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b=X5OwSdP+fi0Uaehu7DjVQsFNIdLpyaoZ93FgUIe97onX9nUceSXf23ESKmpZDFQXS 0rSzmAG2hEV3J8IGKQUGcjWaW2Czv6Te+9TxPQ+bIfomwP55NYIRjWwNR/6+qB3WZN Oo9IA0JTCsQo0QqepLS9wgEyOy+m58czDl8FRh4hGmNuntFlrRwodsKtkjnfPAdMw0 ynBEwbuqOpR5V5Z1YHWSu2R+LlOTGl3ZFa/mG6+x1XgWpgBo8O2Tt+fnaC4kJiVPhL B+e3stH+aDOAFmw7lvKIazYYtEf2fBHP0cjpecKz4ksLX+/cYhQeEyy7STb8L87ScG DyrAh7eZF8sbw== Message-ID: Date: Mon, 1 Jun 2026 10:50:40 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 19/22] mm/page_alloc: implement __GFP_UNMAPPED allocations Content-Language: en-US To: Brendan Jackman , Brendan Jackman , Borislav Petkov , Dave Hansen , Peter Zijlstra , Andrew Morton , David Hildenbrand , Wei Xu , Johannes Weiner , Zi Yan , Lorenzo Stoakes Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, x86@kernel.org, rppt@kernel.org, Sumit Garg , derkling@google.com, reijiw@google.com, Will Deacon , rientjes@google.com, "Kalyazin, Nikita" , patrick.roy@linux.dev, "Itazuri, Takahiro" , Andy Lutomirski , David Kaplan , Thomas Gleixner , Yosry Ahmed References: <20260320-page_alloc-unmapped-v2-0-28bf1bd54f41@google.com> <20260320-page_alloc-unmapped-v2-19-28bf1bd54f41@google.com> <7bfda0d8-2a7a-4337-8b55-d0c158df7839@kernel.org> From: "Vlastimil Babka (SUSE)" Autocrypt: addr=vbabka@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 8bit X-Stat-Signature: xyy5etwj55bed9c3kpb8t1eq3srxq66o X-Rspamd-Queue-Id: 3C7B0A0006 X-Rspam-User: X-Rspamd-Server: rspam10 X-HE-Tag: 1780303848-289826 X-HE-Meta: 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 9RkLk9Y0 kHC/eM97BcG/spd3YfJuPmHLVYRsYdIprcpHBOGAmIyXzpA5vF1t4jYGPIj9uVax87vmKaJ9M0p5vo++Cy2vvCLXJfCL5Ehdp+VjuwYNQ0QpqF+9Ko+ffESj/i6zvq22srUBwfTmSl0OBk8YiYqoPanhvibzyH7Ajvd/OT9WI8DQNN4LX/WxB+klgbaZfw2ZY+qcDdW0c6zMB8u3BdP6wSon2x1xzQfPrN/IT1TpHfWSE9pOkuQJJteHolLiVJD4OpQdfh+3+HCyFi341sQjYCxS8WeGS5Zxvc96bteO+H4aqxG2MaxBWq9yzvowpbkSzrfH4elKi0pcV9+JlCJMvVNp/r825NU37mcPq Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 5/29/26 17:02, Brendan Jackman wrote: > On Fri May 15, 2026 at 4:46 PM UTC, Brendan Jackman wrote: >> On Wed May 13, 2026 at 3:43 PM UTC, Vlastimil Babka (SUSE) wrote: > [...] >>> Uhh, speaking of compaction and reclaim... we rely on finding a whole free >>> pageblock in order to flip it. If that doesn't exist, the whole >>> get_page_from_freelist() will fail, and we might enter the >>> reclaim/compaction cycle in __allow_pages_slowpath(). But since we might >>> ultimately want an order-0 allocation, there won't be any compaction >>> attempted, because that code won't know we failed to flip a pageblock. And >>> the watermarks might look good and prevent reclaim as well I think? We >>> should somehow indicate this, and handle accordingly. Might not be trivial. >>> Or maybe reuse pageblock isolation code to do the migrations directly in >>> __rmqueue_direct_map? >> >> Ah, thanks, I suspect you are right. >> >> I did fear there would be some sort of case where this "not-quite >> reclaim" interacted badly with the actual reclaim, and I tried to test >> it by running some stuff in parallel with stress-ng (allocating >> __GFP_UNMAPPED via secretmem), and I didn't see a difference in the >> effective availability of memory. However, I suspect testing this is >> quite a deep art my "run these two commands that I copy pasted from an >> LLM suggestion" test was just crap. >> >> Do you have any workloads you can suggest for evaluating this kinda >> thing? We would definitely see it in Google prod (I think we see this >> kind of issue with our shrinker-based internal version of ASI distorting >> reclaim behaviour in ways even more subtle than this) but that is not a >> very practical experimental cycle... > > I slop-coded a benchmark: > > https://github.com/bjackman/kernel-benchmarks-nix/tree/master/packages/benchmarks/secretmem-vs-frag > > It does some mmap/munmap patterns to try and generate fragmentation, > then spams secretmem allocations until it gets OOM-killed. > > With this series, I see the OOM-kills happening noticeably sooner on a > 1GiB VM: > > metric: secretmem_allocated_bytes (B) | test: secretmem-vs-frag > +---------------------------------------------+---------+-------------+-------------+-----------------+-------------+-------+ > | kernel_release | samples | mean | min | histogram | max | Δμ | > +---------------------------------------------+---------+-------------+-------------+-----------------+-------------+-------+ > | 7.0.0-rc4-next-20260319 | 4 | 683,147,264 | 643,825,664 | █ | 715,128,832 | | > | 7.0.0-rc4-next-20260319-00028-gf00246eb72cd | 3 | 623,553,195 | 551,550,976 | ███ | 692,060,160 | -8.7% | > +---------------------------------------------+---------+-------------+-------------+-----------------+-------------+-------+ > > So... I think maybe I've reproduced the issue you pointed out? I will > try and fix it and see if this degradation goes away. Since I assume the fragmentating allocations are movable allocations, it might be the case, yeah.