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

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

On Thu, Apr 11, 2024 at 1:42 PM Sergey Senozhatsky
<senozhatsky@chromium.org> wrote:
>
> On (24/03/28 10:48), Barry Song wrote:
> [..]
> > +/*
> > + * Use a temporary buffer to decompress the page, as the decompressor
> > + * always expects a full page for the output.
> > + */
> > +static int zram_bvec_read_multi_pages_partial(struct zram *zram, struct bio_vec *bvec,
> > +                               u32 index, int offset)
> > +{
> > +     struct page *page = alloc_pages(GFP_NOIO | __GFP_COMP, ZCOMP_MULTI_PAGES_ORDER);
> > +     int ret;
> > +
> > +     if (!page)
> > +             return -ENOMEM;
> > +     ret = zram_read_multi_pages(zram, page, index, NULL);
> > +     if (likely(!ret)) {
> > +             atomic64_inc(&zram->stats.zram_bio_read_multi_pages_partial_count);
> > +             void *dst = kmap_local_page(bvec->bv_page);
> > +             void *src = kmap_local_page(page);
> > +
> > +             memcpy(dst + bvec->bv_offset, src + offset, bvec->bv_len);
> > +             kunmap_local(src);
> > +             kunmap_local(dst);
> > +     }
> > +     __free_pages(page, ZCOMP_MULTI_PAGES_ORDER);
> > +     return ret;
> > +}
>
> [..]
>
> > +static int zram_bvec_write_multi_pages_partial(struct zram *zram, struct bio_vec *bvec,
> > +                                u32 index, int offset, struct bio *bio)
> > +{
> > +     struct page *page = alloc_pages(GFP_NOIO | __GFP_COMP, ZCOMP_MULTI_PAGES_ORDER);
> > +     int ret;
> > +     void *src, *dst;
> > +
> > +     if (!page)
> > +             return -ENOMEM;
> > +
> > +     ret = zram_read_multi_pages(zram, page, index, bio);
> > +     if (!ret) {
> > +             src = kmap_local_page(bvec->bv_page);
> > +             dst = kmap_local_page(page);
> > +             memcpy(dst + offset, src + bvec->bv_offset, bvec->bv_len);
> > +             kunmap_local(dst);
> > +             kunmap_local(src);
> > +
> > +             atomic64_inc(&zram->stats.zram_bio_write_multi_pages_partial_count);
> > +             ret = zram_write_page(zram, page, index);
> > +     }
> > +     __free_pages(page, ZCOMP_MULTI_PAGES_ORDER);
> > +     return ret;
> > +}
>
> What type of testing you run on it? How often do you see partial
> reads and writes? Because this looks concerning - zsmalloc memory
> usage reduction is one metrics, but this also can be achieved via
> recompression, writeback, or even a different compression algorithm,
> but higher CPU/power usage/higher requirements for physically contig
> pages cannot be offset easily. (Another corner case, assume we have
> partial read requests on every CPU simultaneously.)

This question brings up an interesting observation. In our actual product,
we've noticed a success rate of over 90% when allocating large folios in
do_swap_page, but occasionally, we encounter failures. In such cases,
instead of resorting to partial reads, we opt to allocate 16 small folios and
request zram to fill them all. This strategy effectively minimizes partial reads
to nearly zero. However, integrating this into the upstream codebase seems
like a considerable task, and for now, it remains part of our
out-of-tree code[1],
which is also open-source.
We're gradually sending patches for the swap-in process, systematically
cleaning up the product's code.

To enhance the success rate of large folio allocation, we've reserved some
page blocks for mTHP. This approach is currently absent from the mainline
codebase as well (Yu Zhao is trying to provide TAO [2]). Consequently, we
anticipate that partial reads may reach 50% or more until this method is
incorporated upstream.

[1] https://github.com/OnePlusOSS/android_kernel_oneplus_sm8550/tree/oneplus/sm8550_u_14.0.0_oneplus11
[2] https://lore.kernel.org/linux-mm/20240229183436.4110845-1-yuzhao@google.com/

Thanks
Barry