Re: [PATCH v5 7/8] mm/shmem, swap: rework swap entry and index calculation for large swapin

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 2025/7/10 11:37, Kairui Song wrote:
> From: Kairui Song <kasong@tencent.com>
> 
> Instead of calculating the swap entry differently in different swapin
> paths, calculate it early before the swap cache lookup and use that
> for the lookup and later swapin. And after swapin have brought a folio,
> simply round it down against the size of the folio.
> 
> This is simple and effective enough to verify the swap value. A folio's
> swap entry is always aligned by its size. Any kind of parallel split or
> race is acceptable because the final shmem_add_to_page_cache ensures
> that all entries covered by the folio are correct, and thus there
> will be no data corruption.
> 
> This also prevents false positive cache lookup. If a shmem read
> request's index points to the middle of a large swap entry,
> previously, shmem will try the swap cache lookup using the large swap
> entry's starting value (which is the first sub swap entry of this
> large entry). This will lead to false positive lookup results if only
> the first few swap entries are cached but the actual requested swap
> entry pointed by the index is uncached. This is not a rare event,
> as swap readahead always tries to cache order 0 folios when possible.
> 
> And this shouldn't cause any increased repeated faults. Instead, no
> matter how the shmem mapping is split in parallel, as long as the
> mapping still contains the right entries, the swapin will succeed.
> 
> The final object size and stack usage are also reduced due to
> simplified code:
> 
> ./scripts/bloat-o-meter mm/shmem.o.old mm/shmem.o
> add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-233 (-233)
> Function                                     old     new   delta
> shmem_swapin_folio                          4040    3807    -233
> Total: Before=33152, After=32919, chg -0.70%
> 
> Stack usage (Before vs After):
> mm/shmem.c:2277:12:shmem_swapin_folio   264     static
> mm/shmem.c:2277:12:shmem_swapin_folio   256     static
> 
> And while at it, round down the index too if swap entry is round down.
> The index is used either for folio reallocation or confirming the
> mapping content. In either case, it should be aligned with the swap
> folio.
> 
> Signed-off-by: Kairui Song <kasong@tencent.com>

Overall, I don't see any obvious issues, but please give me some time to 
run some tests and then get back to you. Thanks.