From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mta0.migadu.com (out-215.mta0.migadu.com [91.218.175.215]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 5D3E3459AE7 for ; Tue, 1 Sep 2026 08:14:08 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.215 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788250452; cv=none; b=Z7Wb2S+D9MZfuu++ek/6N2WMkP6a0ImsdHTiUsT3c9kuqobLePHgfP+Rjh6mMYsCy3GLbnhypb+05xlAkMzmPSgPR2nB1pjgVoy90+RH4uVyWDSGdS22rZWO5FO0Xr4F+emNWyJEfLWLPZhdBLiN4BiB3ZNfnooAXojdDb+B63M= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788250452; c=relaxed/simple; bh=bV4kcBvH/DWH/5vJ1R7+0iB1kjHHNS7RhB31HwS5w6E=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=EzXl2UlLtz7amQKAPHycW1VuZRNLW9QSZ7Y0mmzF6tFHYJmYVZ7kse1FX+GaxKG49e+J0bpUlSb0FQsMd7rFUpj7mfFkKH0nqQpgGQ6IqZrJcNLsjwCk22+BAPM+tsNOjJbB7wV7PMQrhNWvgC6jgLy40rFiLnvTCKKuq1zyRME= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=Z4nRn4e3; arc=none smtp.client-ip=91.218.175.215 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="Z4nRn4e3" X-Envelope-To: linux-kernel@vger.kernel.org DKIM-Signature: a=rsa-sha256; bh=bV4kcBvH/DWH/5vJ1R7+0iB1kjHHNS7RhB31HwS5w6E=; c=simple/simple; d=linux.dev; h=from:to:subject:date:message-id:mime-version:content-type; s=key1; t=1788250447; v=1; x=1788855247; b=Z4nRn4e3So7F5EGRLac5Ypi0WLnBdMDgBE/1FXKDk6TwxkfJDalOb7ZBwDMElSwlFZedtIn6 DDiqnvmXtam/mutNQrfoSRkY0eRVmC7Pcy3SotRRS6bcwKDHQoF1oIKPIWzXSbHsRMm266m6POy gwtwaZwLfCWVuXN3dmf457d4= X-Envelope-To: linux-kernel@vger.kernel.org Received: by smtp.migadu.com with ESMTPS id 569a57e56a310cb2; Tue, 01 Sep 2026 08:13:57 +0000 X-Mizu-Trace-ID: 569a57e56a310cb2 X-Migadu-Flow: FLOW_OUT Date: Tue, 1 Sep 2026 16:13:47 +0800 From: Hao Li To: "Vlastimil Babka (SUSE)" Cc: Harry Yoo , Hyunwoo Kim , Andrew Morton , Christoph Lameter , David Rientjes , Roman Gushchin , Suren Baghdasaryan , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] mm/slab: take n->list_lock for the list_add() in __refill_objects_node() Message-ID: References: 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: On Mon, Aug 31, 2026 at 02:55:06PM +0200, Vlastimil Babka (SUSE) wrote: > On 8/30/26 16:35, Hao Li wrote: > > On Sun, Aug 30, 2026 at 12:45:17PM +0000, Harry Yoo wrote: > >> On Sun, Aug 30, 2026 at 04:25:45PM +0900, Hyunwoo Kim wrote: > >> > In __refill_objects_node(), the list_add(&slab->slab_list, &pc.slabs) that > >> > follows a successful __slab_try_return_freelist() is done without > >> > n->list_lock. > >> > > >> > __slab_try_return_freelist() only succeeds while slab->freelist is NULL. The > >> > slab we are refilling from is taken off pc.slabs by the list_del() at the > >> > top of the loop, so at that point it is on no list. > >> > > >> > If another CPU frees an object of that slab, __slab_free() sees the slab as > >> > full and puts it back on n->partial. If a third CPU then takes that object > >> > in get_from_partial_node(), the freelist becomes NULL again. > >> > >> Ouch. Good catch, Hyunwoo. > >> A classic ABA problem :) > > Ouch, indeed. > > >> > A slab that sits on n->partial with a NULL freelist only exists while > >> > get_from_partial_node() holds n->list_lock, between its cmpxchg and its > >> > remove_partial(). > >> > > >> > A list_add() in that window overwrites slab_list to point into pc.slabs. > >> > The list_del() in remove_partial() then follows the overwritten links, so it > >> > unlinks the slab from pc.slabs and poisons slab_list while leaving the > >> > n->partial side alone. n->partial is left pointing at the poisoned slab. > >> > > >> > CPU0 CPU1 CPU2 > >> > > >> > __refill_objects_node() > >> > get_partial_node_bulk() // n->partial to pc.slabs > >> > list_del() // on no list now > >> > get_freelist_nofreeze() // freelist = NULL > >> > __slab_free() > >> > add_partial() > >> > // back on n->partial > >> > // freelist is not NULL > >> > > >> > get_from_partial_node() > >> > lock > >> > cmpxchg > >> > // freelist = NULL > >> > __slab_try_return_freelist() > >> > list_add(&pc.slabs) // overwrites slab_list > >> > remove_partial() > >> > list_del() > >> > // off pc.slabs > >> > // slab_list = POISON > >> > > >> > panic log: > >> > > >> > list_add corruption. next->prev should be prev > >> > (ffff888100000248), but was dead000000000122. > >> > (next=ffffea000416e410). > >> > kernel BUG at lib/list_debug.c:29! > >> > Oops: invalid opcode: 0000 [#1] SMP NOPTI > >> > CPU: 1 UID: 65534 PID: 144 Comm: poc Not tainted > >> > 7.2.0-16172-gcf72cbb39da8-dirty #1 PREEMPT(lazy) > >> > RIP: 0010:__list_add_valid_or_report+0x80/0xd0 > >> > ... > >> > Call Trace: > >> > alloc_from_new_slab+0x183/0x300 > >> > ___slab_alloc+0x31c/0x890 > >> > __kmalloc_noprof+0x3d4/0x800 > >> > lsm_blob_alloc+0x2d/0x50 > >> > security_msg_msg_alloc+0x26/0x90 > >> > load_msg+0x1aa/0x210 > >> > do_msgsnd+0x91/0x800 > >> > do_syscall_64+0x109/0x5d0 > >> > entry_SYSCALL_64_after_hwframe+0x77/0x7f > >> > ... > >> > Kernel panic - not syncing: Fatal exception > >> > > >> > Do the list_add() under n->list_lock. Reattaching the freelist stays outside > >> > the lock. Once it succeeds the freelist is no longer NULL, so __slab_free() > >> > cannot put the slab back, and by the time the lock is taken remove_partial() > >> > has finished and the slab is on no list. > >> > >> Yeah, this should work correctly. > >> > >> > The lock is held until the block below that returns the remaining slabs to > >> > the partial list. That block already took the same lock on this path, so no > >> > lock/unlock pair is added. > >> > > >> > The unlock is keyed on having taken the lock instead of on pc.slabs being > >> > empty. With CONFIG_DEBUG_LIST or CONFIG_LIST_HARDENED, __list_add() returns > >> > without linking anything if its check fails, which would leave pc.slabs > >> > empty. > >> > >> Well, if the check fails, it has a bug and should be fixed. > >> We should not make the code less readable to handle a bug. > >> > >> I think it's better to have (diff on top of the patch, not tested): > >> > >> diff --git a/mm/slub.c b/mm/slub.c > >> index 7a7f9935c711..eff96b992164 100644 > >> --- a/mm/slub.c > >> +++ b/mm/slub.c > >> @@ -7216,12 +7216,10 @@ __refill_objects_node(struct kmem_cache *s, void **p, gfp_t gfp, unsigned int mi > >> break; > >> } > >> > >> - if (!locked && !list_empty(&pc.slabs)) { > >> - spin_lock_irqsave(&n->list_lock, flags); > >> - locked = true; > >> - } > >> + if (!list_empty(&pc.slabs)) { > >> + if (!locked) > >> + spin_lock_irqsave(&n->list_lock, flags); > >> > >> - if (locked) { > >> list_for_each_entry(slab, &pc.slabs, slab_list) > >> set_node_partial_state(n, slab); > > > > Since introducing a new variable seems unavoidable, what if we temporarily > > stash this slab in a pointer like below, and then add it to pc.slabs once we > > acquire the lock. > > > > struct slab *leftover_slab = NULL; > > > > ... > > ... > > if (__slab_try_return_freelist(s, slab, head, count)) { > > leftover_slab = slab; > > break; > > } > > > > ... > > ... > > if (!list_empty(&pc.slabs)) { > > spin_lock_irqsave(&n->list_lock, flags); > > > > if (leftover_slab) > > list_add(&leftover_slab->slab_list, &pc.slabs); > > ... > > ... > > } > > > > PS: If I recall correctly, Vlastimil's initial patch was actually fine. It was > > my suggestion to save an extra lock/unlock pair that accidentally led to this > > trap... > > Ah, thanks for the reminder. This [1] was the original attempt. > > [1] > https://lore.kernel.org/all/20260421-b4-refill-optimistic-return-v1-1-24f0bfc1acff@kernel.org/ > > I wonder if the fix should be to return to that approach and just have > __slab_try_return_freelist() handle the list_lock. The code would be simpler > with not "bool locked". > > It should be really very rare that we would end up returning a partial list > and also have additional slabs to return on pc.slabs? So I think there would > be no noticeable performance downside to the simpler code potentially ending > up taking the list_lock twice instead of once. Yes, make sense, in fact, when I first looked at this patch, I did consider that maybe the original locking approach was best for readability and safety :) > > >> > >> > Fixes: ba7425312607 ("mm, slab: add an optimistic __slab_try_return_freelist()") > >> > Cc: stable@vger.kernel.org > >> > Signed-off-by: Hyunwoo Kim > >> > --- > >> > mm/slub.c | 9 ++++++++- > >> > 1 file changed, 8 insertions(+), 1 deletion(-) > >> > > >> > diff --git a/mm/slub.c b/mm/slub.c > >> > index f9b56cb439e709..4f6d1a03a8ee46 100644 > >> > --- a/mm/slub.c > >> > +++ b/mm/slub.c > >> > @@ -7260,6 +7260,7 @@ __refill_objects_node(struct kmem_cache *s, void **p, gfp_t gfp, unsigned int mi > >> > struct slab *slab, *slab2; > >> > unsigned int refilled = 0; > >> > unsigned long flags; > >> > + bool locked = false; > >> > void *object; > >> > > >> > pc.flags = gfp; > >> > >> uh, I'm not a big fan of having a new variable to store 'locked' state, > >> but okay, this seems unavoidable with current implementation. > >> > >> > @@ -7297,7 +7298,10 @@ __refill_objects_node(struct kmem_cache *s, void **p, gfp_t gfp, unsigned int mi > >> > void *tail; > >> > > >> > if (__slab_try_return_freelist(s, slab, head, count)) { > >> > + /* get_from_partial_node() may be mid-removal of the slab */ > >> > + spin_lock_irqsave(&n->list_lock, flags); > >> > list_add(&slab->slab_list, &pc.slabs); > >> > + locked = true; > >> > break; > >> > } > >> > > >> > @@ -7312,9 +7316,12 @@ __refill_objects_node(struct kmem_cache *s, void **p, gfp_t gfp, unsigned int mi > >> > break; > >> > } > >> > > >> > - if (!list_empty(&pc.slabs)) { > >> > + if (!locked && !list_empty(&pc.slabs)) { > >> > spin_lock_irqsave(&n->list_lock, flags); > >> > + locked = true; > >> > + } > >> > >> -- > >> Cheers, > >> Harry / Hyeonggon > >> > > >