Re: [PATCH V2] mm/gup: Clear the LRU flag of a page before adding to LRU batch

[Ge Yang <yangge1116@126.com>]

在 2024/7/3 20:02, David Hildenbrand 写道:
> On 22.06.24 08:48, yangge1116@126.com wrote:
>> From: yangge <yangge1116@126.com>
>>
>> If a large number of CMA memory are configured in system (for example, 
>> the
>> CMA memory accounts for 50% of the system memory), starting a virtual
>> virtual machine, it will call pin_user_pages_remote(..., FOLL_LONGTERM,
>> ...) to pin memory.  Normally if a page is present and in CMA area,
>> pin_user_pages_remote() will migrate the page from CMA area to non-CMA
>> area because of FOLL_LONGTERM flag. But the current code will cause the
>> migration failure due to unexpected page refcounts, and eventually cause
>> the virtual machine fail to start.
>>
>> If a page is added in LRU batch, its refcount increases one, remove the
>> page from LRU batch decreases one. Page migration requires the page is 
>> not
>> referenced by others except page mapping. Before migrating a page, we
>> should try to drain the page from LRU batch in case the page is in it,
>> however, folio_test_lru() is not sufficient to tell whether the page is
>> in LRU batch or not, if the page is in LRU batch, the migration will 
>> fail.
>>
>> To solve the problem above, we modify the logic of adding to LRU batch.
>> Before adding a page to LRU batch, we clear the LRU flag of the page so
>> that we can check whether the page is in LRU batch by 
>> folio_test_lru(page).
>> Seems making the LRU flag of the page invisible a long time is no 
>> problem,
>> because a new page is allocated from buddy and added to the lru batch,
>> its LRU flag is also not visible for a long time.
>>
> 
> I think we need to describe the impact of this change in a better way. 
> This example here is certainly interesting, but if pages are new they 
> are also not candidate for immediate reclaim (tail of the LRU).
> 
> The positive thing is that we can more reliably identify pages that are 
> on an LRU batch.
> 
> Further, a page can now only be on exactly one LRU batch.
> 
> But, as long as a page is on a LRU batch, we cannot isolate it, and we 
> cannot check if it's an LRU page. The latter can currently already 
> happen for a shorter time when moving LRU pages, and temporarily 
> clearing the flag.
> 
> I shared some examples where we don't care, because we'd check for 
> additional folio references either way (and the one from the LRU batch).
> 
> But I think we have to identify if there are any LRU folio/page checks 
> that could now be impacted "more". At least we should document it 
> properly to better understand the possible impact (do we maybe have to 
> flush more often?).
> 

Thanks.
I have reviewed a lot of paths using LRU folio/page checks and haven't 
seen more impact. I will documnt possible impact in next version, thanks.