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Thu, 26 Mar 2026 12:48:49 -0700 (PDT) Received: from gourry-fedora-PF4VCD3F ([2620:10d:c091:500::2:e5e8]) by smtp.gmail.com with ESMTPSA id af79cd13be357-8d00e39fbb0sm305859085a.8.2026.03.26.12.48.48 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 26 Mar 2026 12:48:49 -0700 (PDT) Date: Thu, 26 Mar 2026 14:48:47 -0500 From: Gregory Price To: Pedro Falcato Cc: linux-mm@kvack.org, akpm@linux-foundation.org, hughd@google.com, david@kernel.org, ljs@kernel.org, Liam.Howlett@oracle.com, vbabka@kernel.org, rppt@kernel.org, surenb@google.com, mhocko@suse.com, baolin.wang@linux.alibaba.com, linux-kernel@vger.kernel.org, kernel-team@meta.com, stable@vger.kernel.org Subject: Re: [PATCH] mm/shmem: use invalidate_lock to fix hole-punch race Message-ID: References: <20260326162611.693539-1-gourry@gourry.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Stat-Signature: znpj6cw8kb8desyuyramh8qpnxnhihkx X-Rspamd-Queue-Id: A0989180007 X-Rspam-User: X-Rspamd-Server: rspam03 X-HE-Tag: 1774554530-514824 X-HE-Meta: U2FsdGVkX18qeOI+DfRy4RGh5blJLwks0e958w8DYuq0yUipZTiT+1b265s502NBtfPSvh0rPdjN8TpZCqV8Ze6M0RQHqBEOpNLkvjpXcZd2QnnkOU8cM7vgdx1CMLf33u4zDlpxiHKA2/qL0a1G+WxmTFbiQTNew91YtiOxMY82eW1ZDUFUZIj0ByenceMk3yiOQWgwFbqKFTLbF3Q1bX93QCQHZ5TZb9voydknXXtWcslGnycUjOS+o8wt+jjnwrOUYpsqmLh5QArAZGA2F9/VtODFwYmIy21UwXKFIewp+glS62JDCEbb2ycWNAYSQwOh69p9kUUYmIKOdStyU3TAoCKeFTdOEm2XHZ0MOiJBiKMFtmtl79tIPdi+S6PZobYKvR7pvKa1MpE8pgqKXpvxE+/3lm7Rj1Rvb6LoGuNmMu/5FIpJ38nxpEt35hpnlXzXsYeQdu+SnQ2w8xvsEMQcP2ezkCAxTXvlapLK7dlgM/5vRIhyNoiQk3M+yXtwGcUq3pzo6UAzkVs/D7FSPG84kgwTlNnTIiYOBhs7n/GfJ0r02ezEHUv2s77amqOI4jPgsMt8RYtJzlDcKA1vPq2R/tfovcXIY4+VrlcPNJxlKIC1DUpDVL/urxomFzN0e6Xat1bsu2jFJFYYkq8u+mw1TpnbsimQFdQ1TNk9egAleQ447zVh4LeOKu2tBGV23w/371gRFHHeaiJXqwOXSksX5+3zs73a/MSujcJPzDGdpvfk25zT6UqTkSkQSvM718TXm9CONrUdkq90DxXmfnYr9fx0LH/MuzE2Rse9ARXQVkMoSWS2MzdT4L4Hy2hJqe/tyWCkRQDKZpUcNDCq0sdveyb9oyVlswz7neNM1QPONlusDjn/QNLxHsbQ0u8Awb8ecpXdEnl5UDn8mrmqzzYeyRt/ktu2ZvlJU2IBxvdsx+iuLLoyHI09StdNGqYaqy1AENaZkHgPUvoCmPx DyKgp8jj 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 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 07:16:05PM +0000, Pedro Falcato wrote: > > Sure, but can child - parent happen when traversing the i_mmap tree? I don't > think so? (in mm/mmap.c) > /* insert tmp into the share list, just after mpnt */ > vma_interval_tree_insert_after(tmp, mpnt, > &mapping->i_mmap); > > The function itself is somewhat straightforward - find the leftmost node at the > right of 'prev' (our parent) and link ourselves. So an in-order traversal should > always go parent - child. Unless there's some awful tree rotation that can > happen and screw us in the meanwhile. > hm, i think you're right, i have this inverted. But this patch objectively fixed my issue, I no longer see this BUG(), I don't get softlocks, and I don't get the guest corruption I was seeing previously. It could simply be that the contention added makes the race less likely. Let me dig into this and just smoke test your suggestion - but I think your patch would cause some contention issues on unmaps. It's been difficult to generate a reproducer for this without running hundreds of VMs, whatever race is going on here is extremely narrow. > > If this is broken, then every filesystem out there using filemap_fault() and > filemap_fault_around() has to be broken, and I hope that's not true :p > Me too, but i never rule anything out. ~Gregory