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b=U+aPrqxy25vF3Mu+gBiVvrPC56J/hm1a0lxfYbKELx751ipBm4BYCE5Km4DwHGW4q 7GEVDSnr/qkPerbT8DlupMSIzFIX7OyJxLl8ZTrrUbp/wYdh7ddHGO5O1Cp+wBcxVu gjjHJfGfyIanxqctc9ERFgZtw3Y798k1Hney7PQsDXZQtOKEmTmQrhOoBCrTaEIoDs 6ypwSOto+ITP0MnWWR5KgV6qpIfESfNat/QOCjecywgoWMVt/7mBiVHk/Zsw/gPaFj akDlXh9Ipjr7SBlRmqRB+ACQoDFhUnPsOqD0rpiHhxppYJ8oXwfgbNMVSkAvCwvpmR hJVHefBLm1HNA== Date: Thu, 26 Mar 2026 09:38:02 +0000 From: "Lorenzo Stoakes (Oracle)" To: Qi Zheng Cc: akpm@linux-foundation.org, hannes@cmpxchg.org, hughd@google.com, mhocko@suse.com, roman.gushchin@linux.dev, shakeel.butt@linux.dev, muchun.song@linux.dev, david@kernel.org, ziy@nvidia.com, harry.yoo@oracle.com, yosry.ahmed@linux.dev, imran.f.khan@oracle.com, kamalesh.babulal@oracle.com, axelrasmussen@google.com, yuanchu@google.com, weixugc@google.com, chenridong@huaweicloud.com, mkoutny@suse.com, hamzamahfooz@linux.microsoft.com, apais@linux.microsoft.com, lance.yang@linux.dev, bhe@redhat.com, usamaarif642@gmail.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, "Harry Yoo (Oracle)" , Qi Zheng Subject: Re: [PATCH v2 4/4] mm: memcontrol: fix unexpected massive positive number in memcg_state_val_in_pages() Message-ID: <9ee51ac2-c87c-44f6-b983-44ac9007cb35@lucifer.local> References: <54c2b09c-84f8-4118-96a6-acc13ca2f245@lucifer.local> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 7614612000F X-Stat-Signature: ysne5jprwgzsg87eio4dgd9cpawsyets X-Rspam-User: X-HE-Tag: 1774517891-241966 X-HE-Meta: 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 pNnXSxCf ikgiwSkUgJynH43UaSqJau0mPDiOhAZpw3mmHe8X+YB8QtruRDHKC0iC5SOKVS+Dx5VKRQ9jmhBH4MCLWbmEPemBZQdrxD9/avWubpTpZAXRmU3taVrN+zaCYoYi+iaV23nT4XeiAckVy1UMITKY2s0y6823njLUSHV5y5gH3m+vpQNFz5pfsjH37zEWMPw09OP4ebUDyRYTYQOFHRB9DyrvPq/j/zbfAYLk+pgltWmUd6lGzo2hp8AVtnte/3FoaKHWKZD+GokDq7uhOm9yhTygT92ZryNJFBaHNb8pvCj52raktwMpBEL+WsA== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Thu, Mar 26, 2026 at 05:32:05PM +0800, Qi Zheng wrote: > > > On 3/26/26 5:16 PM, Lorenzo Stoakes (Oracle) wrote: > > On Wed, Mar 25, 2026 at 10:13:25PM +0800, Qi Zheng wrote: > > > --- > > > mm/memcontrol.c | 7 +++++-- > > > 1 file changed, 5 insertions(+), 2 deletions(-) > > > > > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > > > index 04076a139dbe3..0c249255ebefb 100644 > > > --- a/mm/memcontrol.c > > > +++ b/mm/memcontrol.c > > > @@ -787,11 +787,14 @@ static int memcg_page_state_unit(int item); > > > static long memcg_state_val_in_pages(int idx, long val) > > > { > > > int unit = memcg_page_state_unit(idx); > > > + long res; > > > > > > if (!val || unit == PAGE_SIZE) > > > return val; > > > - else > > > - return max(val * unit / PAGE_SIZE, 1UL); > > > > Hm this was already fairly horrid, because we're comparing an unsigned long > > value of 1 vs. a ULONG_MAX - abs(val) so this was intended to make 0 -> 1UL > > but not what you'd mathematically think this was which was to make negative > > values (logically < 1) -> 1. > > > > Of course before it was just broken and would promote (val * unit / > > PAGE_SIZE) to unsigned long first (thus massive number) and return that :) > > > > > + > > > + res = max(mult_frac(abs(val), unit, PAGE_SIZE), 1UL); > > > > This is way too compressed into one line and retains the confusing > > behaviour. > > > > Could we split this out and explain what we're doing (sign-extension, > > integer promotion and all of this stuff is confusing - so let's just accept > > that and spell it out): > > > > /* Get the absolute value of (val * unit / PAGE_SIZE). */ > > res = mult_frac(abs(val), unit, PAGE_SIZE); > > /* Round up zero values. */ > > res = res ?: 1; > > /* Retain sign. */ > > return val < 0 ? -res : res; > > > > This is functionally identical, but a lot more readable, I think. > > Make sense, I will update to v3. Thanks! > > If Andrew needs me to merge this patchset into "[PATCH v6 00/33] Eliminate > Dying Memory Cgroup" [1], then I will develop and send v7. > > [1]. > https://lore.kernel.org/all/cover.1772711148.git.zhengqi.arch@bytedance.com/ Oh that's your series too :) That would be ideal unless that's already in mm-stable, as the series ordering will give us strict ordering on patches. Anyway let's wait for Andrew on that! Cheers, Lorenzo