Wu5UykAw6z8H8vYSLSJIxUglmpY1/OJhe/JeWuW5l+JIvYFliIA6MQs= 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 (26/01/07 02:10), Yosry Ahmed wrote: > On Wed, Jan 07, 2026 at 11:06:09AM +0900, Sergey Senozhatsky wrote: > > On (26/01/07 01:56), Yosry Ahmed wrote: > > > > I recall us having exactly this idea when we first introduced > > > > zs_obj_{read,write}_end() functions, and I do recall that it > > > > did not work. Somehow this panics in __memcpy+0xc/0x44. Let > > > > me dig into it again. > > > > > > Maybe because at this point we are trying to memcpy() class->size, which > > > already includes ZS_HANDLE_SIZE. So reading after increasing the offset > > > reads ZS_HANDLE_SIZE after class->size. > > > > Yeah, I guess that falsely hits the spanning path because of extra > > sizeof(unsigned long). > > Or the object could be spanning two pages indeed, but we're copying > extra sizeof(unsigned long), that shouldn't crash tho. It seems there is no second page, it's a pow-of-two size class. So we mis-detect spanning. [ 51.406310] zsmalloc: :: size class 48, orig offt 16336, page size 16384, memcpy sizes 40, 8 [ 51.407571] Unable to handle kernel paging request at virtual address ffffc04000000000 [ 51.420816] pc : __memcpy+0xc/0x44 Second memcpy() of sizeof(unsigned long) traps. > I think the changes need to be shuffled around to avoid this, or just > have a combined patch, which would be less pretty. I think I prefer a shuffle. There is another possible improvement point (UNTESTED): if the first page holds only ZS_HANDLE bytes, then we can avoid memcpy() path and instead just kmap the second page + offset.