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Mon, 23 Mar 2026 04:01:39 -0700 (PDT) Date: Mon, 23 Mar 2026 11:01:38 +0000 In-Reply-To: <554c886f-6678-4208-8566-15aa140f6c4b@intel.com> Mime-Version: 1.0 References: <20260320-page_alloc-unmapped-v2-0-28bf1bd54f41@google.com> <20260320-page_alloc-unmapped-v2-1-28bf1bd54f41@google.com> <554c886f-6678-4208-8566-15aa140f6c4b@intel.com> X-Mailer: aerc 0.21.0 Message-ID: Subject: Re: [PATCH v2 01/22] x86/mm: split out preallocate_sub_pgd() From: Brendan Jackman To: Dave Hansen , Brendan Jackman , Borislav Petkov , Dave Hansen , Peter Zijlstra , Andrew Morton , David Hildenbrand , Vlastimil Babka , Wei Xu , Johannes Weiner , Zi Yan , Lorenzo Stoakes Cc: , , , , Sumit Garg , , , Will Deacon , , "Kalyazin, Nikita" , , "Itazuri, Takahiro" , Andy Lutomirski , David Kaplan , Thomas Gleixner , Yosry Ahmed Content-Type: text/plain; charset="UTF-8" X-Stat-Signature: ydms3mmhcs1sdrstxr3wsms4mydq1eip X-Rspam-User: X-Rspamd-Queue-Id: EB90AC000F X-Rspamd-Server: rspam12 X-HE-Tag: 1774263700-493619 X-HE-Meta: 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 jM9cb8a2 UZyRfuBkKffgGEhYsRrNxiECOfP82NSGtplNw9XAd4lwO18cXPCj2IrTxCix/XhlEEJrBErPEHyCZ42czoyr/M6KJ+S9+OLtJr0uu5gE/tVnuHIYgEL+RPGH1GKvzsDaks0gNkslA4FgkdtwtACC2AimofD/pvwi2c7nojgd1eWh0ZJQ1QnBioalP81Xmn+HGtur6Lcl2XZQ5Yyku0ix3Uv5XStyS7ezBkwEKhkgoB9PZclVBN8A0YCGM0H0/Ere06X2lwwAuTnposmthAncvzLBxSB9Vj4kTVuWt5QLPMIMu5zYrq9mgehMYMna+e02084a4psnRw4wJIPRPTgZXvsieUSakM+8I+1Tp/wdlfRZFn5gWzTIGjet51DIHBQ+CsnQY1T+3NO5AUvv8FMd3+a0KJpkDMVNF/L4lfE1thdiqHSmO62vnbfTte3WTZv70LSHGub5jjdg0IU/drugnQKsPJaoOCHU1Xne1L9mc+k+M/n4GIBOYEbmwHxuYfe35smg5BJUVdtOGnli+i4dtMpyrHmnMzMteEn7ZLHfZanQURf03HhegsrovjGilFM6oN0ALqk08TvWFKPiN5QiFylGzdiQNUNaPXV0x4Jm/gmOHVUVusM0B1a7G7w== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Fri Mar 20, 2026 at 7:42 PM UTC, Dave Hansen wrote: > On 3/20/26 11:23, Brendan Jackman wrote: >> - /* >> - * The goal here is to allocate all possibly required >> - * hardware page tables pointed to by the top hardware >> - * level. > > This comment is pretty important, IMNHO, and you zapped it. > > The problem here is that the per-MM carved out space is PGD-sized. You > want to make sure there are page tables allocated for that space. But, > if you say "go allocate a p4d" then that will collapse down to doing > nothing on a 4-level system. > > So, this is effectively: > > Go allocate a p4d or pud, depending on if it's 4 or 5 level. > Basically, always allocate the level that the hardware PGD > points to. > > Could we put a comment to that effect around somewhere, please? Hm I kinda thought the comments I left in there captured all this stuff, but yeah I can see this is a bit of a weird function so more commentary makes sense. How about I just put a few more words into the top comment: /* * Allocate all possibly requried hardware page tables pointed to ths * top hardware level. In other words, allocate a p4d on 5-level or a * pud on 4-level. */ And then just leave the internal one as it is: /* * On 4-level systems, the P4D layer is folded away and * the above code does no preallocation. Below, go down * to the pud _software_ level to ensure the second * hardware level is allocated on 4-level systems too. */