Re: [PATCH v4 0/7] selftests/cgroup: improve zswap tests robustness and support large page sizes

[Li Wang <liwang@redhat.com>]

On Tue, Mar 24, 2026 at 01:28:12PM -0700, Yosry Ahmed wrote:
> > > Sashiko claims that 512 pages will end up consuming 11K to 15K in
> > > zswap with this setup, do you know what the actual number is?
> >
> > Not very sure, I guess each 64K page contains 1 byte of 'a' and 65535 bytes
> > of zero. A single page like that compresses down to roughly 20–30 bytes
> > (a short literal plus a very long zero run, plus frame/header overhead).
> > So the estimate is roughly 512 × 25 bytes ≈ 12.8 KB, which is where the
> > "11 to 15 kilobytes" ballpark comes from.
> >
> > > Especially with different compressors? If it's close to 64K, this
> > > might be a problem.
> >
> > Yes, good point, when I swith to use 'zstd' compressor, it doesn't work.
> >
> > > Maybe we can fill half of each page with increasing values? It should
> > > still be compressible but not too compressible.
> >
> > I tried, this method works on Lzo algorithm but not Zstd.
> > Anyway, I am still investigating.
> 
> Do you mean the compressibility is still very high on zstd? I vaguely
> remember filling a page with repeating patterns (e.g. alphabet
> letters) seemed to produce a decent compression ratio, but I don't
> remember the specifics.
> 
> I am pretty sure an LLM could figure out what values will work for
> different compression algorithms :)

Well, I have tried many ways for ditry each page of the total, but none
works on zstd compreseor.

e.g,.

--- a/tools/testing/selftests/cgroup/test_zswap.c
+++ b/tools/testing/selftests/cgroup/test_zswap.c
@@ -9,6 +9,7 @@
 #include <string.h>
 #include <sys/wait.h>
 #include <sys/mman.h>
+#include <sys/random.h>

 #include "kselftest.h"
 #include "cgroup_util.h"
@@ -473,8 +474,12 @@ static int test_no_invasive_cgroup_shrink(const char *root)
        if (cg_enter_current(control_group))
                goto out;
        control_allocation = malloc(control_allocation_size);
-       for (int i = 0; i < control_allocation_size; i += page_size)
-               control_allocation[i] = (char)((i / page_size) + 1);
+       unsigned int nr_pages = control_allocation_size/page_size;
+       for (int i = 0; i < nr_pages; i++) {
+               unsigned long off = (unsigned long)i * page_size;
+               memset(&control_allocation[off], 0, page_size);
+               getrandom(&control_allocation[off], nr_pages/2, 0);
+       }
        if (cg_read_key_long(control_group, "memory.stat", "zswapped") < 1)
                goto out;

Even I tried to set random data for all of the pages, they still not
works (zstd). But it can be worked on lzo compressor, I don't know if
the zstd have any addtional configures or I missed anything there.

My current thought is just to satisfy the lzo (default) compressor in
this patch series, and leave the zstd for additional work.

What do you think? any better idea?

-- 
Regards,
Li Wang