linux-kernel - Re: [PATCH 1/1] vfio/spapr_tce: convert get_user_pages() --> pin_user

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Message-ID: <40a4e26f-b89b-a3db-6936-f94b9bb4fc9d@nvidia.com>
Date:   Tue, 26 May 2020 12:45:18 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Souptick Joarder <jrdr.linux@...il.com>
CC:     LKML <linux-kernel@...r.kernel.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Cornelia Huck <cohuck@...hat.com>, <kvm@...r.kernel.org>
Subject: Re: [PATCH 1/1] vfio/spapr_tce: convert get_user_pages() -->
 pin_user_pages()

On 2020-05-26 12:28, Souptick Joarder wrote:
>> @@ -486,7 +486,7 @@ static int tce_iommu_use_page(unsigned long tce, unsigned long *hpa)
>>          struct page *page = NULL;
>>          enum dma_data_direction direction = iommu_tce_direction(tce);
>>
>> -       if (get_user_pages_fast(tce & PAGE_MASK, 1,
>> +       if (pin_user_pages_fast(tce & PAGE_MASK, 1,
>>                          direction != DMA_TO_DEVICE ? FOLL_WRITE : 0,
>>                          &page) != 1)
>>                  return -EFAULT;
> 
> There are few places where nr_pages is passed as 1 to get_user_pages_fast().
> With similar conversion those will be changed to pin_user_pages_fast().
> 
> Does it make sense to add an inline like - pin_user_page_fast(), similar to
> get_user_page_fast_only() ( now merged in linux-next) ?
> 

Perhaps not *just* yet, IMHO. There are only two places so far: here, and
dax_lock_page(). And we don't expect that many places, either, because
pin_user_pages*(), unlike get_user_pages(), is more likely to operate on
a bunch of pages at once. Although, that could change if we look into the
remaining call sites and find more single-page cases that need a gup-to-pup
conversion.

get_user_pages*() has a few more situations (Case 4, in
Documentation/core-api/pin_user_pages.rst: struct page manipulation) in
which it operates on single pages. Those will remain get_user_pages*()
calls, or perhaps change to get_user_page().

thanks,
-- 
John Hubbard
NVIDIA