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t=1789638598; bh=vSU+xrAEcsvkHu26lr1G4pEJ8rlPYCT7TOEWzjoc1DQ=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=ddAunACcGySEBxWygGGBQBOZW5Kc9y9fwixAr9AgU4hqTclBfnONuRYqSKoyfYk0m 3pZboHgJIlAY4irh8yRMsCcdYPYen+IGMChqimfTE1u5zGBOGIejfZBJSR+gnChYGm 6iCXlEWzDGX2UACyLmMePk1m61Vs9ABpZMBL6MvxFje0oUPHy4InNf+E7BBdXrkRzy ONQZ4tWJO/zZsl/JEIAxSFQKoNEAM7JiD0oKrwMNCQAVzr0rqyjLY0mNY23TLcWqS+ P7+WShn0xdG+PeZKjmbKiKqydX/1jLD58t+SOBYr5srCMSoBLfLV9gDXpWzLCZDtuR dFKk13Yg8SgtA== Message-ID: <7eb8f4c0-6e14-4f91-a3b9-77e82f76ed00@kernel.org> Date: Thu, 17 Sep 2026 11:49:52 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 2/2] mm/mempolicy: stop copying the nodemask in the interleave paths To: Gregory Price Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernel-team@meta.com, akpm@linux-foundation.org, ziy@nvidia.com, matthew.brost@intel.com, joshua.hahnjy@gmail.com, rakie.kim@sk.com, byungchul@sk.com, ying.huang@linux.alibaba.com, apopple@nvidia.com, urezki@gmail.com, chenwandun@huawei.com References: <20260829015943.1258774-1-gourry@gourry.net> <20260829015943.1258774-3-gourry@gourry.net> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 1A29080003 X-Stat-Signature: 3f5dkfsnnfzbgry576fg61wdhker9r78 X-HE-Tag: 1789638598-44150 X-HE-Meta: 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 wP/0oOhr Yt7dqYuh7ZnrpzkDViikIgkiuHOLrHLn5GfPvZ/eI6WT2jfwp6nTCUsQWUrSKGZ/NaUw0oXl3FoRBYNjwQjpt8N9bZai/Z53DKbFxJb0xqEoDJyBgU+pR/Plgxjy8G75vDfvBa9aSyl9MWsnrljaODt6StZKAmupm7R3IamY0RoSYCnLROL508dXQNagvb0ooWbwiSd+TmASOrHBGegWew5/Stwot2pbs1yyB3rfzP8ZHpKT3M/qRD/HZBRZAS9rPs/3C23dOeKHi2Q8njXPb47DvCGXVI9+ROdEm/PpqJq4xRZ3aCUffw5FFG0GhGfhnvpgt+s9oK3usS3UsI9Q3Gti5wA== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 9/16/26 18:27, Gregory Price wrote: > On Wed, Sep 16, 2026 at 05:57:22PM +0200, David Hildenbrand (Arm) wrote: >> On 8/29/26 03:59, Gregory Price wrote: >>> The interleave node selectors copy pol->nodes onto the stack so the mask >>> cannot change while they walk it. nodemask_t is 128 bytes at >>> MAX_NUMNODES=1024, and two of the three run per folio fault. >>> >>> The copy only buys consistency between the node count and the walk. >>> Drop the consistency and just bounds check the walk instead. >> >> I think you should document here that accessing the node bitnmap is safe (is RCU >> responsible for that? I think yes), but it can get updated concurrently. >> > > Actually, not quite. > > RCU stabilizes the weights and makes accessing them safe. RCU doesn't > help us with the node bitmasks themselves (we were using a cpuset cookie > to stabilize them to take a copy - but we're killin that here). Ah, my memory comes back. So confusing. > > What makes it safe is that we detect the torn read condition (check for > an empty nodemask) and otherwise don't care about weight skews in the > weighted variant. > > I think it's reasonable to add a couple comments to spell out. > > Are you ok if I just spin an additional commit to tack on rather than > spin a new version just for comments? A resend is probably easiest when modifying the commit message. For adding more code comments an inline patch we can squash is also good enough. (I prefer a resend because that shows the full clean picture, but without actual code changes I don't care that much) -- Cheers, David