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From: Shakeel Butt To: "Harry Yoo (Oracle)" Cc: Vlastimil Babka , Andrew Morton , Hao Li , Christoph Lameter , David Rientjes , Roman Gushchin , "Liam R. Howlett" , Suren Baghdasaryan , Hao Ge , Kees Cook , Pedro Falcato , Danielle Constantino , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH slab/for-next-fixes v3 0/4] mm/slab: fix unbounded recursion in free path with memalloc profiling Message-ID: References: <20260713-kmalloc-no-objext-v3-0-47c7bd138de7@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260713-kmalloc-no-objext-v3-0-47c7bd138de7@kernel.org> X-Migadu-Flow: FLOW_OUT On Mon, Jul 13, 2026 at 11:28:48PM +0900, Harry Yoo (Oracle) wrote: > This is a follow-up fix after the recent discussion [1]. > See patch 4 for the detailed description on the bug. > > Based on slab/for-next-fixes (af9ea231c0b45) and is available at > git.kernel.org [2]. > > Instead preventing cycles by bumping up the allocation size of obj_exts > arrays, it introduces a new kmalloc type called KMALLOC_NO_OBJ_EXT and > disallow formation of cycles between kmalloc types when allocating > obj_exts arrays. obj_exts arrays of normal kmalloc caches are served > from KMALLOC_NO_OBJ_EXT caches (that don't have obj_exts), and all other > obj_exts arrays are served from normal kmalloc caches. > > I tried to reuse SLAB_ALLOC_NO_RECURSE to make kmalloc_slab() select > KMALLOC_NO_OBJ_EXT, but it was not great because it does not allow > sheaves for those caches. So I introduced a new slab alloc flag > SLAB_ALLOC_NO_OBJ_EXT. > > To avoid huge confusion, I had to decouple "disallowing sheaves" > semantics from SLAB_NO_OBJ_EXT and introduced SLAB_NO_SHEAVES. > > While this cannot be directly backported to v6.18 and v6.12 due to lack > of SLAB_ALLOC_* flags and kmalloc_flags(), I don't this will be > particularily challenging to backport it. Instead of a new slab alloc > flag, we can use __GFP_NO_OBJ_EXT to select KMALLOC_NO_OBJ_EXT as > kmalloc caches don't have sheaves in v6.18 anyway. > > [1] https://lore.kernel.org/linux-mm/9a139365-28e6-4f1e-b35b-7f6091e9aa14@kernel.org > > [2] https://git.kernel.org/pub/scm/linux/kernel/git/harry/linux.git/log/?h=kmalloc-no-objext-v3r1 > > To: Vlastimil Babka > To: Andrew Morton > To: Hao Li > To: Christoph Lameter > To: David Rientjes > To: Roman Gushchin > To: Suren Baghdasaryan > To: Hao Ge > To: Kees Cook > To: Pedro Falcato > To: Shakeel Butt > To: Danielle Constantino > To: Liam R. Howlett > Cc: linux-mm@kvack.org > Cc: linux-kernel@vger.kernel.org > Signed-off-by: Harry Yoo (Oracle) I tested next-20260716 which has this series against next-20260707 which does not have the fix. The reproducer were able to trigger the leak/recursion on next-20260707 but not on next-20260716. So, you can add: Tested-by: Shakeel Butt