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 zPdN4X8s DmUMVl8aBBbFAkrOaUIhHRJw9PKIdTxY/UmMxGDHNaRpUO8A+npp1HxmuiIFxptr/gS1TlauD+hgiMJX5JUAIA+q2/jvum13NpFpo8tAEQSn4BKQbJAK+8hQ0XkDMT1t114plk+/gJPSXjoPzmgHtVLh5uwfAih8srWmX8Mijx4sYscJiSlPDzheVyJZWKOsI1N9EH1DRvgOOFVIp5ZCq1IAsQGEX55JzKPJhqseIYMdlZIZEayAKp/qHVOOLLUVpO37iRrsPg8c5NIDUsVtOOn1rxd4fbvuSSQNwD3Qx69iP6JKpjLTw6o4OP/orjcnCOM15cNtlBMtIN5yQIkhbinQrNFClX/DY/mxe6a79Z04cSh2VGR3JSvsC44AH6bedlaHd/ayhKLJVfYNTIMC6sTVRyUxAoe1PCRtF Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 1 Sep 2026, at 16:44, Johannes Weiner wrote: > On Tue, Sep 01, 2026 at 10:01:23PM +0300, Nimrod Oren wrote: >> When THP is enabled, set_recommended_min_free_kbytes() may raise >> min_free_kbytes using a heuristic that scales with pageblock_nr_pages. >> Commit f000565adb77 ("thp: set recommended min free kbytes") added this >> heuristic to help keep pageblocks free and reduce fragmentation for THP >> allocations. > > We've had problems with compaction before when min_free_kbytes was too > small on large machines. Competing free space scanners do a lot of > work only to fight over a very small set of possible target pages. > > So I'm a bit uneasy that you didn't include any benchmark numbers with > this that prove basic functionality on larger hosts isn't regressed. > >> The recommendation scales poorly with larger base page sizes. With the >> default arm64 pageblock sizes, the contribution per eligible zone >> before applying the existing cap of 5% of low memory is: >> >> 4 KiB pages: 2 MiB pageblock, 22 MiB per zone >> 16 KiB pages: 32 MiB pageblock, 352 MiB per zone >> 64 KiB pages: 512 MiB pageblock, 5.5 GiB per zone > > I question whether pageblocks need to be 512M on those machines to > begin with. After this patch, you're still asking the page allocator > to optimize grouping such that 512M pages can be allocated at > runtime. Only now you took away part of the mechanism to do so. > > If you're using 512M THPs, I would kind of assume it's on machines > with a memory size where 5.5G for defrag purposes isn't devastating. > > And if you're not, it would make more sense to lower the pageblock > size to the mTHP size you're actually using. And that would fix the > "excessive" min_free_kbytes issue as well. But lowering pageblock size requires a kernel compilation. That means maintaining two sets of kernels for different needs. The ultimate solution is to enable better compaction to generate THPs bigger than a pageblock size, like Rik's super-pageblock proposal. After removing automatic min_free_kbytes boosting, user can still increase it via sysctl to restore the old free memory head room. It is much easier, right? > >> The automatic min_free_kbytes increase predates proactive compaction >> and many subsequent changes to compaction. Given those changes, >> increasing min_free_kbytes for THP by default is no longer clearly >> justified. > > That's pretty handwavy. How would these changes specifically eliminate > the need for compaction scratch space and allocator fallback options > to stave off fragmentation during placement? Extra free memory is still necessary. min_free_kbytes can be adjusted at machine boot time to achieve it, right? Best Regards, Yan, Zi