Re: [PATCH] mm/cma: add an API to enable/disable concurrent memory allocation for the CMA

Ge Yang
2025-02-09T10:49:28Z

在 2025/2/9 5:34, Barry Song 写道:
> On Sat, Feb 8, 2025 at 9:50 PM Ge Yang <yangge1116@126.com> wrote:
>>
>>
>>
>> 在 2025/1/28 17:58, Barry Song 写道:
>>> On Sat, Jan 25, 2025 at 12:21 AM <yangge1116@126.com> wrote:
>>>>
>>>> From: yangge <yangge1116@126.com>
>>>>
>>>> Commit 60a60e32cf91 ("Revert "mm/cma.c: remove redundant cma_mutex lock"")
>>>> simply reverts to the original method of using the cma_mutex to ensure
>>>> that alloc_contig_range() runs sequentially. This change was made to avoid
>>>> concurrency allocation failures. However, it can negatively impact
>>>> performance when concurrent allocation of CMA memory is required.
>>>
>>> Do we have some data?
>> Yes, I will add it in the next version, thanks.
>>>
>>>>
>>>> To address this issue, we could introduce an API for concurrency settings,
>>>> allowing users to decide whether their CMA can perform concurrent memory
>>>> allocations or not.
>>>
>>> Who is the intended user of cma_set_concurrency?
>> We have some drivers that use cma_set_concurrency(), but they have not
>> yet been merged into the mainline. The cma_alloc_mem() function in the
>> mainline also supports concurrent allocation of CMA memory. By applying
>> this patch, we can also achieve significant performance improvements in
>> certain scenarios. I will provide performance data in the next version.
>> I also feel it is somewhat
>>> unsafe since cma->concurr_alloc is not protected by any locks.
>> Ok, thanks.
>>>
>>> Will a user setting cma->concurr_alloc = 1 encounter the original issue that
>>> commit 60a60e32cf91 was attempting to fix?
>>>
>> Yes, if a user encounters the issue described in commit 60a60e32cf91,
>> they will not be able to set cma->concurr_alloc to 1.
>
> A user who hasn't encountered a problem yet doesn't mean they won't
> encounter it; it most likely just means the testing time hasn't been long
> enough.
>
> Is it possible to implement a per-CMA lock or range lock that simultaneously
> improves performance and prevents the original issue that commit
> 60a60e32cf91 aimed to fix?
>
Using per-CMA locks can improve performance and prevent the original
issue. I am currently preparing the patch. Thanks.
> I strongly believe that cma->concurr_alloc is not the right approach. Let's
> not waste our time on this kind of hack or workaround. Instead, we should
> find a proper fix that remains transparent to users.
>
>>>>
>>>> Fixes: 60a60e32cf91 ("Revert "mm/cma.c: remove redundant cma_mutex lock"")
>>>> Signed-off-by: yangge <yangge1116@126.com>
>>>> Cc: <stable@vger.kernel.org>
>>>> ---
>>>> include/linux/cma.h | 2 ++
>>>> mm/cma.c | 22 ++++++++++++++++++++--
>>>> mm/cma.h | 1 +
>>>> 3 files changed, 23 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/include/linux/cma.h b/include/linux/cma.h
>>>> index d15b64f..2384624 100644
>>>> --- a/include/linux/cma.h
>>>> +++ b/include/linux/cma.h
>>>> @@ -53,6 +53,8 @@ extern int cma_for_each_area(int (*it)(struct cma *cma, void *data), void *data)
>>>>
>>>> extern void cma_reserve_pages_on_error(struct cma *cma);
>>>>
>>>> +extern bool cma_set_concurrency(struct cma *cma, bool concurrency);
>>>> +
>>>> #ifdef CONFIG_CMA
>>>> struct folio *cma_alloc_folio(struct cma *cma, int order, gfp_t gfp);
>>>> bool cma_free_folio(struct cma *cma, const struct folio *folio);
>>>> diff --git a/mm/cma.c b/mm/cma.c
>>>> index de5bc0c..49a7186 100644
>>>> --- a/mm/cma.c
>>>> +++ b/mm/cma.c
>>>> @@ -460,9 +460,17 @@ static struct page *__cma_alloc(struct cma *cma, unsigned long count,
>>>> spin_unlock_irq(&cma->lock);
>>>>
>>>> pfn = cma->base_pfn + (bitmap_no << cma->order_per_bit);
>>>> - mutex_lock(&cma_mutex);
>>>> +
>>>> + /*
>>>> + * If the user sets the concurr_alloc of CMA to true, concurrent
>>>> + * memory allocation is allowed. If the user sets it to false or
>>>> + * does not set it, concurrent memory allocation is not allowed.
>>>> + */
>>>> + if (!cma->concurr_alloc)
>>>> + mutex_lock(&cma_mutex);
>>>> ret = alloc_contig_range(pfn, pfn + count, MIGRATE_CMA, gfp);
>>>> - mutex_unlock(&cma_mutex);
>>>> + if (!cma->concurr_alloc)
>>>> + mutex_unlock(&cma_mutex);
>>>> if (ret == 0) {
>>>> page = pfn_to_page(pfn);
>>>> break;
>>>> @@ -610,3 +618,13 @@ int cma_for_each_area(int (*it)(struct cma *cma, void *data), void *data)
>>>>
>>>> return 0;
>>>> }
>>>> +
>>>> +bool cma_set_concurrency(struct cma *cma, bool concurrency)
>>>> +{
>>>> + if (!cma)
>>>> + return false;
>>>> +
>>>> + cma->concurr_alloc = concurrency;
>>>> +
>>>> + return true;
>>>> +}
>>>> diff --git a/mm/cma.h b/mm/cma.h
>>>> index 8485ef8..30f489d 100644
>>>> --- a/mm/cma.h
>>>> +++ b/mm/cma.h
>>>> @@ -16,6 +16,7 @@ struct cma {
>>>> unsigned long *bitmap;
>>>> unsigned int order_per_bit; /* Order of pages represented by one bit */
>>>> spinlock_t lock;
>>>> + bool concurr_alloc;
>>>> #ifdef CONFIG_CMA_DEBUGFS
>>>> struct hlist_head mem_head;
>>>> spinlock_t mem_head_lock;
>>>> --
>>>> 2.7.4
>>>>
>>>>
>>>
>
> Thanks
> Barry