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

[Sergey Senozhatsky]

On (24/01/06 15:38), Barry Song wrote:
> 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.

Okay, makes sense.
Do you see these problems in real life? I don't recall any reports.

> > 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 it has now been replaced by vmalloc and makes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 :-)

Sure.

> > 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.

Oh, those are not related. TLB shootdowns are what now will be added to
all compressions/decompressions, so it's sort of extra cost.
HW compression (which android may be doing?) is another story.

Did you run any perf tests on zram w/ and w/o the patch?

> 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

Right.

> 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/

Nice.