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Date: Thu, 23 May 2019 11:19:53 -0700
From: Ralph Campbell <rcampbell@...dia.com>
To: Jason Gunthorpe <jgg@...pe.ca>
CC: <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
John Hubbard <jhubbard@...dia.com>,
Ira Weiny <ira.weiny@...el.com>,
Dan Williams <dan.j.williams@...el.com>,
Arnd Bergmann <arnd@...db.de>,
Balbir Singh <bsingharora@...il.com>,
Dan Carpenter <dan.carpenter@...cle.com>,
Matthew Wilcox <willy@...radead.org>,
Souptick Joarder <jrdr.linux@...il.com>,
Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 3/5] mm/hmm: Use mm_get_hmm() in hmm_range_register()
On 5/23/19 5:51 AM, Jason Gunthorpe wrote:
> On Mon, May 06, 2019 at 04:29:40PM -0700, rcampbell@...dia.com wrote:
>> From: Ralph Campbell <rcampbell@...dia.com>
>>
>> In hmm_range_register(), the call to hmm_get_or_create() implies that
>> hmm_range_register() could be called before hmm_mirror_register() when
>> in fact, that would violate the HMM API.
>>
>> Use mm_get_hmm() instead of hmm_get_or_create() to get the HMM structure.
>>
>> Signed-off-by: Ralph Campbell <rcampbell@...dia.com>
>> Cc: John Hubbard <jhubbard@...dia.com>
>> Cc: Ira Weiny <ira.weiny@...el.com>
>> Cc: Dan Williams <dan.j.williams@...el.com>
>> Cc: Arnd Bergmann <arnd@...db.de>
>> Cc: Balbir Singh <bsingharora@...il.com>
>> Cc: Dan Carpenter <dan.carpenter@...cle.com>
>> Cc: Matthew Wilcox <willy@...radead.org>
>> Cc: Souptick Joarder <jrdr.linux@...il.com>
>> Cc: Andrew Morton <akpm@...ux-foundation.org>
>> mm/hmm.c | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/mm/hmm.c b/mm/hmm.c
>> index f6c4c8633db9..2aa75dbed04a 100644
>> +++ b/mm/hmm.c
>> @@ -936,7 +936,7 @@ int hmm_range_register(struct hmm_range *range,
>> range->start = start;
>> range->end = end;
>>
>> - range->hmm = hmm_get_or_create(mm);
>> + range->hmm = mm_get_hmm(mm);
>> if (!range->hmm)
>> return -EFAULT;
>
> I looked for documentation saying that hmm_range_register should only
> be done inside a hmm_mirror_register and didn't see it. Did I miss it?
Yes, hmm_mirror_register() has to be called before hmm_range_register().
Look at Documentation/vm/hmm.rst for "Address space mirroring
implementation and API".
> Can you add a comment?
Sure, although I think your idea to pass hmm_mirror to
hmm_range_register() makes sense and makes it clear.
> It is really good to fix this because it means we can rely on mmap sem
> to manage mm->hmm!
>
> If this is true then I also think we should change the signature of
> the function to make this dependency relationship clear, and remove
> some possible confusing edge cases.
>
> What do you think about something like this? (unfinished)
>
> commit 29098bd59cf481ad1915db40aefc8435dabb8b28
> Author: Jason Gunthorpe <jgg@...lanox.com>
> Date: Thu May 23 09:41:19 2019 -0300
>
> mm/hmm: Use hmm_mirror not mm as an argument for hmm_register_range
>
> Ralf observes that hmm_register_range() can only be called by a driver
> while a mirror is registered. Make this clear in the API by passing
> in the mirror structure as a parameter.
>
> This also simplifies understanding the lifetime model for struct hmm,
> as the hmm pointer must be valid as part of a registered mirror
> so all we need in hmm_register_range() is a simple kref_get.
>
> Suggested-by: Ralph Campbell <rcampbell@...dia.com>
> Signed-off-by: Jason Gunthorpe <jgg@...lanox.com>
>
> diff --git a/include/linux/hmm.h b/include/linux/hmm.h
> index 8b91c90d3b88cb..87d29e085a69f7 100644
> --- a/include/linux/hmm.h
> +++ b/include/linux/hmm.h
> @@ -503,7 +503,7 @@ static inline bool hmm_mirror_mm_is_alive(struct hmm_mirror *mirror)
> * Please see Documentation/vm/hmm.rst for how to use the range API.
> */
> int hmm_range_register(struct hmm_range *range,
> - struct mm_struct *mm,
> + struct hmm_mirror *mirror,
> unsigned long start,
> unsigned long end,
> unsigned page_shift);
> @@ -539,7 +539,8 @@ static inline bool hmm_vma_range_done(struct hmm_range *range)
> }
>
> /* This is a temporary helper to avoid merge conflict between trees. */
> -static inline int hmm_vma_fault(struct hmm_range *range, bool block)
> +static inline int hmm_vma_fault(struct hmm_mirror *mirror,
> + struct hmm_range *range, bool block)
> {
> long ret;
>
> @@ -552,7 +553,7 @@ static inline int hmm_vma_fault(struct hmm_range *range, bool block)
> range->default_flags = 0;
> range->pfn_flags_mask = -1UL;
>
> - ret = hmm_range_register(range, range->vma->vm_mm,
> + ret = hmm_range_register(range, mirror,
> range->start, range->end,
> PAGE_SHIFT);
> if (ret)
> diff --git a/mm/hmm.c b/mm/hmm.c
> index 824e7e160d8167..fa1b04fcfc2549 100644
> --- a/mm/hmm.c
> +++ b/mm/hmm.c
> @@ -927,7 +927,7 @@ static void hmm_pfns_clear(struct hmm_range *range,
> * Track updates to the CPU page table see include/linux/hmm.h
> */
> int hmm_range_register(struct hmm_range *range,
> - struct mm_struct *mm,
> + struct hmm_mirror *mirror,
> unsigned long start,
> unsigned long end,
> unsigned page_shift)
> @@ -935,7 +935,6 @@ int hmm_range_register(struct hmm_range *range,
> unsigned long mask = ((1UL << page_shift) - 1UL);
>
> range->valid = false;
> - range->hmm = NULL;
>
> if ((start & mask) || (end & mask))
> return -EINVAL;
> @@ -946,15 +945,12 @@ int hmm_range_register(struct hmm_range *range,
> range->start = start;
> range->end = end;
>
> - range->hmm = hmm_get_or_create(mm);
> - if (!range->hmm)
> - return -EFAULT;
> -
> /* Check if hmm_mm_destroy() was call. */
> - if (range->hmm->mm == NULL || range->hmm->dead) {
> - hmm_put(range->hmm);
> + if (mirror->hmm->mm == NULL || mirror->hmm->dead)
> return -EFAULT;
> - }
> +
> + range->hmm = mirror->hmm;
> + kref_get(&range->hmm->kref);
>
> /* Initialize range to track CPU page table update */
> mutex_lock(&range->hmm->lock);
>
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