Re: [PATCH V3] mm/gup: Clear the LRU flag of a page before adding to LRU batch

[Ge Yang <yangge1116@126.com>]

在 2024/7/4 4:08, Andrew Morton 写道:
> On Wed,  3 Jul 2024 20:02:33 +0800 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 with device passthrough, 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.
>>
> 
> Thanks.
> 
> I'll add this to the mm-hotfixes branch for additional testing.  Please
> continue to work with David on the changelog enhancements.
> 
> In mm-hotfixes I'd expect to send it to Linus next week.  I could move
> it into mm-unstable (then mm-stable) for merging into 6.11-rc1.  This
> is for additional testing time - it will still be backported into
> earlier kernels.  We can do this with any patch.

Ok, thanks.