From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 5608A1E51EF; Tue, 5 Aug 2025 18:51:26 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1754419886; cv=none; b=LEit9D69X45+Qjxgp3QJurZ1G1YtUwVybFhYxKjKfCfOSNRgwAIJlkz18gwaqXOr/XJ67usbP6Tw5XFAgibLnq/roPBfLSRXjulXjREc2SJvJzZ9rLyhwZh3yKg2EpzOk+F042xc+6HdN5rXlZZ88xV+5NfJKDzZVNiDp51C+G4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1754419886; c=relaxed/simple; bh=1foawzQDKMWTN3rf/TiKTCzlWcGktjNZWFrsshx9B+E=; h=From:To:Cc:Subject:Date:Message-Id:In-Reply-To:References: MIME-Version:Content-Type; b=Sfx2sj58yIy8B/0ww6Y5CWshKo6SA9I52KcOLqSMayV+GYJ2wu16xnr7at75Qny3xjpsdxrw5TQMI8qHW8K59A5+EZXPNindWpEF/rFUpYrvpf4aura8RVmNikKSiJmqTdBc7yeOME6SFGz1Z8SJxwMiI+m+8Azckav/sG+SygA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=IUcOi8O0; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="IUcOi8O0" Received: by smtp.kernel.org (Postfix) with ESMTPSA id EFD44C4CEF0; Tue, 5 Aug 2025 18:51:25 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1754419886; bh=1foawzQDKMWTN3rf/TiKTCzlWcGktjNZWFrsshx9B+E=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=IUcOi8O0X1C8mUjrGriiMDakXjw8/PRPLbCdwUmlxyG4ieRvhIfEtOy9XVHncDd67 WYluf/e6wJSAYelOyk/tHal/cUcRnltRlZRA+XnDqGvBcFvbD1huS+BsnENzrxOdln JVl49eUHVNmh9Y7yDhJ5yBbxe8mQyUKeM5jjoln5ALf/PuWm8NkzSJIorn9c6/2Wub ioy9fyXkceJrBbTEZtXL2CQtrc+emA0r4f9PwgDPLCBQjd1J9c/bEZ16wg2JHq+rYE 1QKQ1uI3HVN4L3HBlL0rEFMaHI9qV/F6mc0st4VXOUOxZXVBHNlF0yEJHS0SelN9hZ qqca8KvbqwPJg== From: SeongJae Park To: Nhat Pham Cc: SeongJae Park , "Liam R. Howlett" , Andrew Morton , Chengming Zhou , David Hildenbrand , Johannes Weiner , Jonathan Corbet , Lorenzo Stoakes , Michal Hocko , Mike Rapoport , Suren Baghdasaryan , Vlastimil Babka , Yosry Ahmed , kernel-team@meta.com, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Takero Funaki Subject: Re: [RFC PATCH v2] mm/zswap: store X-Mailer: git-send-email 2.39.5 In-Reply-To: References: Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On Tue, 5 Aug 2025 11:25:38 -0700 Nhat Pham wrote: > On Mon, Aug 4, 2025 at 5:30 PM SeongJae Park 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. > > > > Keep the LRU order and optimize unnecessary decompression overheads in > > the cases, by storing the original content in zpool as-is. 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'. > > > > So this change is 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 a severe memory pressure comes from memcg's memory.high, storing > > incompressible pages as-is may result in reducing accounted memory > > footprint slower, since the footprint will be reduced only after the > > zswap writeback kicks in. This can incur higher penalty_jiffies and > > degrade the performance. Arguably this is just a wrong setup, but we > > don't want to introduce unnecessary surprises. Add a parameter, namely > > 'save_incompressible_pages', to turn the feature on/off as users want. > > It is turned off by default. > > > > 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 > 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: Same to 1-1, but zswap is enabled. > > - 1-3: Same to 1-2, but save_incompressible_pages is turned on. > > - 2-1: memory.high is set to 1200 MiB, zswap is disabled. > > - 2-2: Same to 2-1, but zswap is enabled. > > - 2-3: Same to 2-2, but save_incompressible_pages is turned on. > > > > 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). > > > > Configurations 2-1, 2-2 and 2-3 are similar to 1-1, 1-2 and 1-3 but to > > show those under a severe level of memory pressure (~20% of the 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 2-1 2-2 2-3 > > perf_normalized 1.0000 0.0057 0.0235 0.0367 0.0031 0.0122 0.0077 > > perf_stdev_ratio 0.0582 0.0652 0.0167 0.0346 0.0404 0.0145 0.0613 > > zswpin 0 0 3548424 1999335 0 2912972 1612517 > > zswpout 0 0 3588817 2361689 0 2996588 2029884 > > zswpwb 0 0 10214 340270 0 34625 382117 > > pswpin 0 485806 772038 340967 540476 874909 790418 > > pswpout 0 649543 144773 340270 692666 275178 382117 > > > > '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). Actually ~10% working set pressure is not a mild one, at least > > on this test setup. > > > > 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 enabling the incompressible pages handling feature that is introduced > > by 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. > > > > Under about 20% of working set memory pressure, which could be extreme, > > the performance drops down to 0.31% of no-pressure one when only the > > normal swap is used (2-1). Enabling zswap significantly improves it, up > > to 1.22%, though again showing a significant number of (non-zswap) swap > > I/O due to incompressible pages. > > > > Enabling the incompressible pages handling feature of this patch (2-3) > > didn't reduce non-zswap swap I/O, because the memory pressure is too > > severe to let nearly all zswap pages including the incompressible pages > > written back by zswap shrinker. And because the memory usage is not > > dropped as soon as incompressible pages are swapped out but only after > > those are written back by shrinker, memory.high apparently applied more > > penalty_jiffies. As a result, the performance became even worse than > > 2-2 about 36.88%, resulting in 0.07% of the no-pressure one. > > > > 20% of working set memory pressure is pretty extreme, but anyway the > > incompressible pages handling feature could make it worse in certain > > setups. Hence add the parameter for turning the feature on/off as > > needed, and disable it by default. > > > > 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 > > Suggested-by: Takero Funaki > > Signed-off-by: SeongJae Park > > --- > > Changes from RFC v1 > > (https://lore.kernel.org/20250730234059.4603-1-sj@kernel.org) > > - Consider PAGE_SIZE-resulting 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. > > > > Documentation/admin-guide/mm/zswap.rst | 9 +++++ > > mm/zswap.c | 53 +++++++++++++++++++++++++- > > 2 files changed, 61 insertions(+), 1 deletion(-) > > > > diff --git a/Documentation/admin-guide/mm/zswap.rst b/Documentation/admin-guide/mm/zswap.rst > > index c2806d051b92..20eae0734491 100644 > > --- a/Documentation/admin-guide/mm/zswap.rst > > +++ b/Documentation/admin-guide/mm/zswap.rst > > @@ -142,6 +142,15 @@ User can enable it as follows:: > > This can be enabled at the boot time if ``CONFIG_ZSWAP_SHRINKER_DEFAULT_ON`` is > > selected. > > > > +If a page cannot be compressed into a size smaller than PAGE_SIZE, it can be > > +beneficial to save the content as is without compression, to keep the LRU > > +order. Users can enable this behavior, as follows:: > > + > > + echo Y > /sys/module/zswap/parameters/save_incompressible_pages > > + > > +This is disabled by default, and doesn't change behavior of zswap writeback > > +disabled case. > > + > > A debugfs interface is provided for various statistic about pool size, number > > of pages stored, same-value filled pages and various counters for the reasons > > pages are rejected. > > diff --git a/mm/zswap.c b/mm/zswap.c > > index 7e02c760955f..6e196c9a4dba 100644 > > --- a/mm/zswap.c > > +++ b/mm/zswap.c > > @@ -129,6 +129,11 @@ static bool zswap_shrinker_enabled = IS_ENABLED( > > CONFIG_ZSWAP_SHRINKER_DEFAULT_ON); > > module_param_named(shrinker_enabled, zswap_shrinker_enabled, bool, 0644); > > > > +/* Enable/disable incompressible pages storing */ > > +static bool zswap_save_incompressible_pages; > > +module_param_named(save_incompressible_pages, zswap_save_incompressible_pages, > > + bool, 0644); > > + > > bool zswap_is_enabled(void) > > { > > return zswap_enabled; > > @@ -937,6 +942,29 @@ static void acomp_ctx_put_unlock(struct crypto_acomp_ctx *acomp_ctx) > > mutex_unlock(&acomp_ctx->mutex); > > } > > > > +/* > > + * Determine whether to save given page as-is. > > + * > > + * If a page cannot be compressed into a size smaller than PAGE_SIZE, it can be > > + * beneficial to saving the content as is without compression, to keep the LRU > > + * order. This can increase memory overhead from metadata, but in common zswap > > + * use cases where there are sufficient amount of compressible pages, the > > + * overhead should be not critical, and can be mitigated by the writeback. > > + * Also, the decompression overhead is optimized. > > + * > > + * When the writeback is disabled, however, the additional overhead could be > > + * problematic. For the case, just return the failure. swap_writeout() will > > + * put the page back to the active LRU list in the case. > > + */ > > +static bool zswap_save_as_is(int comp_ret, unsigned int dlen, > > + struct page *page) > > +{ > > + return zswap_save_incompressible_pages && > > + (comp_ret || dlen == PAGE_SIZE) && > > + mem_cgroup_zswap_writeback_enabled( > > + folio_memcg(page_folio(page))); > > +} > > + > > static bool zswap_compress(struct page *page, struct zswap_entry *entry, > > struct zswap_pool *pool) > > { > > @@ -976,8 +1004,13 @@ 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) > > + if (zswap_save_as_is(comp_ret, dlen, page)) { > > + comp_ret = 0; > > + dlen = PAGE_SIZE; > > + memcpy_from_page(dst, page, 0, dlen); > > + } else if (comp_ret) { > > goto unlock; > > + } > > > > zpool = pool->zpool; > > gfp = GFP_NOWAIT | __GFP_NORETRY | __GFP_HIGHMEM | __GFP_MOVABLE; > > @@ -1001,6 +1034,17 @@ static bool zswap_compress(struct page *page, struct zswap_entry *entry, > > return comp_ret == 0 && alloc_ret == 0; > > } > > > > +/* > > + * If save_incompressible_pages is set and writeback is enabled, incompressible > > + * pages are saved as is without compression. For more details, refer to the > > + * comments of zswap_save_as_is(). > > + */ > > +static bool zswap_saved_as_is(struct zswap_entry *entry, struct folio *folio) > > +{ > > + return entry->length == PAGE_SIZE && zswap_save_incompressible_pages && > > + mem_cgroup_zswap_writeback_enabled(folio_memcg(folio)); > > +} > > Actually, this might not be safe either :( > > What if we have the following sequence: > 1. Initially, the cgroup is writeback enabled. We encounter an > incompressible page, and store it as-is in the zswap pool. > 2. Some userspace agent (systemd or whatever) runs, and disables zswap > writeback on the cgroup. > 3. At fault time, zswap_saved_as_is() returns false, so we'll treat > the page-sized stored object as compressed, and attempt to decompress > it. This is a memory corruption. > > I think you can trigger a similar bug, if you enable > zswap_save_incompressible_pages initially, then disable it later on. Nice catch! Thank you for catching this and giving this nice explanation. I agree your points. > > I think you have to do the following: > 1. At store time, if comp_ret or dlen == PAGE_SIZE, treat it as > compression failure. This means: saving as-is when writeback enabled, > and rejecting when writeback disabled. Basically: > > if (!comp_ret || dlen == PAGE_SIZE) { I saw your reply correcting this to '(comp_ret || dlen == PAGE_SIZE)', and that makes sense to me. > if (zswap_save_incompressible_pages && > mem_cgroup_zswap_writeback_enabled(folio_memcg(page_folio(folio)))) { > /* save as-is */ > } else { > /* rejects */ > } > > } > > 2. At load time, just check that dlen == PAGE_SIZE. We NEVER store > PAGE_SIZE "compressed" page, so we can safely assume that it is the > original, uncompressed data. Thank you for even further giving me this nice suggestion. Again this makes sense to me. I will make this change on the next version. Thanks, SJ