Re: [PATCH v2] mm, zone_device: replace {get, put}_zone_device_page() with a single reference

[Dan Williams]

On Sat, Apr 29, 2017 at 7:18 AM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>
> * Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
>
>> Kirill points out that the calls to {get,put}_dev_pagemap() can be
>> removed from the mm fast path if we take a single get_dev_pagemap()
>> reference to signify that the page is alive and use the final put of the
>> page to drop that reference.
>>
>> This does require some care to make sure that any waits for the
>> percpu_ref to drop to zero occur *after* devm_memremap_page_release(),
>> since it now maintains its own elevated reference.
>>
>> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
>> Cc: JÃrÃme Glisse <jglisse@xxxxxxxxxx>
>> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
>> Reviewed-by: Logan Gunthorpe <logang@xxxxxxxxxxxx>
>> Suggested-by: Kirill Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
>> Tested-by: Kirill Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
>> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
>
> This changelog is lacking an explanation about how this solves the crashes you
> were seeing.
>

Kirill? It wasn't clear to me why the conversion to generic
get_user_pages_fast() caused the reference counts to be off.