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Fri, 20 Feb 2026 11:07:47 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id jaKzBYNAmGmERQAAD6G6ig (envelope-from ); Fri, 20 Feb 2026 11:07:47 +0000 Date: Fri, 20 Feb 2026 11:07:45 +0000 From: Pedro Falcato To: Kalesh Singh Cc: akpm@linux-foundation.org, rostedt@goodmis.org, joel@joelfernandes.org, kernel-team@android.com, android-mm@google.com, "David Hildenbrand (Arm)" , Lorenzo Stoakes , Minchan Kim , Suren Baghdasaryan , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Michal Hocko , Masami Hiramatsu , Mathieu Desnoyers , Jann Horn , Martin Liu , David Rientjes , Zi Yan , Wander Lairson Costa , Petr Mladek , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org Subject: Re: [PATCH] mm/tracing: rss_stat: Ensure curr is false from kthread context Message-ID: <6wljnbdh2tvi5egjd462hel7zkxvxedqgeqbjsdpoibc5n2j43@fsjneenogr3y> References: <20260219233708.1971199-1-kaleshsingh@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260219233708.1971199-1-kaleshsingh@google.com> X-Rspamd-Server: rspam09 X-Stat-Signature: 6c6imijrh5hedmssy5ik93hazgs5g14u X-Rspamd-Queue-Id: A616A1C0009 X-Rspam-User: X-HE-Tag: 1771585670-696333 X-HE-Meta: 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 aI2Rijrm PjzS1lSU7cmJlD/nWRDSP7GRPlptbl3FtiZGY2CzVT/8l4DkKIkjoaRe+i9EDQgsHMjjxm0K+zSC+3fe0voXMZhE0q5fSa9tXjpWoaiqT3cuMVEMrV+gz8mdSs3mYNqJVZTbOn9gMXp3r0IOGv41PORVCY8zTRDfXyPjh+j39UGg5LhXFyNZC8iTylvU7h209MPz8rTcWD/9zly/e2LtSqLwKLYgQ+AJuvK09YDUWJCIz6r7f3NVJjuF0gNA6xsNyHBuJySMHdUEnuy8Q52q9tRs7eMwcEFvlugAZw1fjr7MsiJmj1uUSaJB9iPno6IIeagy6PslbYy5TqbcIYXj4yeuRlbMCH3TX0dGTscjQ6kuhZP92QANX3sxwVFfq6p4kPoYWajDMcCq9Wczcv1eSCI8czsFdkUDusXUDF2IsLuKuhuhaUXhJLqNgW4zzpTGy/+3gOcfqtFN0n9CS51O8z+Kod0owLbbnjNb+kSI7/kmuU6kxNQl7ohJIP4Fg1rzpzTCUhLC8Igomr4sPtIbhQYU9k0JpVdlLFBPHlqnBNin05IaBm1DOxzDHleeCs/hLR9VFbWNjSfGRfkYkOPOmclUsU9TI9nFVKANpBTDfpx/ektx/MZNLINOJK8PqP+UbkQL3qen4JnR30aP/KT/DfE+tYA== 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 Thu, Feb 19, 2026 at 03:36:56PM -0800, Kalesh Singh wrote: > The rss_stat trace event allows userspace tools, like Perfetto [1], > to inspect per-process RSS metric changes over time. > > The curr field was introduced to rss_stat in commit e4dcad204d3a > ("rss_stat: add support to detect RSS updates of external mm"). > It's intent is to indicate whether the RSS update is for the > mm_struct of the current execution context; and is set to false > when operating on a remote mm_struct (e.g., via kswapd or a > direct reclaimer). > > However, an issue arises when a kernel thread temporarily adopts > a user process's mm_struct. Kernel threads do not have their own > mm_struct and normally have current->mm set to NULL. To operate > on user memory, they can "borrow" a memory context using > kthread_use_mm(), which sets current->mm to the user process's mm. > > This can be observed, for example, in the USB Function Filesystem > (FFS) driver. The ffs_user_copy_worker() handles AIO completions > and uses kthread_use_mm() to copy data to a user-space buffer. > If a page fault occurs during this copy, the fault handler executes > in the kthread's context. > > At this point, current is the kthread, but current->mm points to the > user process's mm. Since the rss_stat event (from the page fault) > is for that same mm, the condition current->mm == mm becomes true, > causing curr to be incorrectly set to true when the trace event is > emitted. > > This is misleading because it suggests the mm belongs to the kthread, > confusing userspace tools that track per-process RSS changes and > corrupting their mm_id-to-process association. > > Fix this by ensuring curr is always false when the trace event is > emitted from a kthread context by checking for the PF_KTHREAD flag. > > [1] https://perfetto.dev/ > > Fixes: e4dcad204d3a ("rss_stat: add support to detect RSS updates of external mm") > Cc: Andrew Morton > Cc: "David Hildenbrand (Arm)" > Cc: Joel Fernandes > Cc: Lorenzo Stoakes > Cc: Minchan Kim > Cc: Steven Rostedt > Cc: Suren Baghdasaryan > Signed-off-by: Kalesh Singh Reviewed-by: Pedro Falcato Looks cromulent, thanks :) -- Pedro