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([2405:201:682f:3094:b352:f8fe:e89c:72bd]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-27ed66cf6e7sm124929985ad.2.2025.09.29.02.10.25 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 29 Sep 2025 02:10:26 -0700 (PDT) From: Deepanshu Kartikey To: kent.overstreet@linux.dev Cc: linux-bcachefs@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] bcachefs: Fix deadlocks between fallocate and readahead Date: Mon, 29 Sep 2025 14:40:22 +0530 Message-ID: <20250929091022.340341-1-kartikey406@gmail.com> X-Mailer: git-send-email 2.43.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit I found that bchfs_fallocate() only evicts pagecache when FALLOC_FL_ZERO_RANGE is set: if (mode & FALLOC_FL_ZERO_RANGE) { truncate_pagecache_range(&inode->v, offset, end - 1); } The hole detection code already skips ZERO_RANGE: if (!(mode & FALLOC_FL_ZERO_RANGE)) { bch2_clamp_data_hole(...); // Only runs for basic fallocate } The syzbot reproducer uses mode=0 (basic fallocate), which doesn't evict pages but does run hole detection and hits the deadlock. The fix should be to remove pagecache_block_get() from bch2_fallocate_dispatch() and add it only where pages are actually evicted: In bchfs_fallocate(): if (mode & FALLOC_FL_ZERO_RANGE) { bch2_pagecache_block_get(inode); truncate_pagecache_range(...); // Do allocation work bch2_pagecache_block_put(inode); } Similarly add it to bchfs_fpunch() and bchfs_fcollapse_finsert() around their page eviction calls. This way basic fallocate never holds the lock, avoiding the deadlock. Does this approach look right?