Re: [PATCH 01/19] mm/swap: rename __read_swap_cache_async to swap_cache_alloc_folio

[Kairui Song]

On Mon, Nov 3, 2025 at 4:28 PM Barry Song <baohua@xxxxxxxxxx> wrote:
> > > +/**
> > > + * swap_cache_alloc_folio - Allocate folio for swapped out slot in swap cache.
> > > + * @entry: the swapped out swap entry to be binded to the folio.
> > > + * @gfp_mask: memory allocation flags
> > > + * @mpol: NUMA memory allocation policy to be applied
> > > + * @ilx: NUMA interleave index, for use only when MPOL_INTERLEAVE
> > > + * @new_page_allocated: sets true if allocation happened, false otherwise
> > > + * @skip_if_exists: if the slot is a partially cached state, return NULL.
> > > + *                  This is a workaround that would be removed shortly.
> > > + *
> > > + * Allocate a folio in the swap cache for one swap slot, typically before
> > > + * doing IO (swap in or swap out). The swap slot indicated by @entry must
> > > + * have a non-zero swap count (swapped out). Currently only supports order 0.

Hi Yosry and Barry, thanks for the review.

> >
> > Is it used for swap in? That's confusing because the next sentence
> > mention that it needs to be already swapped out.

Yes, swap in is the typical user, swap_vma_readahead calls this
function directly, allocate a folio then initiate the swap in IO.

I'm not sure why it is confusing. A swapped out slot getting swapped
in seems a very common thing?

> >
> > I suspect you're referring to the zswap writeback use case, but in this
> > case we're still "swapping-in" the folio from zswap to swap it out to
> > disk. I'd avoid mentioning swap in here because it's confusing.

Oh, I thought the zswap writeback is considered a kind of swap out :),
since it's technically writing data from ram to swap device.

>
> I assume you mean avoiding any mention of swap-out? As for swap-out, we’re
> swapping a folio out from the LRU — we’re not allocating a new folio.
>
> BTW, this sentence also feels a bit odd to me. I’d prefer removing
> “swap out” from
> “doing IO (swap in or swap out)”.

How about "doing IO (e.g. swap in or zswap writeback)"? Swap-in is a
very common user, and zswap writeback can be mentioned explicitly.

>
> Thanks
> Barry