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Message-ID: <CAKEwX=Nm08xZVR0Zk2fOvXBmeWe5wggvTXN6+hD84S7T9L=eBQ@mail.gmail.com>
Date: Thu, 7 Aug 2025 16:03:54 -0700
From: Nhat Pham <nphamcs@...il.com>
To: SeongJae Park <sj@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Chengming Zhou <chengming.zhou@...ux.dev>,
David Hildenbrand <david@...hat.com>, Johannes Weiner <hannes@...xchg.org>,
Yosry Ahmed <yosry.ahmed@...ux.dev>, kernel-team@...a.com, linux-kernel@...r.kernel.org,
linux-mm@...ck.org, Takero Funaki <flintglass@...il.com>
Subject: Re: [PATCH] mm/zswap: store <PAGE_SIZE compression failed page as-is
On Thu, Aug 7, 2025 at 11:16 AM SeongJae Park <sj@...nel.org> wrote:
>
> When zswap writeback is enabled and it fails compressing a given page,
> the page is swapped out to the backing swap device. This behavior
> breaks the zswap's writeback LRU order, and hence users can experience
> unexpected latency spikes. If the page is compressed without failure,
> but results in a size of PAGE_SIZE, the LRU order is kept, but the
> decompression overhead for loading the page back on the later access is
> unnecessary.
Maybe add the writeback disabled story - we added to get around this
latency issue, but that solution was not satisfactory either: wasting
cpu cycles retrying incompressible pages, especially when we're under
reclaim/memory pressure, and we've reclaimed most if not all other
sources.
This problem was pointed out by Yosry:
https://lore.kernel.org/all/CAJD7tkZXS-UJVAFfvxJ0nNgTzWBiqepPYA4hEozi01_qktkitg@mail.gmail.com/
>
> Keep the LRU order and optimize unnecessary decompression overheads in
> those cases, by storing the original content as-is in zpool. The length
> field of zswap_entry will be set appropriately, as PAGE_SIZE, Hence
> whether it is saved as-is or not (whether decompression is unnecessary)
> is identified by 'zswap_entry->length == PAGE_SIZE'.
>
> Because the uncompressed data is saved in zpool, same to the compressed
> ones, this introduces no change in terms of memory management including
> movability and migratability of involved pages.
>
> This change is also not increasing per zswap entry metadata overhead.
> But as the number of incompressible pages increases, total zswap
> metadata overhead is proportionally increased. The overhead should not
> be problematic in usual cases, since the zswap metadata for single zswap
> entry is much smaller than PAGE_SIZE, and in common zswap use cases
> there should be a sufficient amount of compressible pages. Also it can
> be mitigated by the zswap writeback.
>
> When the writeback is disabled, the additional overhead could be
> problematic. For the case, keep the current behavior that just returns
> the failure and let swap_writeout() put the page back to the active LRU
> list in the case. It is known to be suboptimal when the incompressible
> pages are cold, since the incompressible pages will continuously be
> tried to be zswapped out, and burn CPU cycles for compression attempts
> that will anyway fails. One imaginable solution for the problem is
> reusing the swapped-out page and its struct page to store in the zswap
> pool. But that's out of the scope of this patch.
>
> Tests
> -----
>
> I tested this patch using a simple self-written microbenchmark that is
> available at GitHub[1]. You can reproduce the test I did by executing
> run_tests.sh of the repo on your system. Note that the repo's
> documentation is not good as of this writing, so you may need to read
> and use the code.
>
> The basic test scenario is simple. Run a test program making artificial
> accesses to memory having artificial content under memory.high-set
> memory limit and measure how many accesses were made in given time.
>
> The test program repeatedly and randomly access three anonymous memory
> regions. The regions are all 500 MiB size, and accessed in the same
> probability. Two of those are filled up with a simple content that can
> easily be compressed, while the remaining one is filled up with a
> content that read from /dev/urandom, which is easy to fail at
> compressing to <PAGE_SIZE size. The program runs for two minutes and
> prints out the number of accesses made every five seconds.
>
> The test script runs the program under below seven configurations.
>
> - 0: memory.high is set to 2 GiB, zswap is disabled.
> - 1-1: memory.high is set to 1350 MiB, zswap is disabled.
> - 1-2: On 1-1, zswap is enabled without this patch.
> - 1-3: On 1-2, this patch is applied.
>
> For all zswap enabled case, zswap shrinker is enabled.
>
> Configuration '0' is for showing the original memory performance.
> Configurations 1-1, 1-2 and 1-3 are for showing the performance of swap,
> zswap, and this patch under a level of memory pressure (~10% of working
> set).
>
> Because the per-5 seconds performance is not very reliable, I measured
> the average of that for the last one minute period of the test program
> run. I also measured a few vmstat counters including zswpin, zswpout,
> zswpwb, pswpin and pswpout during the test runs.
>
> The measurement results are as below. To save space, I show performance
> numbers that are normalized to that of the configuration '0' (no memory
> pressure), only. The averaged accesses per 5 seconds of configuration
> '0' was 36493417.75.
>
> config 0 1-1 1-2 1-3
> perf_normalized 1.0000 0.0057 0.0235 0.0367
> perf_stdev_ratio 0.0582 0.0652 0.0167 0.0346
> zswpin 0 0 3548424 1999335
> zswpout 0 0 3588817 2361689
> zswpwb 0 0 10214 340270
> pswpin 0 485806 772038 340967
> pswpout 0 649543 144773 340270
>
> 'perf_normalized' is the performance metric, normalized to that of
> configuration '0' (no pressure). 'perf_stdev_ratio' is the standard
> deviation of the averaged data points, as a ratio to the averaged metric
> value. For example, configuration '0' performance was showing 5.8%
> stdev. Configurations 1-1 and 1-3 were having about 6.5% and 6.1%
> stdev. Also the results were highly variable between multiple runs. So
> this result is not very stable but just showing ball park figures.
> Please keep this in your mind when reading these results.
>
> Under about 10% of working set memory pressure, the performance was
> dropped to about 0.57% of no-pressure one, when the normal swap is used
> (1-1). Note that ~10% working set pressure is already extreme, at least
> on this test setup. No one would desire system setups that can degrade
> performance to 0.57% of the best case.
>
> By turning zswap on (1-2), the performance was improved about 4x,
> resulting in about 2.35% of no-pressure one. Because of the
> incompressible pages in the third memory region, a significant amount of
> (non-zswap) swap I/O operations were made, though.
>
> By applying this patch (1-3), about 56% performance improvement was
> made, resulting in about 3.67% of no-pressure one. Reduced pswpin of
> 1-3 compared to 1-2 let us see where this improvement came from.
Nice.
>
> Related Works
> -------------
>
> This is not an entirely new attempt. Nhat Pham and Takero Funaki tried
> very similar approaches in October 2023[2] and April 2024[3],
> respectively. The two approaches didn't get merged mainly due to the
> metadata overhead concern. I described why I think that shouldn't be a
> problem for this change, which is automatically disabled when writeback
> is disabled, at the beginning of this changelog.
>
> This patch is not particularly different from those, and actually built
> upon those. I wrote this from scratch again, though. Hence adding
> Suggested-by tags for them. Actually Nhat first suggested this to me
> offlist.
>
> [1] https://github.com/sjp38/eval_zswap/blob/master/run.sh
> [2] https://lore.kernel.org/20231017003519.1426574-3-nphamcs@gmail.com
> [3] https://lore.kernel.org/20240706022523.1104080-6-flintglass@gmail.com
>
> Suggested-by: Nhat Pham <nphamcs@...il.com>
> Suggested-by: Takero Funaki <flintglass@...il.com>
> Signed-off-by: SeongJae Park <sj@...nel.org>
> ---
> Changes from RFC v2
> (https://lore.kernel.org/20250805002954.1496-1-sj@kernel.org)
> - Fix race conditions at decompressed pages identification.
> - Remove the parameter and make saving as-is the default behavior.
> - Open-code main changes.
> - Clarify there is no memory management changes on the cover letter.
> - Remove 20% pressure case from test results, since it is arguably too
> extreme and only adds confusion.
> - Drop RFC tag.
>
> Changes from RFC v1
> (https://lore.kernel.org/20250730234059.4603-1-sj@kernel.org)
> - Consider PAGE_SIZE compression successes as failures.
> - Use zpool for storing incompressible pages.
> - Test with zswap shrinker enabled.
> - Wordsmith changelog and comments.
> - Add documentation of save_incompressible_pages parameter.
>
> mm/zswap.c | 29 +++++++++++++++++++++++++++--
> 1 file changed, 27 insertions(+), 2 deletions(-)
>
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 3c0fd8a13718..2db2da130ec4 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -976,8 +976,25 @@ static bool zswap_compress(struct page *page, struct zswap_entry *entry,
> */
> comp_ret = crypto_wait_req(crypto_acomp_compress(acomp_ctx->req), &acomp_ctx->wait);
> dlen = acomp_ctx->req->dlen;
> - if (comp_ret)
> - goto unlock;
> +
> + /*
> + * If a page cannot be compressed into a size smaller than PAGE_SIZE,
> + * save the content as is without a compression, to keep the LRU order
> + * of writebacks. If writeback is disabled, reject the page since it
> + * only adds metadata overhead. swap_writeout() will put the page back
> + * to the active LRU list in the case.
> + */
> + if (comp_ret || dlen >= PAGE_SIZE) {
> + if (mem_cgroup_zswap_writeback_enabled(
> + folio_memcg(page_folio(page)))) {
> + comp_ret = 0;
> + dlen = PAGE_SIZE;
> + memcpy_from_page(dst, page, 0, dlen);
I wonder if we can save one memcpy here. Would it be safe to map the page:
dst = kmap_local_page(page);
then, after we're done with storing (i.e after zpool_obj_write()), do:
if (dlen == PAGE_SIZE)
kunmap(dst);
(don't forget to unmap the page in the failure paths too!)
> + } else {
> + comp_ret = comp_ret ? : -EINVAL;
Does this keep the value of comp_ret if comp_ret != 0 lol. Syntax
looks weird to me.
> + goto unlock;
> + }
> + }
Also, can we fix the counter value?
I assume we want:
else if (comp_ret || dlen == PAGE_SIZE)
zswap_reject_compress_fail++;
or something like that.
And what happened to the incompressible page stored counter? :)
>
> zpool = pool->zpool;
> gfp = GFP_NOWAIT | __GFP_NORETRY | __GFP_HIGHMEM | __GFP_MOVABLE;
> @@ -1012,6 +1029,14 @@ static bool zswap_decompress(struct zswap_entry *entry, struct folio *folio)
> acomp_ctx = acomp_ctx_get_cpu_lock(entry->pool);
> obj = zpool_obj_read_begin(zpool, entry->handle, acomp_ctx->buffer);
>
> + /* zswap entries of PAGE_SIZE are not compressed. */
of length?
> + if (entry->length == PAGE_SIZE) {
> + memcpy_to_folio(folio, 0, obj, entry->length);
> + zpool_obj_read_end(zpool, entry->handle, obj);
> + acomp_ctx_put_unlock(acomp_ctx);
> + return true;
> + }
> +
> /*
> * zpool_obj_read_begin() might return a kmap address of highmem when
> * acomp_ctx->buffer is not used. However, sg_init_one() does not
>
> base-commit: 2ec534125ae474292175ae198483c545eac2161d
> --
> 2.39.5
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