linux-kernel - Re: [PATCH v5 02/12] crypto: acomp - Define new interfaces for compress/decompress batching.

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Message-ID: <CAGsJ_4yAX8QbCBe+zfNj1i8USZGLXOomB7pFzm5F_+aQ=ukyAQ@mail.gmail.com>
Date: Sat, 22 Feb 2025 21:21:00 +1300
From: Barry Song <21cnbao@...il.com>
To: Herbert Xu <herbert@...dor.apana.org.au>
Cc: Yosry Ahmed <yosry.ahmed@...ux.dev>, Minchan Kim <minchan@...nel.org>, 
	Sergey Senozhatsky <senozhatsky@...omium.org>, 
	"Sridhar, Kanchana P" <kanchana.p.sridhar@...el.com>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "linux-mm@...ck.org" <linux-mm@...ck.org>, 
	"hannes@...xchg.org" <hannes@...xchg.org>, "nphamcs@...il.com" <nphamcs@...il.com>, 
	"chengming.zhou@...ux.dev" <chengming.zhou@...ux.dev>, 
	"usamaarif642@...il.com" <usamaarif642@...il.com>, "ryan.roberts@....com" <ryan.roberts@....com>, 
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>, 
	"linux-crypto@...r.kernel.org" <linux-crypto@...r.kernel.org>, "davem@...emloft.net" <davem@...emloft.net>, 
	"clabbe@...libre.com" <clabbe@...libre.com>, "ardb@...nel.org" <ardb@...nel.org>, 
	"ebiggers@...gle.com" <ebiggers@...gle.com>, "surenb@...gle.com" <surenb@...gle.com>, 
	"Accardi, Kristen C" <kristen.c.accardi@...el.com>, 
	"Feghali, Wajdi K" <wajdi.k.feghali@...el.com>, "Gopal, Vinodh" <vinodh.gopal@...el.com>
Subject: Re: [PATCH v5 02/12] crypto: acomp - Define new interfaces for
 compress/decompress batching.

On Sat, Feb 22, 2025 at 8:23 PM Herbert Xu <herbert@...dor.apana.org.au> wrote:
>
> On Sat, Feb 22, 2025 at 08:13:13PM +1300, Barry Song wrote:
> >
> > Somehow, I find your comment reasonable. Another point I want
> > to mention is the semantic difference. For example, in a system
> > with only one algorithm, a dst_buf overflow still means a successful
> > swap-out. However, other errors actually indicate an I/O failure.
> > In such cases, vmscan.c will log the relevant error in pageout() to
> > notify the user.
>
> I'm talking specifically about the error from the Crypto API,
> not any other error.  So if you werer using some sort of an
> offload device to do the compression, that could indeed fail
> due to an IO error (perhaps the PCI bus is on fire :)
>
> But because that's reported through the Crypto API, it should
> not be treated any differently than an incompressible page,
> except for reporting purposes.

I'm referring more to the mm subsystem :-)

Let me provide a concrete example. Below is a small program that will
swap out 16MB of memory to zRAM:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/mman.h>
#include <unistd.h>

#define MB (1024 * 1024)
#define SIZE (16 * MB)

int main() {
    void *addr = mmap(NULL, SIZE, PROT_READ | PROT_WRITE, MAP_ANON |
MAP_PRIVATE, -1, 0);
    if (addr == MAP_FAILED) {
        perror("mmap failed");
        return 1;
    }

    for (size_t i = 0; i < SIZE / sizeof(int); i++) {
        ((int*)addr)[i] = rand();
    }

    if (madvise(addr, SIZE, MADV_PAGEOUT) != 0) {
        perror("madvise failed");
        return 1;
    }

    while (1);

    return 0;
}

For errors other than dst_buf overflow, we receive:

/ # ./a.out &
/ # free
               total        used        free      shared  buff/cache   available
Mem:          341228       77036      251872           0       20600      264192
Swap:        2703356           0     2703356
[1]+  Done                       ./a.out

/ # cat /proc/vmstat | grep swp
pswpin 0
pswpout 0
...

No memory has been swapped out, the swap-out counter is zero, and
the swap file is not used at all.

If this is an incompressible page(I mean dst_buf overflow error), there is
no actual issue, and we get the following:

/ #
/ # free
               total        used        free      shared  buff/cache   available
Mem:          341228       92948      236248           0       20372      248280
Swap:        2703356       16384     2686972

/ # cat /proc/vmstat | grep swp
pswpin 0
pswpout 4096
...


>
> Cheers,
> --
> Email: Herbert Xu <herbert@...dor.apana.org.au>
> Home Page: http://gondor.apana.org.au/~herbert/
> PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
>

Thanks
Barry