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SubjectRe: [PATCH 1/5] mm/hmm: Update HMM documentation
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On 6/6/19 7:02 AM, Jason Gunthorpe wrote:
> On Mon, May 06, 2019 at 04:29:38PM -0700, rcampbell@nvidia.com wrote:
>> From: Ralph Campbell <rcampbell@nvidia.com>
>>
>> Update the HMM documentation to reflect the latest API and make a few minor
>> wording changes.
>>
>> Signed-off-by: Ralph Campbell <rcampbell@nvidia.com>
>> Reviewed-by: Jérôme Glisse <jglisse@redhat.com>
>> Cc: John Hubbard <jhubbard@nvidia.com>
>> Cc: Ira Weiny <ira.weiny@intel.com>
>> Cc: Dan Williams <dan.j.williams@intel.com>
>> Cc: Arnd Bergmann <arnd@arndb.de>
>> Cc: Balbir Singh <bsingharora@gmail.com>
>> Cc: Dan Carpenter <dan.carpenter@oracle.com>
>> Cc: Matthew Wilcox <willy@infradead.org>
>> Cc: Souptick Joarder <jrdr.linux@gmail.com>
>> Cc: Andrew Morton <akpm@linux-foundation.org>
>> Documentation/vm/hmm.rst | 139 ++++++++++++++++++++-------------------
>> 1 file changed, 73 insertions(+), 66 deletions(-)
>
> Okay, lets start picking up hmm patches in to the new shared hmm.git,
> as promised I will take responsibility to send these to Linus. The
> tree is here:
>
> https://git.kernel.org/pub/scm/linux/kernel/git/rdma/rdma.git/log/?h=hmm
>
> This looks fine to me with one minor comment:
>
>> diff --git a/Documentation/vm/hmm.rst b/Documentation/vm/hmm.rst
>> index ec1efa32af3c..7c1e929931a0 100644
>> +++ b/Documentation/vm/hmm.rst
>>
>> @@ -151,21 +151,27 @@ registration of an hmm_mirror struct::
>>
>> int hmm_mirror_register(struct hmm_mirror *mirror,
>> struct mm_struct *mm);
>> - int hmm_mirror_register_locked(struct hmm_mirror *mirror,
>> - struct mm_struct *mm);
>>
>> -
>> -The locked variant is to be used when the driver is already holding mmap_sem
>> -of the mm in write mode. The mirror struct has a set of callbacks that are used
>> +The mirror struct has a set of callbacks that are used
>> to propagate CPU page tables::
>>
>> struct hmm_mirror_ops {
>> + /* release() - release hmm_mirror
>> + *
>> + * @mirror: pointer to struct hmm_mirror
>> + *
>> + * This is called when the mm_struct is being released.
>> + * The callback should make sure no references to the mirror occur
>> + * after the callback returns.
>> + */
>
> This is not quite accurate (at least, as the other series I sent
> intends), the struct hmm_mirror is valid up until
> hmm_mirror_unregister() is called - specifically it remains valid
> after the release() callback.
>
> I will revise it (and the hmm.h comment it came from) to read the
> below. Please let me know if you'd like something else:
>
> /* release() - release hmm_mirror
> *
> * @mirror: pointer to struct hmm_mirror
> *
> * This is called when the mm_struct is being released. The callback
> * must ensure that all access to any pages obtained from this mirror
> * is halted before the callback returns. All future access should
> * fault.
> */
>
> The key task for release is to fence off all device access to any
> related pages as the mm is about to recycle them and the device must
> not cause a use-after-free.
>
> I applied it to hmm.git
>
> Thanks,
> Jason
>

Yes, I agree this is better.

Also, I noticed the sample code for hmm_range_register() is wrong.
If you could merge this minor change into this patch, that
would be appreciated.

diff --git a/Documentation/vm/hmm.rst b/Documentation/vm/hmm.rst
index dc8fe4241a18..b5fb9bc02aa2 100644
--- a/Documentation/vm/hmm.rst
+++ b/Documentation/vm/hmm.rst
@@ -245,7 +245,7 @@ The usage pattern is::
hmm_range_wait_until_valid(&range, TIMEOUT_IN_MSEC);
goto again;
}
- hmm_mirror_unregister(&range);
+ hmm_range_unregister(&range);
return ret;
}
take_lock(driver->update);
@@ -257,7 +257,7 @@ The usage pattern is::

// Use pfns array content to update device page table

- hmm_mirror_unregister(&range);
+ hmm_range_unregister(&range);
release_lock(driver->update);
up_read(&mm->mmap_sem);
return 0;
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