Re: [PATCH 4/4] mm: swap: entirely map large folios found in swapcache

["Huang, Ying" <ying.huang@intel.com>]

Barry Song <21cnbao@gmail.com> writes:

> On Mon, Apr 8, 2024 at 7:20 PM Huang, Ying <ying.huang@intel.com> wrote:
>>
>> Barry Song <21cnbao@gmail.com> writes:
>>
>> > From: Chuanhua Han <hanchuanhua@oppo.com>
>> >
>> > When a large folio is found in the swapcache, the current implementation
>> > requires calling do_swap_page() nr_pages times, resulting in nr_pages
>> > page faults. This patch opts to map the entire large folio at once to
>> > minimize page faults. Additionally, redundant checks and early exits
>> > for ARM64 MTE restoring are removed.
>>
>> For large folios in reclaiming, it makes sense to restore all PTE
>> mappings to the large folio to reduce the number of page faults.
>>
>
> Indeed, this patch addresses the refault case first, much less controversial
> then :-)
>
>> But for large folios swapped in, I think that it's better to map one PTE
>> which triggers the page fault only.  Because this makes us get the
>> opportunity to trap the page accesses to the sub-pages of the large
>> folio that is swapped in ahead (kind of swap readahead).  Then we can
>> decide the order of large folio swapin based on the readahead window
>> information.  That is, we may need to check PageReadahead() to decide
>> whether to map all PTEs in the future.
>
> Another scenario occurs when a process opts to utilize large folios for
> swap_readahead. Subsequently, another process encounters the large
> folios introduced by the former process. In this case, would it be optimal
> to fully map them just like the refault case?

We only need to trap the first access to the readahead sub-page.  So, we
can map PTE for all sub-pages without PageReadahead().

IIUC, now readahead flag is per-folio, we may need to change it to
per-sub-page when needed.

--
Best Regards,
Huang, Ying