Re: [PATCH] zram: easy the allocation of zcomp_strm's buffers with 2 pages

[Barry Song]

On Sat, Jan 6, 2024 at 9:30 AM Sergey Senozhatsky
<senozhatsky@xxxxxxxxxxxx> wrote:
>
> On (24/01/03 13:30), Barry Song wrote:
> > There is no need to keep zcomp_strm's buffers contiguous physically.
> > And rarely, 1-order allocation can fail while buddy is seriously
> > fragmented.
>
> Dunno. Some of these don't sound like convincing reasons, I'm afraid.
> We don't allocate compression streams all the time, we do it once
> per-CPU. And if the system is under such a terrible memory pressure

We actually do it many times actually because we free it while unplugging and
re-allocate it during hotplugging. this can happen quite often for systems like
Android using hotplug for power management.

> then one probably should not use zram at all, because zsmalloc needs
> pages for its pool.

In my humble opinion, 1-order allocation and 0-order allocation are different
things, 1-order is still more difficult though it is easier than
2-order which was
a big pain causing allocation latency for tasks' kernel stacks and negatively
affecting user experience. it has now been replaced by vmalloc and makes
life easier :-)

>
> I also wonder whether Android uses HW compression, in which case we
> may need to have physically contig pages. Not to mention TLB shootdowns
> that virt contig pages add to the picture.

I don't understand how HW compression and TLB shootdown are related as zRAM
is using a traditional comp API.
We are always passing a virtual address, traditional HW drivers use their own
buffers to do DMA.

int crypto_comp_compress(struct crypto_comp *comp,
const u8 *src, unsigned int slen,
u8 *dst, unsigned int *dlen);
int crypto_comp_decompress(struct crypto_comp *comp,
  const u8 *src, unsigned int slen,
  u8 *dst, unsigned int *dlen);

In new acomp API, we are passing a sg - users' buffers to drivers directly,
sg_init_one(&input, src, entry->length);
sg_init_table(&output, 1);
sg_set_page(&output, page, PAGE_SIZE, 0);
acomp_request_set_params(acomp_ctx->req, &input, &output, entry->length, dlen);
ret = crypto_wait_req(crypto_acomp_decompress(acomp_ctx->req),
&acomp_ctx->wait);

but i agree one-nents sg might have some advantage in scompress case
after we move
to new acomp APIs if we have this patch I sent recently [patch 3/3],
https://lore.kernel.org/linux-mm/20240103095006.608744-1-21cnbao@xxxxxxxxx/

For the current zRAM code, I guess HW compression/TLB is not a concern.

>
> [..]
> > @@ -37,7 +38,7 @@ static void zcomp_strm_free(struct zcomp_strm *zstrm)
> >  {
> >       if (!IS_ERR_OR_NULL(zstrm->tfm))
> >               crypto_free_comp(zstrm->tfm);
> > -     free_pages((unsigned long)zstrm->buffer, 1);
> > +     vfree(zstrm->buffer);
> >       zstrm->tfm = NULL;
> >       zstrm->buffer = NULL;
> >  }
> > @@ -53,7 +54,7 @@ static int zcomp_strm_init(struct zcomp_strm *zstrm, struct zcomp *comp)
> >        * allocate 2 pages. 1 for compressed data, plus 1 extra for the
> >        * case when compressed size is larger than the original one
> >        */
> > -     zstrm->buffer = (void *)__get_free_pages(GFP_KERNEL | __GFP_ZERO, 1);
> > +     zstrm->buffer = vzalloc(2 * PAGE_SIZE);
> >       if (IS_ERR_OR_NULL(zstrm->tfm) || !zstrm->buffer) {
> >               zcomp_strm_free(zstrm);
> >               return -ENOMEM;

Thanks
Barry