Re: [PATCH RFC 2/2] zram: support compression at the granularity of multi-pages

[Barry Song <21cnbao@gmail.com>]

On Thu, Nov 7, 2024 at 5:23 AM Usama Arif <usamaarif642@gmail.com> wrote:
>
>
>
> On 22/10/2024 00:28, Barry Song wrote:
> >> From: Tangquan Zheng <zhengtangquan@oppo.com>
> >>
> >> +static int zram_bvec_write_multi_pages(struct zram *zram, struct bio_vec *bvec,
> >> +                       u32 index, int offset, struct bio *bio)
> >> +{
> >> +    if (is_multi_pages_partial_io(bvec))
> >> +            return zram_bvec_write_multi_pages_partial(zram, bvec, index, offset, bio);
> >> +    return zram_write_page(zram, bvec->bv_page, index);
> >> +}
> >> +
>
> Hi Barry,
>
> I started reviewing this series just to get a better idea if we can do something
> similar for zswap. I haven't looked at zram code before so this might be a basic
> question:
> How would you end up in zram_bvec_write_multi_pages_partial if using zram for swap?

Hi Usama,

There’s a corner case where, for instance, a 32KiB mTHP is swapped
out. Then, if userspace
performs a MADV_DONTNEED on the 0~16KiB portion of this original mTHP,
it now consists
of 8 swap entries(mTHP has been released and unmapped). With
swap0-swap3 released
due to DONTNEED, they become available for reallocation, and other
folios may be swapped
out to those entries. Then it is a combination of the new smaller
folios with the original 32KiB
mTHP.

>
> We only swapout whole folios. If ZCOMP_MULTI_PAGES_SIZE=64K, any folio smaller
> than 64K will end up in zram_bio_write_page. Folios greater than or equal to 64K
> would be dispatched by zram_bio_write_multi_pages to zram_bvec_write_multi_pages
> in 64K chunks. So for e.g. 128K folio would end up calling zram_bvec_write_multi_pages
> twice.

In v2, I changed the default order to 2, allowing all anonymous mTHP
to benefit from this
feature.

>
> Or is this for the case when you are using zram not for swap? In that case, I probably
> dont need to consider zram_bvec_write_multi_pages_partial write case for zswap.
>
> Thanks,
> Usama

Thanks
barry