Subject: Re: [PATCH 09/13] gup: Don't allow FOLL_LONGTERM pinning of FS DAX pages

From: Alistair Popple <[email-masked]>
Date: Tue, 02 Jul 2024 09:47:03 +1000

David Hildenbrand <[email-masked]> writes:

> On 27.06.24 02:54, Alistair Popple wrote:
>> Longterm pinning of FS DAX pages should already be disallowed by
>> various pXX_devmap checks. However a future change will cause these
>> checks to be invalid for FS DAX pages so make
>> folio_is_longterm_pinnable() return false for FS DAX pages.
>> Signed-off-by: Alistair Popple <[email-masked]>
>> ---
>>   include/linux/memremap.h | 11 +++++++++++
>>   include/linux/mm.h       |  4 ++++
>>   2 files changed, 15 insertions(+)
>> diff --git a/include/linux/memremap.h b/include/linux/memremap.h
>> index 6505713..19a448e 100644
>> --- a/include/linux/memremap.h
>> +++ b/include/linux/memremap.h
>> @@ -193,6 +193,17 @@ static inline bool folio_is_device_coherent(const struct folio *folio)
>>   	return is_device_coherent_page(&folio->page);
>>   }
>>   +static inline bool is_device_dax_page(const struct page *page)
>> +{
>> +	return is_zone_device_page(page) &&
>> +		page_dev_pagemap(page)->type == MEMORY_DEVICE_FS_DAX;
>> +}
>> +
>> +static inline bool folio_is_device_dax(const struct folio *folio)
>> +{
>> +	return is_device_dax_page(&folio->page);
>> +}
>> +
>>   #ifdef CONFIG_ZONE_DEVICE
>>   void zone_device_page_init(struct page *page);
>>   void *memremap_pages(struct dev_pagemap *pgmap, int nid);
>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>> index b84368b..4d1cdea 100644
>> --- a/include/linux/mm.h
>> +++ b/include/linux/mm.h
>> @@ -2032,6 +2032,10 @@ static inline bool folio_is_longterm_pinnable(struct folio *folio)
>>   	if (folio_is_device_coherent(folio))
>>   		return false;
>>   +	/* DAX must also always allow eviction. */
>> +	if (folio_is_device_dax(folio))
>> +		return false;
>> +
>>   	/* Otherwise, non-movable zone folios can be pinned. */
>>   	return !folio_is_zone_movable(folio);
>>   
>
> Why is the check in check_vma_flags() insufficient? GUP-fast maybe?

Right. This came up when I was changing the code for GUP-fast, but also
they shouldn't be longterm pinnable and adding the case to
folio_is_longterm_pinnable() is an excellent way of documenting that.