Re: [PATCH 1/5] mm: Do not reclaim private data from pinned page

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On 2/14/23 05:06, Jan Kara wrote:
> On Mon 13-02-23 01:55:04, Christoph Hellwig wrote:
>> I think we need to distinguish between short- and long terms pins.
>> For short term pins like direct I/O it doesn't make sense to take them
>> off the lru, or to do any other special action.  Writeback will simplify
>> have to wait for the short term pin.
>>
>> Long-term pins absolutely would make sense to be taken off the LRU list.
> 
> Yeah, I agree distinguishing these two would be nice as we could treat them
> differently then. The trouble is a bit with always-crowded struct page. But
> now it occurred to me that if we are going to take these long-term pinned
> pages out from the LRU, we could overload the space for LRU pointers with
> the counter (which is what I think John originally did). So yes, possibly
> we could track separately long-term and short-term pins. John, what do you
> think? Maybe time to revive your patches from 2018 in a bit different form?
> ;)
> 

Oh wow, I really love this idea. We kept running into problems because
long- and short-term pins were mixed up together (except during
creation), and this, at long last, separates them. Very nice. I'd almost
forgotten about the 2018 page.lru adventures, too. ha :)

One objection might be that pinning is now going to be taking a lot of
space in struct page / folio, but I think it's warranted, based on the
long-standing, difficult problems that it would solve.

We could even leave most of these patches, and David Howells' patches,
intact, by using an approach similar to the mm_users and mm_count
technique: maintain a long-term pin count in one of the folio->lru
fields, and any non-zero count there creates a single count in
folio->_pincount.

I could put together something to do that.


thanks,
-- 
John Hubbard
NVIDIA




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