Re: [RFC PATCH 0/2] mm: filemap: add filemap_grab_folios

[Nikita Kalyazin <kalyazin@amazon.com>]

On 10/01/2025 17:01, David Hildenbrand wrote:
> On 10.01.25 16:46, Nikita Kalyazin wrote:
>> Based on David's suggestion for speeding up guest_memfd memory
>> population [1] made at the guest_memfd upstream call on 5 Dec 2024 [2],
>> this adds `filemap_grab_folios` that grabs multiple folios at a time.
>>
> 
> Hi,

Hi :)

> 
>> Motivation
>>
>> When profiling guest_memfd population and comparing the results with
>> population of anonymous memory via UFFDIO_COPY, I observed that the
>> former was up to 20% slower, mainly due to adding newly allocated pages
>> to the pagecache.  As far as I can see, the two main contributors to it
>> are pagecache locking and tree traversals needed for every folio.  The
>> RFC attempts to partially mitigate those by adding multiple folios at a
>> time to the pagecache.
>>
>> Testing
>>
>> With the change applied, I was able to observe a 10.3% (708 to 635 ms)
>> speedup in a selftest that populated 3GiB guest_memfd and a 9.5% (990 to
>> 904 ms) speedup when restoring a 3GiB guest_memfd VM snapshot using a
>> custom Firecracker version, both on Intel Ice Lake.
> 
> Does that mean that it's still 10% slower (based on the 20% above), or
> were the 20% from a different micro-benchmark?

Yes, it is still slower:
  - isolated/selftest: 2.3%
  - Firecracker setup: 8.9%

Not sure why the values are so different though.  I'll try to find an 
explanation.

>>
>> Limitations
>>
>> While `filemap_grab_folios` handles THP/large folios internally and
>> deals with reclaim artifacts in the pagecache (shadows), for simplicity
>> reasons, the RFC does not support those as it demonstrates the
>> optimisation applied to guest_memfd, which only uses small folios and
>> does not support reclaim at the moment.
> 
> It might be worth pointing out that, while support for larger folios is
> in the works, there will be scenarios where small folios are unavoidable
> in the future (mixture of shared and private memory).
> 
> How hard would it be to just naturally support large folios as well?

I don't think it's going to be impossible.  It's just one more dimension 
that needs to be handled.  `__filemap_add_folio` logic is already rather 
complex, and processing multiple folios while also splitting when 
necessary correctly looks substantially convoluted to me.  So my idea 
was to discuss/validate the multi-folio approach first before rolling 
the sleeves up.

> We do have memfd_pin_folios() that can deal with that and provides a
> slightly similar interface (struct folio **folios).
> 
> For reference, the interface is:
> 
> long memfd_pin_folios(struct file *memfd, loff_t start, loff_t end,
>                       struct folio **folios, unsigned int max_folios,
>                       pgoff_t *offset)
> 
> Maybe what you propose could even be used to further improve
> memfd_pin_folios() internally? However, it must do this FOLL_PIN thingy,
> so it must process each and every folio it processed.

Thanks for the pointer.  Yeah, I see what you mean.  I guess, it can 
potentially allocate/add folios in a batch and then pin them?  Although 
swap/readahead logic may make it more difficult to implement.

> -- 
> Cheers,
> 
> David / dhildenb