linux-kernel - Re: [PATCH 07/32] mm: Bring back vmalloc

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Date:   Tue, 20 Jun 2023 15:43:02 -0700
From:   Nadav Amit <nadav.amit@...il.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        Kent Overstreet <kent.overstreet@...ux.dev>,
        Mark Rutland <mark.rutland@....com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org,
        "linux-bcachefs@...r.kernel.org" <linux-bcachefs@...r.kernel.org>,
        Kent Overstreet <kent.overstreet@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Uladzislau Rezki <urezki@...il.com>,
        "hch@...radead.org" <hch@...radead.org>,
        linux-mm <linux-mm@...ck.org>, Kees Cook <keescook@...omium.org>,
        the arch/x86 maintainers <x86@...nel.org>
Subject: Re: [PATCH 07/32] mm: Bring back vmalloc_exec


> On Jun 20, 2023, at 3:32 PM, Andy Lutomirski <luto@...nel.org> wrote:
> 
>> // out needs to be zeroed first
>> void unpack(struct uncompressed *out, const u64 *in, const struct 
>> bitblock *blocks, int nblocks)
>> {
>>    u64 *out_as_words = (u64*)out;
>>    for (int i = 0; i < nblocks; i++) {
>>        const struct bitblock *b;
>>        out_as_words[b->target] |= (in[b->source] & b->mask) << 
>> b->shift;
>>    }
>> }
>> 
>> void apply_offsets(struct uncompressed *out, const struct uncompressed *offsets)
>> {
>>    out->a += offsets->a;
>>    out->b += offsets->b;
>>    out->c += offsets->c;
>>    out->d += offsets->d;
>>    out->e += offsets->e;
>>    out->f += offsets->f;
>> }
>> 
>> Which generates nice code: https://godbolt.org/z/3fEq37hf5
> 
> Thinking about this a bit more, I think the only real performance issue with my code is that it does 12 read-xor-write operations in memory, which all depend on each other in horrible ways.

If you compare the generated code, just notice that you forgot to initialize b in unpack() in this version.

I presume you wanted it to say "b = &blocks[i]”.