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AJvYcCW15XzSkVeQ0QScvTBgzmMReXoYFeU4VoUR/LNxF6H0rPdy5rGGR7nJBOlFphijO2ycm8fhM4pSdHlaBMNgfmBJ6kA= X-Gm-Message-State: AOJu0YysOryiFJ3WLiDgwkO5kqD/gq60QzeJBljMLVdnQkhatHvPPyES svRVMuKQgDVOT362lQZWDRGnux+ZxU0FR7fI7PjU+CaMO+nOEtWGCW/eU8Aqghp87KDMMWn0Gel bM7GfFFJ0Soomr/E4xqhlwmFiJfojiSXJCcza X-Google-Smtp-Source: AGHT+IFqKcZjrBqDZxphr6WtcE3CcU/enUigjZklXRpwMJNBGBhuMM1g1fWCr028B2gZnKgiwIq3tDnZ1lacMhNhsNQ= X-Received: by 2002:a05:6000:2a2:b0:366:eb45:6d54 with SMTP id ffacd0b85a97d-366eb456e70mr15954495f8f.48.1719524549539; Thu, 27 Jun 2024 14:42:29 -0700 (PDT) MIME-Version: 1.0 References: <1a48d05c-539b-4d5d-9375-f2aaa5fa0dd4@I-love.SAKURA.ne.jp> In-Reply-To: <1a48d05c-539b-4d5d-9375-f2aaa5fa0dd4@I-love.SAKURA.ne.jp> From: Axel Rasmussen Date: Thu, 27 Jun 2024 14:41:53 -0700 Message-ID: Subject: Re: [PATCH] mm: mmap_lock: replace get_memcg_path_buf() with on-stack buffer To: Tetsuo Handa Cc: bpf , linux-mm , Andrew Morton , Nicolas Saenz Julienne , LKML Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Server: rspam03 X-Rspam-User: X-Rspamd-Queue-Id: 673FE40010 X-Stat-Signature: iogdz9um17ndqmhtzfmpo5mqpxdmgbpz X-HE-Tag: 1719524551-429714 X-HE-Meta: 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 /sVbiGjA JPeKcF6CsrvdApBIr7xjj1qPwTqGA6HDkye5RtSZD6AaAdxtu/C14jkrSn47YmlXs1Yf+3HKuCNAm/MOjlXSXAydPZUErJaO+ZrV9EvJZba0NaXI1KGPCHwCUuxunjrrYZPc/oxa/uj7EAVyiLapHTuULXp7bhtCBM3pDTI+ynKeTP08JWI6Rs6/N7WX6QYOD7nE9LxHP1z5xRyakXIMdl4HAgCIB09zB9t4Sg8USr1iXwrCjYDYgg+vgA+HMmNr6KzO2ZRBtsLjavmhB5Wf5nkIpPwXygINChyZAXxqWBd0cHKU= X-Bogosity: Ham, tests=bogofilter, spamicity=0.005998, 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 Fri, Jun 21, 2024 at 10:57=E2=80=AFPM Tetsuo Handa wrote: > > On 2024/06/22 8:03, Axel Rasmussen wrote: > > No objections. Looking back all the way to the first version [1] the > > buffers were already percpu, instead of on the stack like this. IOW, > > there was no on-list discussion about why this shouldn't go on the > > stack. It has been a while, but if memory serves I opted to do it that > > way just out of paranoia around putting large buffers on the stack. > > But, I agree 256 bytes isn't all that large. > > > > That v1 patch wasn't all that complex, but then again it didn't deal > > with various edge cases properly :) so it has grown significantly more > > complex over time. Reconsidering the approach seems reasonable now, > > given how much code this removes. > > > > This change looks straightforwardly correct to me. You can take: > > > > Reviewed-by: Axel Rasmussen > > Thank you. One question. CONTEXT_COUNT was defined as below. > > >> -/* > >> - * How many contexts our trace events might be called in: normal, sof= tirq, irq, > >> - * and NMI. > >> - */ > >> -#define CONTEXT_COUNT 4 > > Is there possibility that this function (or in general, trace events) is = called from NMI > context? If yes, I worry that functions called from get_mm_memcg_path() a= re not NMI-safe. > Original change at > https://lkml.kernel.org/r/3e9b2a54-73d4-48cb-a510-d17984c97a45@I-love.SAK= URA.ne.jp was > posted due to worrying about NMI safety. I think it's unlikely, but I'm not certain. Although trace events *in general* can be called from NMI context. This code was added based on the discussion here [1] from v4 of this patchset. At the time I don't think we had any certainty that this *was* called from NMI context, but rather just that if CONTEXT_COUNT was 4 when it only had to be 3 in practice, it wasn't a huge deal, but getting it wrong the other way would be much worse. [1]: https://lore.kernel.org/r/20201020184746.300555-2-axelrasmussen@google= .com >