Re: [PATCH 4/4] mm/gup: remove get_user_pages_locked()

[John Hubbard <jhubbard@nvidia.com>]

On 1/31/22 04:05, Jan Kara wrote:
> On Sun 30-01-22 21:17:52, John Hubbard wrote:
>> Unraveling the rat's nest set of APIs in mm/gup.c a bit more.
>> get_user_pages_locked() was not helping at all, so remove it.
>>
>> Also, lookup_node() has only a single caller, but it is still worth
>> having a clearer locking policy there. Changing it so that the caller
>> both takes and releases the mmap_lock, thus leaving lookup_node() with
>> the sole job of translating a virtual address into a numa node ID.
>>
>> Signed-off-by: John Hubbard <jhubbard@nvidia.com>
> 
> Well, the point of _locked() GUP variants is that we can unlock mmap_sem
> when reading a page from the disk during a page fault (hidden behind
> VM_FAULT_RETRY). So as such _locked() variants are about reducing mmap_sem
> latency rather than code readability.  In this particular case, I don't
> think using _locked() variant in lookup_node() is very beneficial
> (generally I would not expect to take a fault there) but at least a
> justification in the commit message should be different :).
> 
> 								Honza

I'll rewrite this commit description to cover this point properly.
Jason also suggested using gup-fast, which I like.
  thanks,
-- 
John Hubbard
NVIDIA