Re: [PATCH v2 13/16] mm/slab: allow __GFP_NOMEMALLOC and __GFP_NOWARN for kmalloc_nolock()

From: Vlastimil Babka (SUSE)

Date: Fri Jun 12 2026 - 09:04:48 EST

On 6/12/26 14:52, hu.shengming@xxxxxxxxxx wrote:
> Vlastimil Babka (SUSE) wrote:
>> The two flags are added internally so there's no point for warning if
>> they are passed by the caller as well, so allow them. This will allow
>> simplifying obj_ext allocation under kmalloc_nolock().
>>
>> Also it's not necessary to have the extra alloc_gfp variable for adding
>> the two flags. The original gfp_flags parameter is not used anywhere
>> except for the warning. So remove alloc_gfp and directly modify and use
>> gfp_flags everywhere.
>>
>> Signed-off-by: Vlastimil Babka (SUSE) <vbabka@xxxxxxxxxx>
>> ---
>> include/linux/slab.h | 3 ++-
>> mm/slub.c | 19 ++++++++++---------
>> 2 files changed, 12 insertions(+), 10 deletions(-)
>>
>> diff --git a/include/linux/slab.h b/include/linux/slab.h
>> index ce1c867dc0ba..b955f3cbb732 100644
>> --- a/include/linux/slab.h
>> +++ b/include/linux/slab.h
>> @@ -1040,7 +1040,8 @@ void *_kmalloc_nolock_noprof(DECL_TOKEN_PARAMS(size, token), gfp_t gfp_flags, in
>> * kmalloc_nolock - Allocate an object of given size from any context.
>> * @size: size to allocate
>> * @gfp_flags: GFP flags. Only __GFP_ACCOUNT, __GFP_ZERO, __GFP_NO_OBJ_EXT
>> - * allowed.
>> + * allowed. Also __GFP_NOWARN and __GFP_NOMEMALLOC are allowed but added
>> + * internally thus not necessary.
>> * @node: node number of the target node.
>> *
>> * Return: pointer to the new object or NULL in case of error.
>> diff --git a/mm/slub.c b/mm/slub.c
>> index 6845e15c148a..847cad5203b2 100644
>> --- a/mm/slub.c
>> +++ b/mm/slub.c
>> @@ -5388,7 +5388,6 @@ EXPORT_SYMBOL(__kmalloc_noprof);
>>
>> void *_kmalloc_nolock_noprof(DECL_TOKEN_PARAMS(size, token), gfp_t gfp_flags, int node)
>> {
>> - gfp_t alloc_gfp = __GFP_NOWARN | __GFP_NOMEMALLOC | gfp_flags;
>> size_t orig_size = size;
>> unsigned int alloc_flags = SLAB_ALLOC_TRYLOCK;
>> struct kmem_cache *s;
>> @@ -5396,7 +5395,9 @@ void *_kmalloc_nolock_noprof(DECL_TOKEN_PARAMS(size, token), gfp_t gfp_flags, in
>> void *ret;
>>
>> VM_WARN_ON_ONCE(gfp_flags & ~(__GFP_ACCOUNT | __GFP_ZERO |
>> - __GFP_NO_OBJ_EXT));
>> + __GFP_NO_OBJ_EXT | __GFP_NOWARN | __GFP_NOMEMALLOC));
>> +
>> + gfp_flags |= __GFP_NOWARN | __GFP_NOMEMALLOC;
>>
>
> Hi Vlastimil,
>
> While reviewing your patch, I spotted a potential GFP flag mismatch along the kmalloc_flags()
> -> __kmalloc_nolock_noprof() call path. I cloned the slab/for-next branch to verify this and
> successfully triggered a VM_WARN_ON_ONCE() in the __kmalloc_nolock_noprof() path.
>
> Here is the observed trace:
>
> [ 57.283791] ------------[ cut here ]------------
> [ 57.284226] WARNING: mm/slub.c:5407 at __kmalloc_nolock_noprof+0x3ec/0x450, CPU#6: insmod/379
> [ 57.285060] Modules linked in: slub_nolock(O+)
> [ 57.285494] CPU: 6 UID: 0 PID: 379 Comm: insmod Tainted: G O 7.1.0-rc3-00036-g5ade53586fae-dirty #5 PREEMPT(lazy)
> [ 57.286608] Tainted: [O]=OOT_MODULE
> [ 57.286941] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.12.1-0-ga5cab58e9a3f-prebuilt.qemu.org 04/01/2014
> [ 57.288018] RIP: 0010:__kmalloc_nolock_noprof+0x3ec/0x450
> [ 57.288543] Code: 01 00 8b 4c 24 04 41 89 c2 e9 dd fc ff ff b8 10 00 00 00 e9 f9 fd ff ff 90 0f 0b 90 f7 04 24 ff de b7 ff 0f 84 54
> [ 57.290252] RSP: 0018:ffffa82840c73b68 EFLAGS: 00010206
> [ 57.290752] RAX: 0000000000000000 RBX: 0000000000000003 RCX: ffffa82840c73bc8
> [ 57.291428] RDX: 0000000000000000 RSI: 00000000002c2100 RDI: 0000000000000100
> [ 57.292124] RBP: 0000000000000000 R08: ffffffffb853c968 R09: 0000000000009ffb
> [ 57.292821] R10: 00000000000001d7 R11: ffffffffb850c980 R12: 00000000002c2100
> [ 57.293485] R13: 0000000000000000 R14: ffffa82840c73bc8 R15: 0000000000000003
> [ 57.294159] FS: 0000000006fa8880(0000) GS:ffff8f4ef9d5a000(0000) knlGS:0000000000000000
> [ 57.294914] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [ 57.295464] CR2: 0000000006fac528 CR3: 000000003f9b6003 CR4: 0000000000770ef0
> [ 57.296124] PKRU: 55555554
> [ 57.296443] Call Trace:
> [ 57.296702] <TASK>
> [ 57.296914] ? vprintk_emit+0x22e/0x280
> [ 57.297295] __kmalloc_flags_noprof+0x216/0x530
> [ 57.297735] ? _printk+0x56/0x70
> [ 57.298053] ? alloc_slab_obj_exts+0x89/0x1e0
> [ 57.298474] alloc_slab_obj_exts+0x89/0x1e0
> [ 57.298876] new_slab+0x2bc/0x660
> [ 57.299200] ___slab_alloc+0x2ae/0x660
> [ 57.299603] __kmalloc_nolock_noprof+0x151/0x450
> [ 57.300050] _kmalloc_nolock_noprof+0x47/0x70
> [ 57.300490] ? slub_nolock_init+0x127/0xff0 [slub_nolock]
> [ 57.301013] slub_nolock_init+0x127/0xff0 [slub_nolock]
> [ 57.301566] ? __pfx_slub_nolock_init+0x10/0x10 [slub_nolock]
> [ 57.302197] do_one_initcall+0x44/0x220
> [ 57.302598] ? do_init_module+0x1e/0x240
> [ 57.302990] do_init_module+0x5f/0x240
> [ 57.303357] __do_sys_init_module+0x162/0x190
> [ 57.303783] do_syscall_64+0xf7/0x550
> [ 57.304159] entry_SYSCALL_64_after_hwframe+0x77/0x7f
> [ 57.304648] RIP: 0033:0x4b8839
> [ 57.304954] Code: 00 f3 c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 40
> [ 57.306676] RSP: 002b:00007ffea38fdce8 EFLAGS: 00000246 ORIG_RAX: 00000000000000af
> [ 57.307376] RAX: ffffffffffffffda RBX: 00007ffea38fe080 RCX: 00000000004b8839
> [ 57.308044] RDX: 000000000062d4b5 RSI: 0000000000002790 RDI: 0000000006fa9d20
> [ 57.308710] RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000002790
> [ 57.309366] R10: 0000000006fa9cc0 R11: 0000000000000246 R12: 00007ffea38fe088
> [ 57.310040] R13: 000000000062d4b5 R14: 0000000000000000 R15: 0000000000000000
> [ 57.310737] </TASK>
> [ 57.310963] ---[ end trace 0000000000000000 ]---
>
> With "# CONFIG_KMALLOC_PARTITION_CACHES is not set":
> RSI: 00000000002c2100 -> gfp_flags
>
> Decoding the GFP bits:
>
> 0x00000100 = __GFP_ZERO
> 0x00002000 = __GFP_NOWARN
> 0x00080000 = __GFP_NOMEMALLOC
> 0x00040000 = __GFP_COMP
> 0x00200000 = __GFP_THISNODE
>
> The root cause:
>
> Previously, the obj_ext allocation path(alloc_slab_obj_exts()) called kmalloc_nolock() with a
> fixed set of flags(__GFP_ZERO | __GFP_NO_OBJ_EXT). After switching to kmalloc_flags(), the
> full GFP mask from the upper allocation path propagates through the call chain.
>
> In detail, ___slab_alloc() may append __GFP_THISNODE, and allocate_slab() adds __GFP_COMP
> via s->allocflags. Both flags flow down unchanged into __kmalloc_nolock_noprof().
>
> This patch only permits four flags (__GFP_ACCOUNT, __GFP_ZERO, __GFP_NOWARN and __GFP_NOMEMALLOC).
> Since neither __GFP_THISNODE nor __GFP_COMP is in the allowed set, the warning is triggered.
>
> Would the following fix be acceptable?
>
> +#define KMALLOC_NOLOCK_ALLOWED_GFP \
> + (__GFP_ACCOUNT | __GFP_ZERO | __GFP_NOWARN | __GFP_NOMEMALLOC)
> +
> /*
> * The only version of kmalloc_node() that takes alloc_flags and thus can
> * determine on its own whether to handle the allocation via kmalloc_nolock() or
> @@ -5548,6 +5564,7 @@ void *__kmalloc_flags_noprof(DECL_TOKEN_PARAMS(size, token), gfp_t flags,
> return __do_kmalloc_node(size, NULL, flags, node,
> PASS_TOKEN_PARAM(token), &ac);
> } else {
> + flags &= KMALLOC_NOLOCK_ALLOWED_GFP;
> return __kmalloc_nolock_noprof(PASS_TOKEN_PARAMS(size, token),
> flags, node, &ac);
> }

Thanks, Hao Li pointed out __GFP_THISNODE and I filtered it from nested
allocations like this:
https://lore.kernel.org/all/49f1bf1e-fcaf-48fa-a7b1-f8ee78b19762@xxxxxxxxxx/
It makes sense to filter out __GFP_COMP too as that's a page allocator flag
and not useful for (nested) kmalloc allocations, so I'll add that