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Mon, 27 Jan 2025 16:37:26 -0800 (PST) Received: from google.com ([2401:fa00:8f:203:566d:6152:c049:8d3a]) by smtp.gmail.com with ESMTPSA id d2e1a72fcca58-72f8a77c892sm7807869b3a.151.2025.01.27.16.37.23 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 27 Jan 2025 16:37:26 -0800 (PST) Date: Tue, 28 Jan 2025 09:37:20 +0900 From: Sergey Senozhatsky To: Yosry Ahmed Cc: Sergey Senozhatsky , Andrew Morton , Minchan Kim , Johannes Weiner , Nhat Pham , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [RFC PATCH 5/6] zsmalloc: introduce handle mapping API Message-ID: References: <20250127080254.1302026-1-senozhatsky@chromium.org> <20250127080254.1302026-6-senozhatsky@chromium.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Queue-Id: BA5BB1C0008 X-Stat-Signature: fs93q9yqyg743hwmnfrfufduqjo7f7nh X-Rspamd-Server: rspam08 X-Rspam-User: X-HE-Tag: 1738024647-695403 X-HE-Meta: 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 Mrh23/6d sScpgMDSb/fuFaVMQf3rOlATMCIswFt3pLCHo3oKuYDRut0Ol6FB00DXC7CPUUwEN9mMdN1kXhke7VzupL1zFXjdrHd9811u+b4xfIhTeUplbQK1lRyL9Bd5y/knPmDNWSqWdZR6DRLieGWl+HNTdRe2+BCQlKaxqICl0Eo1JqhheL9Wc2lEuCAKHRvxD03Lq3Go9mO6p8DoyBKAQUjzH1qkBydmXDLZsALOWw7O3uUsj3WU0gcpoL79D2MAX1xGetDh+yLxNhE8o2qMpKmTs3Tt9eDVdbBqpfg7FbuL+Qb9j1xin6XD2xLvsQiw+0RcHRnMgbli4YlpTR7a97imt6udKFFsOz881nDqyKWKyrfrRl0E= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000263, 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 (25/01/27 21:26), Yosry Ahmed wrote: > On Mon, Jan 27, 2025 at 04:59:30PM +0900, Sergey Senozhatsky wrote: > > Introduce new API to map/unmap zsmalloc handle/object. The key > > difference is that this API does not impose atomicity restrictions > > on its users, unlike zs_map_object() which returns with page-faults > > and preemption disabled > > I think that's not entirely accurate, see below. Preemption is disabled via zspage-s rwlock_t - zs_map_object() returns with it being locked and it's being unlocked in zs_unmap_object(). Then the function disables pagefaults and per-CPU local lock (protects per-CPU vm-area) additionally disables preemption. > [..] > > @@ -1309,12 +1297,14 @@ void *zs_map_object(struct zs_pool *pool, unsigned long handle, > > goto out; > > } > > > > - /* this object spans two pages */ > > - zpdescs[0] = zpdesc; > > - zpdescs[1] = get_next_zpdesc(zpdesc); > > - BUG_ON(!zpdescs[1]); > > + ret = area->vm_buf; > > + /* disable page faults to match kmap_local_page() return conditions */ > > + pagefault_disable(); > > Is this accurate/necessary? I am looking at kmap_local_page() and I > don't see it. Maybe that's remnant from the old code using > kmap_atomic()? No, this does not look accuare nor neccesary to me. I asume that's from a very long time ago, but regardless of that I don't really understand why that API wants to resemblwe kmap_atomic() (I think that was the intention). This interface if expected to be gone so I didn't want to dig into it and fix it.