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MKl5rj4KzBNn8BjLFy0KkL+66nc6GGeCkpRlSwMwaUenNyLWR89+Iv6w6DSSsLzMVhbaFjbf/dZu9rbJpsbDtsZCbv/AxTQAOo+eoUCj6e+uM8g7grHD5N0xZKYnuPcXaDanr5oohC8LP4NETPPpFW9VqoyWWtgryIAQHVVQwsUTlFSEu0DCMOq18HzSfWL/oxmtWG23Hxc9uxuEAOMam92XjupK9qzQgFaGTCY0X2uZoDbOmKNX3NRp1K1Y7hWVjT+w4EJfHjs4Ekr3y7eDZFIsn7UYFYs4Xz6CqqhqDPm/j8BzI8x8Ao5o/7uogdm+gh8vNuHtaGVV5CjeQ8NJecq0rFjPbu+xhTNxl8UrzC9hQtlY= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 19 Feb 2026, at 11:00, David Hildenbrand (Arm) wrote: >> >> I see 1G THPs being opportunistically used ideally at the start of the= application >> or by the allocator (jemalloc/tcmalloc) when there is plenty of free m= emory >> available and a greater chance of getting 1G THPs. >> >> Splitting strategy >> =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D >> >> When PUD THP must be break -- for COW after fork, partial munmap, mpro= tect on >> a subregion, or reclaim -- it splits directly from PUD to PTE level, c= onverting >> 1 PUD entry into 262,144 PTE entries. The ideal solution would be to s= plit to >> PMDs and only the necessary PMDs to PTEs. This is something that would= hopefully >> be possible with Davids proposal [3]. With mapping of folios > PMD with PMDs, you can use non uniform split to = keep after-split folios as large as possible. > > There once was this proposal where we would, instead of splitting a THP= , migrate all memory away instead. That means, instead of splitting the 1= GiB THP, you would instead return it to the page allocator where somebod= y else could use it. This sounds more reasonable than splitting 1GB itself. > > However, we cannot easily do the same when remapping a 1 GiB THP to be = mapped by PMDs etc. I think there are examples where that just doesn't wo= rk or is not desired. > > But I considered that in general (avoid folio_split()) an interesting a= pproach. The remapping part is a bit different though. If HW can support multiple TLB entries translating to the same physical f= rame and allow translation priority of TLB entries, this remapping would be ea= sy and we can still keep the 1GB PUD mapping. Basically, we can have 1GB TLB= entry pointing to the 1GB folio and another 4KB TLB entry pointing to the remap= ped region and overriding the part in the original 1GB vaddr region. Without that, SW will need to split the PUD into PMDs and PTEs. Best Regards, Yan, Zi