[PATCH] slub: Clear __GFP_COMP flag when allocating 0 order page

[PATCH] slub: Clear __GFP_COMP flag when allocating 0 order page

[Haifeng Xu]

We encounter warning messages when failing to create a new slab like
this:

page allocation failure: order:0, mode:0x1004000(GFP_NOWAIT|__GFP_COMP),
nodemask=(null)

It's a bit confusing for users because __GFP_COMP flag is used to create
compound page which implies the order should not be 0. This is because
minimum order will be tried if higher-order allocation fails and the
minimum order is 0. It's pointless to allocate 0 order page with __GFP_COMP
flag. Therefore, clear the __GFP_COMP flag when falling back to 0 order
allocation which makes the order and gfp flags matched.

Signed-off-by: Haifeng Xu <haifeng.xu@shopee.com>
---
 mm/slub.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/mm/slub.c b/mm/slub.c
index a307d319e82c..d3e03dcb9ff2 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -1875,6 +1875,13 @@ static inline struct slab *alloc_slab_page(gfp_t flags, int node,
 	struct slab *slab;
 	unsigned int order = oo_order(oo);
 
+	/*
+	 * If fallback to the minimum order allocation and the order is 0,
+	 * clear the __GFP_COMP flag.
+	 */
+	if (order == 0)
+		flags = flags & ~__GFP_COMP;
+
 	if (node == NUMA_NO_NODE)
 		folio = (struct folio *)alloc_pages(flags, order);
 	else
-- 
2.25.1

Re: [PATCH] slub: Clear __GFP_COMP flag when allocating 0 order page

[Christoph Lameter (Ampere)]

On Thu, 11 Apr 2024, Haifeng Xu wrote:

> @@ -1875,6 +1875,13 @@ static inline struct slab *alloc_slab_page(gfp_t flags, int node,
> 	struct slab *slab;
> 	unsigned int order = oo_order(oo);
>
> +	/*
> +	 * If fallback to the minimum order allocation and the order is 0,
> +	 * clear the __GFP_COMP flag.
> +	 */
> +	if (order == 0)
> +		flags = flags & ~__GFP_COMP;


This would be better placed in allocate_slab() when the need for a
fallback to a lower order is detected after the first call to alloc_slab_page().

Re: [PATCH] slub: Clear __GFP_COMP flag when allocating 0 order page

[Vlastimil Babka]

On 4/11/24 6:51 PM, Christoph Lameter (Ampere) wrote:
> On Thu, 11 Apr 2024, Haifeng Xu wrote:
> 
>> @@ -1875,6 +1875,13 @@ static inline struct slab *alloc_slab_page(gfp_t flags, int node,
>> 	struct slab *slab;
>> 	unsigned int order = oo_order(oo);
>>
>> +	/*
>> +	 * If fallback to the minimum order allocation and the order is 0,
>> +	 * clear the __GFP_COMP flag.
>> +	 */
>> +	if (order == 0)
>> +		flags = flags & ~__GFP_COMP;
> 
> 
> This would be better placed in allocate_slab() when the need for a
> fallback to a lower order is detected after the first call to alloc_slab_page().

Yeah. Although I don't really see the harm of __GFP_COMP with order-0 in the
first place, if the only issue is that the error output might be confusing.
I'd also hope we should eventually get rid of those odd non-__GFP_COMP
high-order allocations and then can remove the flag.

Re: [PATCH] slub: Clear __GFP_COMP flag when allocating 0 order page

[Matthew Wilcox]

On Fri, Apr 12, 2024 at 10:01:29AM +0200, Vlastimil Babka wrote:
> On 4/11/24 6:51 PM, Christoph Lameter (Ampere) wrote:
> > On Thu, 11 Apr 2024, Haifeng Xu wrote:
> > 
> >> @@ -1875,6 +1875,13 @@ static inline struct slab *alloc_slab_page(gfp_t flags, int node,
> >> 	struct slab *slab;
> >> 	unsigned int order = oo_order(oo);
> >>
> >> +	/*
> >> +	 * If fallback to the minimum order allocation and the order is 0,
> >> +	 * clear the __GFP_COMP flag.
> >> +	 */
> >> +	if (order == 0)
> >> +		flags = flags & ~__GFP_COMP;
> > 
> > 
> > This would be better placed in allocate_slab() when the need for a
> > fallback to a lower order is detected after the first call to alloc_slab_page().
> 
> Yeah. Although I don't really see the harm of __GFP_COMP with order-0 in the
> first place, if the only issue is that the error output might be confusing.
> I'd also hope we should eventually get rid of those odd non-__GFP_COMP
> high-order allocations and then can remove the flag.

The patch seems pointless to me.  I wouldn't clear the flag.  If
somebody finds it confusing, that's really just their expectations being
wrong.  folio_alloc() sets __GFP_COMP on all allocations, whether or not
they're order 0.

Re: [PATCH] slub: Clear __GFP_COMP flag when allocating 0 order page

[Haifeng Xu]

On 2024/4/12 20:17, Matthew Wilcox wrote:
> On Fri, Apr 12, 2024 at 10:01:29AM +0200, Vlastimil Babka wrote:
>> On 4/11/24 6:51 PM, Christoph Lameter (Ampere) wrote:
>>> On Thu, 11 Apr 2024, Haifeng Xu wrote:
>>>
>>>> @@ -1875,6 +1875,13 @@ static inline struct slab *alloc_slab_page(gfp_t flags, int node,
>>>> 	struct slab *slab;
>>>> 	unsigned int order = oo_order(oo);
>>>>
>>>> +	/*
>>>> +	 * If fallback to the minimum order allocation and the order is 0,
>>>> +	 * clear the __GFP_COMP flag.
>>>> +	 */
>>>> +	if (order == 0)
>>>> +		flags = flags & ~__GFP_COMP;
>>>
>>>
>>> This would be better placed in allocate_slab() when the need for a
>>> fallback to a lower order is detected after the first call to alloc_slab_page().
>>
>> Yeah. Although I don't really see the harm of __GFP_COMP with order-0 in the
>> first place, if the only issue is that the error output might be confusing.
>> I'd also hope we should eventually get rid of those odd non-__GFP_COMP
>> high-order allocations and then can remove the flag.
> 
> The patch seems pointless to me.  I wouldn't clear the flag.  If
> somebody finds it confusing, that's really just their expectations being
> wrong.  folio_alloc() sets __GFP_COMP on all allocations, whether or not
> they're order 0.

If we don't care about the warnings at all, then higher-order and lower-order allocations can set
__GFP_COMP when creating a new slab, just like folio_alloc(). If so, there is no need to check 
the order in calculate_sizes() and we can set __GFP_COMP in kmem_cache by default.

diff --git a/mm/slub.c b/mm/slub.c
index e7bf1a1a31a8..49a3ebefab86 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -4461,9 +4461,7 @@ static int calculate_sizes(struct kmem_cache *s)
        if ((int)order < 0)
                return 0;

-       s->allocflags = 0;
-       if (order)
-               s->allocflags |= __GFP_COMP;
+       s->allocflags = __GFP_COMP;

        if (s->flags & SLAB_CACHE_DMA)
                s->allocflags |= GFP_DMA;

Re: [PATCH] slub: Clear __GFP_COMP flag when allocating 0 order page

[Matthew Wilcox]

On Fri, Apr 12, 2024 at 10:14:39PM +0800, Haifeng Xu wrote:
> diff --git a/mm/slub.c b/mm/slub.c
> index e7bf1a1a31a8..49a3ebefab86 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -4461,9 +4461,7 @@ static int calculate_sizes(struct kmem_cache *s)
>         if ((int)order < 0)
>                 return 0;
> 
> -       s->allocflags = 0;
> -       if (order)
> -               s->allocflags |= __GFP_COMP;
> +       s->allocflags = __GFP_COMP;
> 
>         if (s->flags & SLAB_CACHE_DMA)
>                 s->allocflags |= GFP_DMA;

Resend this with a proper changelog and you can add:

Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>

Re: [PATCH] slub: Clear __GFP_COMP flag when allocating 0 order page

[Haifeng Xu]

On 2024/4/12 00:51, Christoph Lameter (Ampere) wrote:
> On Thu, 11 Apr 2024, Haifeng Xu wrote:
> 
>> @@ -1875,6 +1875,13 @@ static inline struct slab *alloc_slab_page(gfp_t flags, int node,
>>     struct slab *slab;
>>     unsigned int order = oo_order(oo);
>>
>> +    /*
>> +     * If fallback to the minimum order allocation and the order is 0,
>> +     * clear the __GFP_COMP flag.
>> +     */
>> +    if (order == 0)
>> +        flags = flags & ~__GFP_COMP;
> 
> 
> This would be better placed in allocate_slab() when the need for a
> fallback to a lower order is detected after the first call to alloc_slab_page().

Yes. Thanks for your suggestion.