Re: [PATCH] mm/gup: don't check page lru flag before draining it

[David Hildenbrand <david@redhat.com>]

>> Some random thoughts about some folio_test_lru() users:
>>
>> mm/khugepaged.c: skips pages if !folio_test_lru(), but would fail skip
>> it either way if there is the unexpected reference from the LRU batch!
>>
>> mm/compaction.c: skips pages if !folio_test_lru(), but would fail skip
>> it either way if there is the unexpected reference from the LRU batch!
>>
>> mm/memory.c: would love to identify this case and to a lru_add_drain()
>> to free up that reference.
>>
>> mm/huge_memory.c: splitting with the additional reference will fail
>> already. Maybe we'd want to drain the LRU batch.
> 
> Agree.
> 
>>
>> mm/madvise.c: skips pages if !folio_test_lru(). I wonder what happens if
>> we have the same page twice in an LRU batch with different target goals ...
> 
> IIUC, LRU batch can ignore this folio since it's LRU flag is cleared by
> folio_isolate_lru(), then will call folios_put() to frop the reference.
> 

I think what's interesting to highlight in the current design is that a 
folio might end up in multiple LRU batches, and whatever the result will 
be is determined by the sequence of them getting flushed. Doesn't sound 
quite right but maybe there was a reason for it (which could just have 
been "simpler implementation").

> 
>> Some other users (there are not that many that don't use it for sanity
>> checks though) might likely be a bit different.

There are also some PageLRU checks, but not that many.

> 
> mm/page_isolation.c: fail to set pageblock migratetype to isolate if
> !folio_test_lru(), then alloc_contig_range_noprof() can be failed. But
> the original code could set pageblock migratetype to isolate, then
> calling drain_all_pages() in alloc_contig_range_noprof() to drop
> reference of the LRU batch.
> 
> mm/vmscan.c: will call lru_add_drain() before calling
> isolate_lru_folios(), so seems no impact.

lru_add_drain() will only drain the local CPU. So if the folio would be 
stuck on another CPU's LRU batch, right now we could isolate it. When 
processing that LRU batch while the folio is still isolated, it would 
currently simply skip the operation.

So right now we can call isolate_lru_folios() even if the folio is stuck 
on another CPU's LRU batch.

We cannot really reclaim the folio as long is it is in another CPU's LRU 
batch, though (unexpected reference).

> 
> BTW, we also need to look at the usage of folio_isolate_lru().

Yes.

> 
> It doesn’t seem to have major obstacles, but there are many details to
> analyze :)

Yes, we're only scratching the surface.

Having a way to identify "this folio is very likely some CPU's LRU 
batch"  could end up being quite valuable, because likely we don't want 
to blindly drain the LRU simply because there is some unexpected 
reference on a folio [as we would in this patch].

-- 
Cheers,

David / dhildenb