Re: [RFC 2/6] mm/migrate_pages: split unmap_and_move() to _unmap() and _move()

John Hubbard
2022-09-28T01:45:05Z

On 9/27/22 18:41, Huang, Ying wrote:
>>>> I also agree that we cannot make any rules such as "do not lock > 1 page
>>>> at the same time, elsewhere in the kernel", because it is already
>>>> happening, for example in page-writeback.c, which locks PAGEVEC_SIZE
>>>> (15) pages per batch [1].
>>
>> That's not really the case though. The inner loop of write_cache_page()
>> only ever locks one page at a time, either directly via the
>> unlock_page() on L2338 (those goto's are amazing) or indirectly via
>> (*writepage)() on L2359.
>>
>> So there's no deadlock potential there because unlocking any previously
>> locked page(s) doesn't depend on obtaining the lock for another page.
>> Unless I've missed something?
>
> Yes. This is my understanding too after checking ext4_writepage().
>

Yes, I missed the ".writepage() shall unlock the page" design point. Now
it seems much more reasonable and safer. :)

thanks,

--
John Hubbard
NVIDIA